Methods of titrimetric analysis are divided according to the titration option and according to the chemical reactions that are chosen to determine the substance (component). In modern chemistry there are quantitative and

Types of classification

Titrimetric analysis methods are selected for a specific chemical reaction. Depending on the type of interaction, there is a division of titrimetric determination into separate types.

Analysis methods:

• Redox titration; The method is based on changing the oxidation state of elements in a substance.
• Complexation is a complex chemical reaction.
• Acid-base titration involves complete neutralization of the reacting substances.

Neutralization

Acid-base titration allows you to determine the amount of inorganic acids (alkalimetry), as well as calculate bases (acidimetry) in the desired solution. Using this method, substances that react with salts are determined. When using organic solvents (acetone, alcohol), it became possible to determine a larger number of substances.

Complexation

What is the essence of the titrimetric analysis method? It is assumed that substances are determined by precipitation of the desired ion as a poorly soluble compound or its binding into a slightly dissociated complex.

Redoximetry

Redox titration is based on reduction and oxidation reactions. Depending on the titrated reagent solution used in analytical chemistry, the following are distinguished:

• permanganatometry, which is based on the use of potassium permanganate;
• iodometry, which is based on oxidation with iodine, as well as reduction with iodide ions;
• bichromatometry, which uses oxidation with potassium bichromate;
• bromatometry based on the oxidation of potassium bromate.

Redox methods of titrimetric analysis also include processes such as cerimetry, titanometry, and vanadometry. They involve the oxidation or reduction of ions of the corresponding metal.

By titration method

There is a classification of titrimetric analysis methods depending on the titration method. In the direct version, the ion being determined is titrated with the selected reagent solution. The titration process in the substitution method is based on determining the equivalence point in the presence of unstable chemical compounds. Titration by residue (reverse method) is used when it is difficult to select an indicator, as well as when the chemical reaction proceeds slowly. For example, when determining calcium carbonate, a sample of the substance is treated with an excess amount of titrated

Analysis value

All methods of titrimetric analysis assume:

• accurate determination of the volume of one or each of the reacting chemicals;
• the presence of a titrated solution, thanks to which the titration procedure is performed;
• identification of analysis results.

Titration of solutions is the basis of analytical chemistry, so it is important to consider the basic operations performed when conducting an experiment. This section is closely related to everyday practice. Having no idea about the presence of main components and impurities in a raw material or product, it is difficult to plan a technological chain in the pharmaceutical, chemical, and metallurgical industries. Fundamentals of analytical chemistry are applied to complex economic issues.

Research methods in analytical chemistry

This branch of chemistry is the science of determining a component or substance. Fundamentals of titrimetric analysis - methods used to carry out the experiment. With their help, the researcher draws a conclusion about the composition of the substance and the quantitative content of individual parts in it. It is also possible, during analytical analysis, to identify the oxidation state in which the component of the substance being studied is located. When classifying chemistry, they take into account exactly what action is supposed to be performed. To measure the mass of the resulting sediment, a gravimetric research method is used. When analyzing the intensity of a solution, photometric analysis is necessary. Based on the EMF value, the constituent components of the test drug are determined by potentiometry. Titration curves clearly demonstrate the experiment being carried out.

Analytical Methods Division

If necessary, analytical chemistry uses physicochemical, classical (chemical), and physical methods. Chemical methods are usually understood as titrimetric and gravimetric analysis. Both methods are classical, proven, and widely used in analytical chemistry. involves determining the mass of the desired substance or its constituent components, which are isolated in a pure state, as well as in the form of insoluble compounds. The volumetric (titrimetric) method of analysis is based on determining the volume of the reagent consumed in a chemical reaction, taken in a known concentration. There is a division of chemical and physical methods into separate groups:

• optical (spectral);
• electrochemical;
• radiometric;
• chromatographic;
• mass spectrometric.

Specifics of titrimetric research

This branch of analytical chemistry involves measuring the amount of reagent required to carry out a complete chemical reaction with a known amount of the desired substance. The essence of the technique is that a reagent with a known concentration is added dropwise to a solution of the test substance. Its addition continues until its amount is equivalent to the amount of the analyte that reacts with it. This method allows for high-speed quantitative calculations in analytical chemistry.

The French scientist Gay-Lusac is considered the founder of the technique. The substance or element determined in this sample is called the substance being determined. These may include ions, atoms, functional groups, and bound free radicals. Reagents are gaseous or liquid substances that react with a specific chemical substance. The titration process involves adding one solution to another with constant mixing. A prerequisite for the successful implementation of the titration process is the use of a solution with a specified concentration (titrant). To carry out calculations, they use the number of gram equivalents of the substance contained in 1 liter of solution. Titration curves are constructed after calculations.

Chemical compounds or elements interact with each other in clearly defined weight quantities corresponding to their gram equivalents.

Options for preparing a titrated solution using a weighed portion of the starting substance

As the first method of preparing a solution with a given concentration (certain titer), you can consider dissolving a sample of the exact mass in water or another solvent, as well as diluting the prepared solution to the required volume. The titer of the resulting reagent can be determined by the known mass of the pure compound and the volume of the finished solution. This technique is used to prepare titrated solutions of those chemical substances that can be obtained in pure form, the composition of which does not change during long-term storage. To weigh the substances used, bottles with closed lids are used. This method of preparing solutions is not suitable for substances that are highly hygroscopic, as well as for compounds that react chemically with carbon monoxide (4).

The second technology for preparing titrated solutions is used at specialized chemical plants and in special laboratories. It is based on the use of solid pure compounds weighed in precise quantities, as well as on the use of solutions with a certain normality. The substances are placed in glass ampoules and then sealed. Those substances that are inside glass ampoules are called fixans. During the actual experiment, the ampoule with the reagent is broken over a funnel that has a punching device. Next, the entire component is transferred to a volumetric flask, then the required volume of working solution is obtained by adding water.

A certain algorithm of actions is also used for titration. The burette is filled with the prepared working solution to the zero mark so that there are no air bubbles in its lower part. Next, the analyzed solution is measured with a pipette, then it is placed in a conical flask. Add a few drops of indicator to it. Gradually add the working solution drop by drop from a burette to the prepared solution and monitor the color change. When a stable color appears that does not disappear after 5-10 seconds, the titration process is judged to be complete. Next, they begin calculations, calculate the volume of solution consumed with a given concentration, and draw conclusions based on the experiment performed.

Conclusion

Titrimetric analysis allows you to determine the quantitative and qualitative composition of the analyzed substance. This method of analytical chemistry is necessary for various industries; it is used in medicine and pharmaceuticals. When choosing a working solution, be sure to take into account its chemical properties, as well as the ability to form insoluble compounds with the substance being studied.

Laboratory work No. 8

TITRIMETRIAN ANALYSIS

Purpose of the work: to become familiar with the basics of titrimetric analysis, to study the basic methods and techniques of titration.

THEORETICAL PART

1. The essence of titrimetric analysis. Basic concepts.

Titrimetric (volumetric) analysis is one of the most important types of quantitative analysis. Its main advantages are accuracy, speed of execution and the ability to be used for determining a wide variety of substances. Determination of the content of a substance in titrimetric analysis is carried out as a result of the reaction of a precisely known amount of one substance with an unknown amount of another, followed by calculation of the amount of the substance being determined using the reaction equation. The reaction that occurs must be stoichiometric, that is, substances must react strictly quantitatively, according to the coefficients in the equation. Only if this condition is met can the reaction be used for quantitative analysis.

The main operation of titrimetric analysis is titration– gradual mixing of substances until the reaction is complete. Typically, solutions of substances are used in titrimetric analysis. During titration, a solution of one substance is gradually added to a solution of another substance until the substances react completely. The solution that is poured is called titrant, the solution to which the titrant is added is called titrated solution. The volume of a titrated solution that is subjected to titration is called aliquot part or aliquot volume.

Equivalence point is the point during titration when the reactants have completely reacted. At this point they are in equivalent quantities , i.e., sufficient for the reaction to proceed completely, without residue.

For titration, solutions with precisely known concentrations are used, which are called standard or titrated. There are several types of standard solutions.

Primary standard is a solution with a precisely known concentration, prepared by accurately weighing the substance. The substance for the preparation of the primary standard must have a certain composition and be of a certain degree of purity. The content of impurities in it should not exceed established standards. Often, to prepare standard solutions, the substance undergoes additional purification. Before weighing, the substance is dried in a desiccator over a drying agent or kept at elevated temperature. The sample is weighed on an analytical balance and dissolved in a certain volume of solvent. The resulting standard solution should not change its properties during storage. Standard solutions are stored in tightly closed containers. If necessary, they are protected from direct sunlight and exposure to high temperatures. Standard solutions of many substances (HCl, H2SO4, Na2B4O7, etc.) can be stored for years without changing the concentration.

Due to the fact that preparing a substance for preparing a standard solution is a long and labor-intensive process, the chemical industry produces so-called. fixed channels. Fixanal is a glass ampoule in which a certain portion of the substance is sealed. The ampoule is broken, and the substance is transferred quantitatively into a volumetric flask, then bringing the volume of liquid to the mark. The use of fixation channels greatly simplifies the process and reduces the preparation time of the standard solution.

Some substances are difficult to obtain in chemically pure form (for example, KMnO4). Due to the impurity content, it is often impossible to take an accurate sample of a substance. In addition, solutions of many substances change their properties during storage. For example, alkali solutions are able to absorb carbon dioxide from the air, as a result of which their concentration changes over time. In these cases, secondary standards are used.

Secondary standard is a solution of a substance with a precisely known concentration, which is established according to the primary standard. Secondary standards (for example, solutions of KMnO4, NaOH, etc.) are stored under the same conditions as primary standards, but their concentration is periodically checked against standard solutions of the so-called setting substances.

2. Methods and types of titration.

During the titration process, an aliquot of the solution is usually taken into a flask, then the titrant solution is added to it from a burette in small portions until the equivalence point is reached. At the equivalence point, the volume of titrant consumed to titrate the solution is measured. Titration can be carried out in several ways.

Direct titration is that the solution of the analyte A titrate with standard titrant solution IN. The direct titration method is used to titrate solutions of acids, bases, carbonates, etc.

At reverse titrating an aliquot of a standard solution IN titrated with a solution of the analyte A. Reverse titration is used if the analyte is unstable under the conditions under which the titration is performed. For example, the oxidation of nitrites with potassium permanganate occurs in an acidic environment.

NO2- + MnO2- + 6H+ ® NO3- + Mn2+ + 3H2O

But nitrites themselves are unstable in an acidic environment.

2NaNO2 + H2SO4 ® Na2SO4 + 2HNO2

Therefore, a standard solution of permanganate, acidified with sulfuric acid, is titrated with a solution of nitrite, the concentration of which is to be determined.

Back titration used in cases where direct titration is not applicable: for example, due to a very low content of the analyte, the inability to determine the equivalence point, when the reaction is slow, etc. During back titration to an aliquot of the analyte A pour in a precisely measured volume of a standard solution of the substance IN taken in excess. Unreacted excess substance IN determined by titration with a standard solution of the excipient WITH. Based on the difference in the initial amount of the substance IN and its amount remaining after the reaction, determine the amount of substance IN, which reacted with the substance A, on the basis of which the substance content is calculated A.

Indirect titration or titration by substituent. Based on the fact that it is not the substance being determined that is titrated, but the product of its reaction with the auxiliary substance WITH.

Substance D must be formed strictly quantitatively in relation to the substance A. Having determined the content of the reaction product D titration with a standard solution of the substance IN, Using the reaction equation, the content of the analyte is calculated A.

Reactions used in titrimetric analysis must be strictly stoichiometric, proceed fairly quickly and, if possible, at room temperature. Depending on the type of reaction occurring, there are:

Acid-base titration, which is based on a neutralization reaction.

Redox titration, based on redox reactions.

Complexometric titration, based on complexation reactions.

3. Acid-base titration.

The basis of acid-base titration is the neutralization reaction between an acid and a base. As a result of the neutralization reaction, salt and water are formed.

HAn + KtOH ® KtAn + H2O

The neutralization reaction occurs almost instantly at room temperature. Acid-base titration is used to determine acids, bases, and many salts of weak acids: carbonates, borates, sulfites, etc. Using this method, mixtures of various acids or bases can be titrated, determining the content of each component separately.

When an acid is titrated with a base or vice versa, a gradual change in the acidity of the medium occurs, which is expressed by the pH value. Water is a weak electrolyte that dissociates according to the equation.

H2O ® H+ + OH-

The product of the concentration of hydrogen ions and the concentration of hydroxyl ions is a constant value and is called ionic product of water.

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In a neutral environment, the concentrations of hydrogen ions and hydroxide ions are equal and amount to 10-7 m/l. The ionic product of water remains constant when an acid or base is added to water. When an acid is added, the concentration of hydrogen ions increases, which leads to a shift in the dissociation equilibrium of water to the left, resulting in a decrease in the concentration of hydroxide ions. For example, if = 10-3 m./l., then = 10-11 m./l. The ionic product of water will remain constant.

If you increase the concentration of alkali, the concentration of hydroxide ions will increase, and the concentration of hydrogen ions will decrease, the ionic product of water will also remain constant. For example, = 10-2, = 10-12

pH value is called the negative decimal logarithm of the hydrogen ion concentration.

pH = - log. (2)

Based on equation (1), we can conclude that in a neutral environment pH = 7.

pH = - log 10-7 = 7.

In an acidic pH environment< 7, в щелочной рН >7. The formula for pOH is derived similarly from equation (1).

pOH = - log = 14 – pH. (3)

During acid-base titration, the pH of the solution changes with each portion of added titrant. At the equivalence point, the pH reaches a certain value. At this point in time, the titration must be stopped and the volume of titrant used for titration must be measured. To determine pH at the equivalence point, build titration curve– graph of the dependence of the pH of the solution on the volume of added titrant. The titration curve can be constructed experimentally by measuring the pH at various points in the titration, or calculated theoretically using formulas (2) or (3). As an example, consider the titration of a strong acid HCl with a strong base NaOH.

Table 1. Titration of 100 ml of 0.1 M HCl solution with 0.1 M NaOH solution.

	nNaOH (mol)	nHCl (mol) reacted.	nHCl remaining in solution (mol)
	1,00 10-2	1,00 10-2

As alkali is added to an acid solution, the amount of acid decreases and the pH of the solution increases. At the equivalence point, the acid is completely neutralized by the alkali and pH = 7. The reaction of the solution is neutral. With further addition of alkali, the pH of the solution is determined by the excess amount of NaOH. When adding 101 and 110 ml. NaOH solution excess alkali is 1 and 10 ml, respectively. The amount of NaOH at these two points, based on the formula for the molar concentration of the solution, is equal to mol and 1 10-3 mol, respectively

Based on formula (3) for a titrated solution with an excess of alkali of 1 and 10 ml. we have pH values ​​of 10 and 11, respectively. Based on the calculated pH values, we construct a titration curve.

The titration curve shows that at the beginning of titration, the pH of the solution is determined by the presence of hydrochloric acid in the solution and changes slightly when an alkali solution is added. Near the equivalence point, a sharp jump in pH occurs when a very small amount of alkali is added. At the equivalence point, only salt and water are present in the solution. The salt of a strong base and a strong acid does not undergo hydrolysis and therefore the reaction of the solution is neutral pH = 7. Further addition of alkali leads to an increase in the pH of the solution, which also changes slightly depending on the volume of the added titrant, as at the beginning of the titration. In the case of titration of strong acids with strong bases and vice versa, the equivalence point coincides with the neutrality point of the solution.

When titrating a weak acid with a strong base, a slightly different picture is observed. Weak acids in solutions do not dissociate completely and equilibrium is established in the solution.

HAn ® H+ + An-.

The constant of this equilibrium is called the acid dissociation constant.

(4)

Since a weak acid does not dissociate completely, the concentration of hydrogen ions cannot be reduced to the total concentration of the acid in the solution, as was the case with the titration of a strong acid. (6)

When a solution of alkali is added to a solution of a weak acid, a salt of the weak acid is formed in the solution. Solutions containing a weak electrolyte and its salt are called buffer solutions. Their acidity depends not only on the concentration of the weak electrolyte, but also on the concentration of salt. Using formula (5), you can calculate the pH of buffer solutions.

СKtAn – salt concentration in the buffer solution.

KD – dissociation constant of a weak electrolyte

CHАn is the concentration of a weak electrolyte in solution.

Buffer solutions have the property of maintaining a certain pH value when an acid or base is added (hence their name). Adding a strong acid to a buffer solution results in the displacement of the weak acid from its salt and, consequently, in the binding of hydrogen ions:

KtAn + H+ ® Kt+ + HAn

When a strong base is added, the latter is immediately neutralized by the weak acid present in the solution to form a salt,

HAn + OH-® HOH + An-

which also leads to stabilization of the pH of the buffer solution. Buffer solutions are widely used in laboratory practice in cases where it is necessary to create an environment with a constant pH value.

As an example, consider the titration of 100 ml. 0.1M. acetic acid solution CH3COOH, 0.1M. NaOH solution.

When alkali is added to a solution of acetic acid, a reaction occurs.

CH3COON + NaOH ® CH3COONa + H2O

From the reaction equation it is clear that CH3COOH and NaOH react in a 1:1 ratio, therefore the amount of acid that reacted is equal to the amount of alkali contained in the poured titrant. The amount of sodium acetate CH3COONa formed is also equal to the amount of alkali added to the solution during titration.

At the equivalence point, acetic acid is completely neutralized and sodium acetate is present in solution. However, the reaction of the solution at the equivalence point is not neutral, since sodium acetate, as a salt of a weak acid, undergoes hydrolysis at the anion.

CH3COO - + H+OH- ® CH3COOH + OH-.

It can be shown that the concentration of hydrogen ions in a solution of a salt of a weak acid and a strong base can be calculated using the formula.

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CH3COOH reacted.

CH3COOH remaining in solution

1,00 10-2

1,00 10-2

0 ,100

Using the data obtained, we construct a titration curve of a weak acid with a strong base.

The titration curve shows that the equivalence point when titrating a weak acid with a strong base does not coincide with the neutrality point and lies in the region of the alkaline reaction of the solution.

Titration curves allow you to accurately determine the pH of a solution at the equivalence point, which is important for determining the end point of the titration. Determination of the equivalence point can be done instrumentally, directly measuring the pH of the solution using a pH meter, but more often acid-base indicators are used for these purposes. Indicators by their nature are organic substances that change their color depending on the pH of the environment. The indicators themselves are weak acids or bases that dissociate reversibly according to the equation:

НInd ® H+ + Ind-

The molecular and ionic forms of the indicator have different colors and transform into each other at a certain pH value. The pH range within which the indicator changes color is called the indicator transition interval. For each indicator, the transition interval is strictly individual. For example, the methyl red indicator changes color in the pH range = 4.4 – 6.2. At pH< 4,4 индикатор окрашен в красный цвет, при рН >6.2, in yellow. Phenolphthalein is colorless in an acidic environment, but in the pH range = 8 – 10 it acquires a crimson color. In order to choose the right indicator, it is necessary to compare its transition interval with the pH jump on the titration curve. The indicator transition interval should, if possible, coincide with the pH jump. For example, when titrating a strong acid with a strong base, a pH jump is observed in the range of 4-10. This interval includes the transition intervals of indicators such as methyl red (4.4 - 6.2), phenolphthalein (8 - 10), litmus (5 - 8). All of these indicators are suitable for establishing the equivalence point in this type of titration. Indicators such as alizarin yellow (10 – 12), thymol blue (1.2 – 2.8) are completely unsuitable in this case. Their use will give completely incorrect analysis results.

When choosing an indicator, it is desirable that the color change be as contrasting and sharp as possible. For this purpose, mixtures of various indicators or mixtures of indicators with dyes are sometimes used.

3. Oxidation-reduction titration.

(redoximetry, oxidimetry.)

Redox methods include a wide group of titrimetric analysis methods based on the occurrence of redox reactions. Redox titrations use various oxidizing and reducing agents. In this case, it is possible to determine reducing agents by titration with standard solutions of oxidizing agents, and vice versa, determining oxidizing agents with standard solutions of reducing agents. Due to the wide variety of redox reactions, this method makes it possible to determine a large number of different substances, including those that do not directly exhibit redox properties. In the latter case, back titration is used. For example, when determining calcium, its ions precipitate oxalate - an ion

Ca2+ + C2O42- ® CaC2O4¯

The excess oxalate is then titrated with potassium permanganate.

Redox titration has a number of other advantages. Redox reactions occur quite quickly, allowing titration to be carried out in just a few minutes. Many of them occur in acidic, neutral and alkaline environments, which significantly expands the possibilities of using this method. In many cases, fixing the equivalence point is possible without the use of indicators, since the titrant solutions used are colored (KMnO4, K2Cr2O7) and at the equivalence point the color of the titrated solution changes from one drop of titrant. The main types of redox titrations are distinguished by the oxidizing agent used in the reaction.

Permanganatometry.

In this redox titration method, the oxidizing agent is potassium permanganate KMnO4. Potassium permanganate is a strong oxidizing agent. It is capable of reacting in acidic, neutral and alkaline environments. In different environments, the oxidizing ability of potassium permanganate is not the same. It is most pronounced in an acidic environment.

MnO4- + 8H+ +5e ® Mn+ + 4H2O

MnO4- + 2H2O + 3e ® MnO2¯ + 4OH-

MnO4- + e ® MnO42-

The permanganatometric method can determine a wide variety of substances: Fe2+, Cr2+, Mn2+, Cl-, Br-, SO32-, S2O32-, NO2,- Fe3+, Ce4+, Cr2O72+, MnO2, NO3-, ClO3-, etc. Many organic substances: phenols, amino sugars, aldehydes, oxalic acid, etc.

Permanganatometry has many advantages.

1. Potassium permanganate is a cheap and readily available substance.

2. Permanganate solutions are colored crimson, so the equivalence point can be established without the use of indicators.

3. Potassium permanganate is a strong oxidizing agent and is therefore suitable for the determination of many substances that are not oxidized by other oxidizing agents.

4. Titration with permanganate can be carried out with different reactions of the medium.

Permanganatometry also has some disadvantages.

1. Potassium permanganate is difficult to obtain in chemically pure form. Therefore, it is difficult to prepare a standard solution based on an accurate weighing of the substance. For titration, secondary permanganate standards are used, the concentration of which is established using standard solutions of other substances: (NH4)2C2O4, K4, H2C2O4, etc., which are called setting substances.

2. Permanganate solutions are unstable and during long-term storage they change their concentration, which must be periodically checked against solutions of the setting substances.

3. Oxidation of many substances with permanganate at room temperature proceeds slowly and heating of the solution is required to carry out the reaction.

Iodometry.

In iodometric titration, the oxidizing agent is iodine. Iodine oxidizes many reducing agents: SO32-, S2O32-, S2-, N2O4, Cr2+, etc. But the oxidizing ability of iodine is much less than that of permanganate. Iodine is poorly soluble in water, so it is usually dissolved in a KI solution. The concentration of a standard iodine solution is set with a standard solution of sodium thiosulfate Na2S2O3.

2S2O32- + I2 ® S4O62- + 2I-

For iodometric determination, various titration methods are used. Substances that are easily oxidized by iodine are titrated directly with a standard iodine solution. This is how they define: CN-, SO32-, S2O32-, etc.

Substances that are more difficult to oxidize with iodine are titrated using the back titration method: an excess of iodine solution is added to the solution of the substance being determined. After the reaction is completed, excess iodine is titrated with a standard thiosulfate solution. The indicator in iodometric titration is usually starch, which gives a characteristic blue color with iodine, by the appearance of which one can judge the presence of free iodine in the solution.

Many oxidizing agents are determined by indirect iodometric titration: a certain volume of a standard potassium iodide solution is added to the oxidizing solution, free iodine is released, which is then titrated with a standard thiosulfate solution. Cl2, Br2, O3, KMnO4, BrO32-, etc. are determined by the indirect titration method.

Advantages of the iodometric method.

1. The iodometric method is very accurate and superior in accuracy to other redox titration methods.

2. Iodine solutions are colored, which allows in some cases to determine the equivalence point without the use of indicators.

3. Iodine is highly soluble in organic solvents, which allows it to be used for titration of non-aqueous solutions.

Iodometry also has some disadvantages.

1. Iodine is a volatile substance and during titration it may be lost due to evaporation. Therefore, iodometric titration should be carried out quickly and, if possible, in the cold.

2. Iodide ions are oxidized by atmospheric oxygen, for this reason iodometric titration must be carried out quickly.

3. Define the concepts: primary standard, secondary standard, titrant, aliquot volume, titration.

4. What types of titrimetric analysis exist, what is their classification based on?

5. List the main types of redox titration. Give a brief description of permanganatometry and iodometry.

6. What is called the equivalence point? What methods exist for establishing it, and which of them were used in this laboratory work?

7. What are titration curves used for? What are the principles of their construction in acid-base and redox titrations?

In titrimetric analysis, the quantitative determination of a substance is made based on the volume of a solution of known concentration spent on the reaction with a certain substance.

The process of determining the content of a substance or the exact concentration of a solution by volumetric analytical means is called titration. This most important operation of titrimetric analysis consists in the fact that another solution of a precisely known concentration is slowly added to the test solution in an amount equivalent to the amount of the compound being determined.

The volumes of solutions that react quantitatively with each other are inversely proportional to the normal concentrations of these solutions:

V 1 = N 2 or V 1 x N 1 = N 2 x V 2 V 1 x N 1 = V 2 x N 2

Where V is the volume of the reacting solution, l; N – concentration, n.

This position underlies titrimetric analysis. In order to determine the concentration of one of the solutions, it is necessary to know the exact volumes of the reacting solutions, the exact concentration of the other solution, and the moment when the two substances will react in equivalent quantities. The conditions for titrimetric determination are:

a) accurate measurement of volumes of reacting substances;

b) preparation of solutions of precisely known concentration, with the help of which titration is carried out, the so-called working solutions (titranes)(often such solutions of known concentration are called standard (titrated);

c) determining the end of the reaction.

Titrimetric determination takes much less time than gravimetric determination. Instead of many lengthy operations of gravimetric analysis (sedimentation, filtration, weighing, etc.), titrimetric determination involves only one operation - titration.

The accuracy of titrimetric determinations is somewhat less than the accuracy of gravimetric analysis, but the difference is small, therefore, where possible, they try to carry out the determination using a faster method.

In order for a particular reaction to serve as a basis for titration, it must satisfy a number of requirements.

1. The reaction must proceed quantitatively according to a certain equation without side reactions. You need to be confident. That the added reagent is spent exclusively on the reaction with the substance being determined.

2. The end of the reaction should be accurately recorded so that the amount of reagent is

equivalent to the amount of the substance being determined. The calculation of analytical results is based on the equivalence of the reacting substances.

3. The reaction must proceed at a sufficient speed and be practically irreversible. It is almost impossible to accurately fix the equivalence point for slow reactions.

TITRATION METHODS

According to the method of titration, a distinction is made between direct, reverse or indirect titration (substitution method).

In direct titration, the titrant is directly added to the solution of the analyte. To carry out analysis using this method, one working solution is sufficient. For example, to determine an acid, a working solution of an alkali is required, and to determine an oxidizing agent, a solution of a reducing agent is required.

During back titration, a known volume of working solution, taken in excess, is added to the solution of the analyte. After this, the remainder of the first working solution is titrated with another working solution and the amount of reagent that reacted with the analyte is calculated. For example, to determine chloride ions, a known volume of AqNO 3 solution taken in excess is added to the analyzed chloride solution. A reaction occurs

Aq + +Cl = AqCl↓.

The excess of the AqNO 3 solution is determined using another working solution - ammonium thiocyanate NH 4 SCN:

Aq + + SCN - = AqSCN↓.

In indirect titration, an excess of a reagent is added to the solution being analyzed, which reacts with the substance being determined. One of the reaction products is then determined by titration. For example, to determine hydrocyanic acid, add a solution of AqNO 3 in excess. A reaction occurs

HCN + AqNO 3 = AqCN↓ + HNO 3

Nitric acid is then easily determined using a working solution of alkali NaOH:

HNO 3 + NaOH = NaNO 3 + H 2 O

In this case, the weak hydrocyanic acid is replaced in equivalent quantities by the strong one.

3. CLASSIFICATION OF TITRIMETRY METHODS

ANALYSIS

In titrimetric analysis, reactions of various types are used (acid-base interaction, complexation, etc.), satisfying the requirements for titrimetric reactions. Some titrimetric methods are named according to the type of main reaction that occurs during titration or the name of the titrant (for example, in argentometric methods the titrant is an AqNO 3 solution, in permanganometric methods - a KMnO 4 solution, etc.) According to the method of fixing the equivalence point, titration methods are distinguished with color indicators, methods of potentiometric titration, conductometric, photometric, etc. When classifying according to the type of main reaction occurring during titration, the following methods of titrimetric analysis are usually distinguished:

1. Acid-base titration methods based on reactions associated with the proton transfer process:

H + + OH - = H 2 O, CH 3 COOH +OH - = CH 3 COO - + H 2 O,

CO 3 2- + H + = HCO - 3;

2. complexation methods using reactions of formation of coordination compounds (for example, complexometry):

Mg 2+ + H 2 V 2- = MgV 2_ + 2H +

WhereV 2 = CH 2 – N /

׀ / CH 2 – COO-

3. Precipitation methods based on reactions of formation of poorly soluble

connections:

Aq + + Cl - + AqCl↓ (argentometry),

Hg 2 2+ +2Cl - = Hg 2 Cl 2 ↓ (mercurometry);

4.methods of redox titration. based

on redox reactions (oxidimetry):

MnO 4 - + 5Fe 2+ + 8H + = Mn 2+ + 5Fe 3+ + 4H 2 O (permanganatometry);

2S 2 O 3 2- + l 2 = S 4 O 6 2- + 2l - (iodometry);

5NO - 2 + 2MnO 4 - + 6H + + 5NO - 3 + 2Mn 2+ +3H 2 O (nitritometry);

3SbCl 4 - + Br - 3 + 6H + + 6Cl - = 3SbCl 6 - + Br _ + 3H 2 O (bromatometry).

A wide variety of reactions are used in titrimetry. Depending on what reaction underlies the titration, the following methods of titrimetric analysis are distinguished.

Acid-base methods, which are based on the neutralization reaction:

H + + OH - → H 2 O

This method determines the amount of acids, bases, and some salts.

Oxidation-reduction methods(oxidimetry). These methods are based on oxidation-reduction reactions. Using a solution of an oxidizing agent, the amount of a substance that is a reducing agent is determined and vice versa.

Precipitation and complexation methods are based on the precipitation of ions in the form of sparingly soluble compounds and on the binding of ions into a slightly dissociated complex.

The following are distinguished: titration methods:

direct, when during titration a reaction occurs between the analyte and the titrant;

the opposite, to when a obviously excess, but precisely measured volume of a solution of known concentration is added to the solution to be determined and the excess reagent is titrated with a titrant;

substituent titration when the product of the reaction of the analyte with any reagent is titrated with a titrant.

TITRANTS

Titrant is the solution used for titrimetric determination, i.e. titration solution. To carry out determination using a titrant, you need to know its exact concentration. There are two methods for preparing titrated solutions, i.e. solutions of precisely known concentration.

1. An accurate sample taken on an analytical balance is dissolved in a volumetric flask, i.e. A solution is prepared in which the amount of dissolved substance and the volume of the solution are known. In this case, the solutions are called solutions with prepared titer.

2. The solution is prepared to approximately the desired concentration, and the exact concentration is determined by titration, having another solution with the prepared titer. Titrated solutions, the exact concentration of which is found as a result of titration, are called solutions with a set titer.

Titrants are usually prepared to approximately the desired concentration, and their exact concentration is determined. It must be remembered that the titer of solutions changes over time and must be checked at certain intervals (from 1 to 3 weeks, depending on the substance from which the solution is prepared). Therefore, if the titrant is prepared from a precisely taken sample, then its titer corresponds to that prepared only for a limited time.

One of the rules of titrimetric analysis is the following: titrant titers must be set under the same conditions under which the analysis will be performed.

To determine the exact concentration of the titrant (“titre setting.” or standardization), use the so-called starting or setting substance.

The accuracy of determining the titrant titer, and, consequently, the accuracy of all subsequent analyzes depends on the properties of the establishing substance. The installation substance must satisfy the following requirements.

Correspondence of the composition of a substance to its chemical formula.

Chemical purity - the total amount of impurities should not exceed 0.1% - Stability in air, i.e. carbon dioxide.

Stability in solution (does not oxidize or decompose).

Perhaps a large equivalent mass reduces the relative error in determination.

Good solubility in water.

The ability to react with a solution, the titer of which is established according to a strictly defined equation and at high speed.

To set the titrant titer from the setting substance prepare an exact solution using a precisely taken sample. The solution is prepared in a volumetric flask. The volumetric flask should be washed with a chrome mixture until it drains completely, rinsed many times with tap water and then 3 to 4 times with distilled water. The funnel should be clean, dry and fit freely into the neck of the flask.

A sample of the setting substance is weighed on an analytical balance in a weighing bottle. You can weigh out the exact calculated amount, or you can take an amount close to the calculated amount, but accurately weighed. In the first case, the solution will be of exactly the specified concentration, and in the second, the exact concentration is calculated.

The sample taken is carefully transferred through a funnel into a volumetric flask. The remains from the bottle are thoroughly washed into a funnel with distilled water from the rinse. Then they wash the inner walls of the funnel and, slightly lifting it, wash the outer part of the tube. It is necessary to ensure that the total amount of water used to wash the beaker and funnel does not occupy more than half of the flask. Using a gentle rotational motion, stir the contents of the flask until the sample is completely dissolved. Then use distilled water from the washing machine to bring the contents of the flask to the mark. To do this, pour water approximately 1 cm below the mark. Place the flask so that the mark is at eye level and carefully add water drop by drop until the lower part of the meniscus touches the mark on the neck of the flask (Fig. 1). Carefully close the flask with a stopper and, turning the flask over, stir the solution 12-15 times. Solutions for titration must be freshly prepared.

To obtain titrated solutions, they often use fixed channels, which are sealed glass ampoules with precisely weighed reagents. Each ampoule has an inscription showing what substance and in what quantity is in the ampoule.

A funnel is inserted into the volumetric flask, also thoroughly washed and rinsed with distilled water. If the ampoule contains not a solution, but a dry substance, then the funnel should be dry. Then a special glass striker (usually included in the box with fixings) is inserted into the funnel, also rinsed with distilled water. The ampoule is wiped with ethyl alcohol to remove the inscription and washed with distilled water. Then it is inserted into the funnel so that it touches the striker with its thin, inwardly curved bottom, lift it and lightly hit the end of the striker. In this case, the contents of the ampoule enter the flask through a funnel (Fig. 2). There is a recess on the side or top of the ampoule, in which a hole is punched with a glass rod with a pointed end. Through this hole, the inner walls of the ampoule are washed with distilled water from the washing machine. You need to rinse many times in small portions. After this, the outer walls of the ampoule are rinsed and the ampoule is discarded. Rinse the funnel and striker, then lift the funnel and wash the outer

Part of a funnel tube. Wash the top of the neck of the volumetric flask. When performing all these washing operations, make sure that the amount of water in the volumetric flask at the end of all operations does not exceed 2-3 times the volume of the flask. Gently stir the contents of the flask with a rotating motion. If the fixanal contained a dry substance, stir it until completely dissolved. Then use distilled water to bring the contents of the flask to the mark. Carefully close the flask and stir the solution 12-15 times.

To establish the titrant of the titrant, separate portions of the solution are taken with a pipette and titrated. You can also take separate weighed portions of the starting substance and, having dissolved each of them in an arbitrary amount of water, titrate the entire resulting solution. This method gives more accurate results than the first, but is too labor-intensive. Therefore, in the laboratory, practically when performing analyzes, they use the first method.

5. DETERMINING THE POINT OF EQUIVALENCE AND END

REACTIONS

When titrating, do not use an excess of the reagent, but an amount equivalent to the amount of the substance being determined. A necessary condition for determining the content of a substance titrimetrically is the precise determination of the moment when the reaction between the titrated substance and the titrant ends, that is, fixation of the point equivalence. The more precisely the end of the reaction is determined, the more accurate the analysis result will be.

To determine the end of a reaction, special reagents, so-called indicators, are used. The action of indicators usually boils down to the fact that, after the completion of the reaction between the titrated substance and the titrant in the presence of a small excess of the latter, they undergo changes and change the color of the solution or precipitate. When so much titrant has been added from the burette that a noticeable change in the color of the titrated solution is observed, it is said that the titrant has been achieved. end point of titration.

In most cases, indicators are added to a solution of the test substance and titration occurs in the presence of the indicator. These are the so-called internal indicators. In some cases, they do it differently: as the titration proceeds, a drop of the solution is taken from the titrated solution with a capillary, to which a drop of an indicator is added on a porcelain plate. Thus, the reaction with the indicator occurs outside the titrated solution. The indicators used in this case are called external.

There are separate indicators for each titrimetric method. In acid-base titration, indicators change color as the pH of the solution changes. In precipitation methods, the equivalence point is determined by the cessation of sediment formation. The indicators used in these methods form a brightly colored precipitate or solution with an excess of titrant. Sometimes, if you are titrating with a brightly colored solution, for example a KMnO 4 solution, the end of the titration can be seen without an indicator, since the first drop of titrant that does not react with a certain substance changes the color of the titrated solution.

Titrimetric analysis

History and principle of the method

Titrimetric analysis (titrimetry) is the most important chemical analysis method. It originated in the 18th century, initially as an empirical way of testing the quality of various materials, such as vinegar, soda, and bleaching solutions. At the turn of the 18th and 19th centuries, burettes and pipettes were invented (F. Decroisille). Of particular importance were the works of J. Gay-Lussac, who introduced the basic terms of this method: titration, titrant and others derived from the word “title”. Titer is the mass of dissolved substance (in grams) contained in one milliliter of solution. In Gay-Lussac's time, analysis results were calculated using titers. However, the titer as a way of expressing the concentration of a solution turned out to be less convenient than other characteristics (for example, molar concentrations), therefore, in modern chemistry analytics, calculations using titers are quite rare. On the contrary, various terms derived from the word "title" are used very widely.

In the middle of the 19th century, the German chemist K. Mohr summarized all the titrimetric methods created by that time and showed that the basis of any method is the same principle. A solution with a precisely known concentration of the reagent R (titrant) is always added to the sample solution containing the component X to be determined. This process is called titration. When performing a titration, the analyst monitors the progress of the chemical reaction between X and the added R. Upon reaching the equivalence point (eq.), when the number of moles of equivalents of the introduced R is exactly equal to the number of moles of equivalents of the substance X present in the sample, the titration is stopped and the volume of titrant consumed is measured. The moment the titration ends is called the end point of titration (e.t.t.), it, like t.eq., is expressed in units of volume, usually in milliliters. In the ideal case, V t.t.t = V t.eq. , but in practice an exact match is not achieved for various reasons; the titration is completed a little earlier or, conversely, a little later than the t.eq. is achieved. Naturally, the titration should be carried out so that the difference between V t.eq. and V k.t.t. would be as small as possible.

Since the mass or concentration of X is calculated from the volume of titrant spent on titrating the sample (according to V c.t.t.), in the past titrimetry was called volumetric analysis. This name is often used today, but the term titrimetric analysis more accurate. The fact is that the operation of gradual addition of the reagent (titration) is characteristic of any technique of this type, and the consumption of the titrant can be assessed not only by measuring the volume, but also in other ways. Sometimes the added titrant is weighed (measuring mass on an analytical balance gives less relative error than measuring volume). Sometimes the time it takes for the titrant to be introduced is measured (at a constant injection rate).

Since the end of the 19th century, titrimetric techniques began to be used in research, factory, and other laboratories. Using the new method, it was possible to determine milligram and even microgram amounts of a wide variety of substances. The widespread use of titrimetry was facilitated by the simplicity of the method, low cost and versatility of the equipment. Titrimetry began to be used especially widely in the 50s of the 20th century, after the Swiss analyst G. Schwarzenbach created a new version of this method (complexometry). At the same time, widespread use of instrumental methods for monitoring the c.t.t. began. By the end of the 20th century, the importance of titrimetry decreased somewhat due to the competition of more sensitive instrumental methods, but today titrimetry remains a very important method of analysis. It allows you to quickly, easily and accurately determine the content of most chemical elements, individual organic and inorganic substances, the total content of substances of the same type, as well as general composition indicators (water hardness, milk fat content, acidity of petroleum products).

Technique for titrimetric analysis

The principle of the method will become clearer after the technique of its implementation is described. So, let them bring you an alkali solution of unknown concentration, and your task is to establish its exact concentration. For this you will need reagent solution, or titrant- a substance that reacts chemically with an alkali, and the concentration of the titrant must be precisely known. Obviously, to establish the concentration of alkali, we use an acid solution as a titrant.

1. Using a pipette, select the exact volume of the analyzed solution - it is called aliquot. Typically, the aliquot volume is 10-25 ml.

2. Transfer an aliquot to a titration flask, dilute with water and add an indicator.

3. Fill the burette with titrant solution and perform titration is the slow, dropwise addition of titrant to an aliquot of the test solution.

4. We complete the titration at the moment when the indicator changes color. This moment is called end point of titration – k.t.t. K.t.t., as a rule, coincides with the moment when the reaction between the analyte and the titrant is completed, i.e. exactly equivalent amount of titrant is added to the aliquot - this moment is called equivalence point, i.e. Thus i.e. and k.t.t. - these are two characteristics of the same moment, one is theoretical, the other is experimental, depending on the selected indicator. Therefore, it is necessary to choose the indicator correctly so that the c.t.t. coincided as closely as possible with i.e.

5. Measure the volume of titrant used for titration and calculate the concentration of the test solution.

Types of titrimetric analysis

Titrimetric methods can be classified according to several independent criteria: namely: 1) according to the type of reaction between X and R, 2) according to the method of titration and calculation of results, 3) according to the method of monitoring eq.

Classification by type of chemical reaction– the most important. Let us remember that not all chemical reactions can be used for titrations.

First, as in other chemical methods, the component to be determined (analyte) must react quantitatively with the titrant.

Secondly, it is necessary that the reaction equilibrium be established as quickly as possible. Reactions in which, after adding the next portion of titrant, the establishment of equilibrium requires at least several minutes, are difficult or even impossible to use in titrimetry.

Thirdly, the reaction must correspond to a single and previously known stoichiometric equation. If a reaction leads to a mixture of products, the composition of this mixture will change during the titration and depend on the reaction conditions. It will be very difficult to fix the equivalence point, and the result of the analysis will be inaccurate. The combination of these requirements is met by protolysis (neutralization) reactions, many complexation and oxidation-reduction reactions, as well as some precipitation reactions. Accordingly, titrimetric analysis distinguishes:

Neutralization method

Complexometry,

Redoxmetric methods

Precipitation methods.

Within each method, its individual variants are distinguished (Table 1). Their names come from the names of the reagents used in each option as a titrant (permanganatometry, iodometry, chromatometry, etc.).

Table 1.

Classification of titrimetric techniques according to the type of chemical reaction used

Reaction	Method	Reagent (titrant)	Method option	Determined substances
Protolysis	Neutralization method	H Cl, HClO 4, HNO 3	Acidimetry	Os new
		KOH, NaOH, etc.	Alkalimetry	Acids
Complexity-education	Complexometry	EDTA	Complexometry	Metals and their compounds
			Fluoridometry, cyanidometry	Some metals, organic substances
Oxidation-reduction	Redox metry	KMnO4 K 2 C r 2 O 7	Permanganatometry chromatometry	Restorers
		KJ and Na 2 S 2 O 3	Iodometry	Reducing agents, oxidizing agents, acids
		Ascorbic acid	Ascorbinometry	Oxidizing agents
Precipitation	Sedimetry	AgNO3	Argentometry	Halides
		Hg 2 (NO 3) 2	Mercurimetry
		KSCN	Rhodanometry	Some metals
		Ba(NO3)2	Bariemetry	Sulfates

Classification by titration method. Usually there are three methods: direct, reverse and substitution titration. Direct titration involves the direct addition of titrant to the sample solution. Sometimes a different order of mixing reagents is used - a sample solution in which they want to determine the concentration of X is gradually added to a known amount of R; but this is also a direct titration. In both cases, the analysis results are calculated using the same formulas based on the law of equivalents.

ν X = ν R

where ν X and ν R are the number of moles of equivalents X and R. Calculation formulas based on the ratio, as well as examples of calculations, will be given below.

Direct titration is a convenient and most common type of titrimetry. It is more accurate than others. After all, random errors mainly arise when measuring the volume of solutions, and in this titration method the volume is measured only once. However, direct titration is not always possible. Many reactions between X and R do not proceed quickly enough, and after adding the next portion of the titrant, equilibrium does not have time to establish in the solution. Sometimes direct titration is not possible due to adverse reactions or the lack of a suitable indicator. In such cases, more complex reverse or substitution titration schemes are used. They involve at least two chemical reactions.

Back titration carried out according to a two-stage scheme:

X + R 1 =Y 1

R 1 + R 2 = Y 2

Auxiliary reagent R 1 is introduced in a precisely known quantity. The volume and concentration of the solution R1 is chosen so that R1 remains in excess after completion of the reaction. The unreacted portion of R 1 is then titrated with titrant R 2 . An example would be permanganatometric titration of organic substances. It is not possible to titrate many substances “directly” with permanganate due to the slowness of their oxidation and for other reasons. But you can first add a known (excess) amount of KMnO 4 to the sample being analyzed, acidify and heat the resulting solution. This will lead to complete and rapid completion of the oxidation of organic substances. Then the remaining permanganate is titrated with some active reducing agent, for example, a solution of SnCl 2 or FeSO 4 .

Calculation of the results of back titration is carried out based on the obvious relationship:

ν X =ν R 1 - ν R 2

Since the volumes in this case are measured twice (first the volume of the reagent solution R1, then the volume of the titrant R2), the random error of the analysis result is slightly higher than with direct titration. The relative error of the analysis increases especially strongly with a small excess of the auxiliary reagent, when ν R 1 ≈ν R 2 .

Classification according to the method of control t.eq. Several such methods are known. The simplest is indicatorless titration, the most common is titration with color indicators, and the most accurate and sensitive are instrumental titrimetry options.

Indicatorless titration is based on the use of reactions that are accompanied by a change in the visible properties of the titrated solution. As a rule, one of the reagents (X or R) has a visible color. The progress of such a reaction is monitored without special instruments and without the addition of indicator reagents. Thus, colorless reducing agents are titrated in an acidic medium with a violet solution of an oxidizing agent - potassium permanganate (KMnO 4). Each portion of the added titrant will immediately become discolored, turning into Mn 2+ ions under the action of the reducing agent. This will continue until t.eq. However, the very first “extra” drop of titrant will turn the titrated solution pink-violet; the color will not disappear even when the solution is stirred. When a persistent color appears, the titration is stopped and the volume of titrant consumed is measured ( V k.t.t.). The end of titration can be detected not only by the appearance of color in the titrated solution, as in the example considered, but also by the discoloration of the previously colored sample solution, as well as by the appearance of any precipitate, its disappearance, or a change in appearance. Indicatorless titration is used quite rarely, since only a few reactions are accompanied by a change in the visible properties of the solution.

Instrumental titration. The progress of the reaction between X and R can be monitored not just “by eye” (visually), but also with the help of instruments that measure some physical property of the solution. Options for instrumental titrimetry are distinguished depending on what property of the solution is being controlled. You can use any property depending on the qualitative and quantitative composition of the titrated solution. Namely, you can measure the electrical conductivity of a solution (this option is called conductometric titration), the potential of the indicator electrode immersed in the titrated solution ( potentiometric titration), light absorption by the titrated solution ( photometric titration), etc. Titration can be stopped when a certain pre-selected value of the property being measured is reached. For example, an acid solution is titrated with an alkali until pH = 7 is reached. However, more often they do it differently - the selected property of the solution is measured repeatedly (or even continuously) as the titrant is introduced, not only before, but also after the expected temperature .eq. Based on the data obtained, a graphical dependence of the measured property on the volume of added titrant is plotted ( titration curve). Near the equivalence point, a sharp change in the composition and properties of the titrated solution is observed, and a jump or kink is recorded on the titration curve. For example, a jump in the potential of an electrode immersed in a solution. The position of the t.eq. is assessed by the position of the inflection on the curve. This type of analysis is more labor-intensive and time-consuming than conventional titration, but gives more accurate results. In one titration it is possible to determine the individual concentrations of a number of components.

More than a dozen variants of instrumental titrimetry are known. The American analyst I. Kolthoff played an important role in their creation. The corresponding techniques differ in the property of the solution being measured, in the equipment used and in the analytical capabilities, but they are all more sensitive and selective than indicator-based or indicator-free visual titrimetry options. Instrumental control is especially important when indicators cannot be used, for example, when analyzing turbid or intensely colored solutions, as well as when determining microimpurities and when analyzing mixtures. However, instrumental titrimetry requires equipping the laboratory with special instruments, preferably self-recording or fully automated, which is not always economically feasible. In many cases, fairly accurate and reliable results can be obtained in a simpler and cheaper way, based on the use of indicators.

Using indicators. A small amount of a special reagent can be added to the titrated sample in advance - indicator. The titration will need to be stopped at the moment when the indicator changes its visible color under the influence of the introduced titrant; this is the end point of the titration. It is important that the color change does not occur gradually, as a result of adding just one “extra” drop of titrant. In some cases, the indicator does not change its color, solubility or character of luminescence. However, such indicators (adsorption, fluorescent, chemiluminescent, etc.) are used much less frequently than color indicators. A change in color of any indicator occurs due to the chemical interaction of the indicator with the titrant, leading to the transition of the indicator to a new form. The properties of indicators need to be considered in more detail.

Indicators

In analytical laboratories, several hundred color indicators of various types are used (acid-base, metallochromic, adsorption, etc.). Once upon a time, tinctures obtained from plants were used as indicators - from violet flowers or from a special type of lichen (litmus). R. Boyle was the first to use such indicators. Currently, natural indicators are not used, since they are always a mixture of different substances, so the transition of their color is not clearly expressed. Modern indicators are specially synthesized individual organic compounds. As a rule, indicators are compounds of the aromatic series, the molecules of which contain several functional groups (substituents). Many such compounds are known, but only some of them can be used as color indicators. The proposed indicator must meet a number of requirements:

· the indicator should dissolve well, giving solutions that are stable during storage;

· In solution, the indicator must exist in several forms, different in molecular structure. A mobile chemical equilibrium must be established between the forms. For example, the acidic form of the indicator goes into the basic form (and vice versa), the oxidized form into the reduced form (and vice versa); a metallochromic indicator reversibly binds into a complex with metal ions, etc.;

· color indicator must be intense absorb light in the visible region of the spectrum. The color of its solution should be distinguishable even at very low concentrations (10 -6 - 10 -7 mol/l). In this case, it will be possible to introduce very small amounts of indicator into the titrated solution, which helps to obtain more accurate analytical results;

· different forms of the indicator must be different in their color, that is, in the absorption spectrum in the visible region. In this case, a contrasting color transition will be observed during titration. For example, the color transition of the indicator from pink to emerald green is clearly visible to the eye. It is much more difficult to fix the end point of titration (e.t.t.) by the transitional pink or orange or violet color. It is very important how different the absorption spectra of the two forms of the indicator are. If one of the indicator forms maximally absorbs light with wavelength λ 1, and the other with wavelength λ 2, then the difference ∆λ = λ 1 - λ 2 characterizes the contrast of the color transition. The larger ∆λ, the better the color transition of the indicator is perceived by eye. To increase the visual contrast of a color transition, mixtures of different indicators are sometimes used or a foreign inert dye is added to the indicator;

· the transition of the indicator from one form to another when the composition of the solution changes should occur very quickly, in a fraction of a second;

· the transition must be caused by a single factor, the same for all indicators of this type. Thus, a change in the color of an acid-base indicator should not occur due to reactions of another type, for example, when interacting with oxidizing agents, or metal ions, or proteins! On the contrary, redox indicators should change their color only due to interaction with oxidizing agents and reducing agents, and this should happen at a certain potential specific to each redox indicator. The color of these indicators and the transition potential should not depend on the pH of the solution. Unfortunately, in practice, the transition potential of many redox indicators depends on pH.

To weaken the influence of side processes, sometimes the indicator is not introduced into the titrated solution, but, on the contrary, during the titration, a drop of the titrated solution is periodically taken, mixed on a watch glass with a drop of the indicator solution and observed what color is obtained. This technique allows the use of irreversibly reacting indicators. It is more convenient to work with an “external indicator” if you soak the paper in advance.

The end point of the titration, determined by the color transition of the indicator, may not coincide with the equivalence point. Mismatch V k.t.t. And V t.eq leads to a systematic error in the analysis result. The magnitude of the error is determined by the nature of the indicator, its concentration and the composition of the titrated solution.

The principle of selecting indicators is very simple and universal : the transition characteristic of the indicator (pT titration index, transition potential, etc.) must correspond to the expected composition of the titrated solution at the equivalence point. Thus, if an analyst titrates an aqueous solution of a strong acid with a strong base, at the equivalence point the solution will have a pH = 7. Therefore, it is necessary to use an acid-base indicator that changes color at approximately pH 7 (bromothymol blue, etc.). Required information about pT - titration indicators for indicators of various types is in the reference literature.

Calculation of titrimetric analysis results

It is not recommended to calculate the results of titrimetric analysis directly from the reaction equation, for example, using proportions. This “school” method of solving calculation problems is irrational and, as a rule, does not provide the required accuracy. The results of titrimetric analysis are calculated using one of several ready-made algebraic formulas derived on the basis of the law of equivalents. The initial data will be the volume of titrant consumed (in milliliters) and titrant concentration (in mol/liter); they must be established with the required accuracy.

The calculation method does not depend on the type of chemical reaction occurring during titration and the method of controlling the equivalence point (indicator, device, etc.). The choice of calculation formula is determined by which titration method (direct, reverse, substitution) is used during the analysis. When choosing a formula, two cases should be distinguished: a) calculation of the concentration of solution X; b) determination of the mass fraction of the component (percentage of X in the sample).

The calculation formulas look most simple if the concentrations of the component being determined and the titrant are expressed as the number of moles of their equivalents per liter of the corresponding solutions, i.e. use the concentration of the component being determined ( N x ) and titrant (N T ), expressed as the number of moles equivalent per liter of solution. Previously, these concentrations were called normal. Now this term is not recommended, but in practice it is used very widely, especially in redoxmetry. But in complexometry and some other methods, where 1 mole of the analyte X always reacts with 1 mole of titrant, the normal concentrations coincide with the usual molar concentrations ( C x and C T ), and therefore there is no need to use normal concentrations and equivalents when calculating the results.

Unlike ordinary molar concentrations, the normal concentration is determined taking into account the chemistry of the reaction occurring during the titration. It is useful to remember that the normal concentration of X in a solution is either equal to its molar concentration or exceeds it several (2,3,4...) times, depending on how many protons (or electrons) are involved in the reaction, per particle X. When writing the reaction equation, determining equivalents and calculating normal concentrations, one should take into account the conditions under which the titration takes place, and even the choice of indicator.

Weighttitrated Xatdirect titration equal (in mg):

m x =N T . V T . E x , (1),

where E x - molar mass of the equivalent of X, corresponding to one proton (in acid-base reactions), one electron (in redox reactions), one ligand (in complexation reactions), etc. V T – titrant volume (in ml). In complexometry, the mass of the analyte (in mg) is best calculated using a formula that includes the quantity M x -molar mass X:

m x = C T . V T . M x (2).

From (4.11) it follows that the mass fraction of X in the sample, expressed in %, is equal to:

%X = N T . V T . E x . 100%/m S , (3),

where m S - mass of the sample in mg. Usually, the result of titration does not depend on the volume of water in which the sample was dissolved before titration, and this volume is not taken into account in the calculations. If you titrate not the entire sample, but only some of it (an aliquot), then you need to take into account an additional coefficient TO , equal to the ratio V 0 - volume of solution into which this sample was transferred and from which aliquots were taken, to V aliq - volumetric aliquot:

m x = K. N T . V T . E x , (4).

When calculating concentrationsaccording to the method of direct (or substitution) titration, a simple formula is used, directly following from the law of equivalents:

N x . V x =N T . V T (5).

analysis, but in factory laboratories they also use other calculation methods.

Preparation of working solutions in titrimetry

Working solutions of precisely known concentration used in titrimetric analysis are prepared in several ways:

· by precise weighing of a chemical reagent taken on an analytical balance. This sample is dissolved in a small amount of solvent, and then the volume of the resulting solution is adjusted to the mark in a volumetric flask. The resulting solutions are called standard, and the corresponding reagents are called primary standards. Only a few substances can be primary standards - they must be pure chemicals of constant and precisely known composition, solid at room temperature, stable in air, and not hygroscopic or volatile. Examples include potassium dichromate, complexone III, oxalic acid. On the contrary, from a sample it is impossible to prepare a standard solution of hydrochloric acid (the “hydrochloric acid” reagent is a liquid with an inaccurately known composition), ferrous chloride (rapidly oxidizes in air), sodium hydroxide (hygroscopic) and many other substances.

· from fixed channels. This term refers to a sealed glass ampoule that contains a certain amount of reagent, usually 0.1000 mol equivalent. Fixans are prepared in the factory. If in the laboratory you quantitatively transfer the contents of the fixanal into a 1000 ml volumetric flask and bring it to the mark with solvent, you will get a liter of exactly 0.1000 N solution. Preparation of fixing solutions not only saves the analyst’s time, but also allows one to prepare solutions with precisely known concentrations from substances that do not have the complex of properties required for primary standards (for example, fixing solutions of hydrochloric acid, ammonia or iodine).

· according to an approximately known weighed portion of the chemical reagent taken on a technical scale. This sample is dissolved in an approximately known amount of solvent. Then an additional operation is carried out - standardization of the resulting solution. For example, an exact weighed portion of another substance (primary standard) is titrated with the resulting solution. You can do it another way: take a known volume (aliquot) of the prepared solution and titrate it with a suitable standard solution. Based on the volume used for titration, the exact concentration of the prepared solution is calculated. Such solutions are called standardized. For example, a KOH solution is standardized using a weighed portion of oxalic acid or using a fixed solution of hydrochloric acid. If a substance in the laboratory is available in the form of a concentrated solution of approximately known concentration (for example, hydrochloric acid), then instead of weighing it, a certain pre-calculated volume of the concentrated solution is measured. This requires knowledge of the density of the original solution. Then, as in the previous case, the resulting solution is standardized.

The concentration of solutions should not change spontaneously during storage. In this case, pre-prepared (standard or standardized) solutions can be used for titrations without any additional operations. It should be noted that the more diluted the solution, the less stable it is during storage (hydrolysis of the dissolved substance, its oxidation with oxygen air, adsorption on the inner surface of glassware, etc.). Therefore, working solutions with low concentrations, as a rule, are not prepared in advance. They are prepared only as needed, on the day of use. To do this, the original (standard, fixed or standardized) solutions are diluted with a pure solvent in a precisely known number of times (usually the solution is diluted 5 or 10 times in one operation). If even more dilute solutions are required, this operation is repeated. For example, from a 0.1 M solution 0.01 M is prepared, from that - 0.001 M, etc.

The preparation of solutions with precisely known concentrations requires the use of a whole set of special measuring utensils that allow volumes to be measured with the required accuracy. These are volumetric flasks, pipettes and burettes. The manuals for laboratory work provide descriptions of measuring glassware and rules for working with it.

Titration methods

Method of separate samples and method of aliquots. To reduce the influence of random errors, titrations are usually repeated several times and then the results are averaged. Repeated analyzes can be carried out in two different ways: by the method of individual samples or by the aliquot method. Both methods are used both for standardization of working solutions and for direct analysis of real objects.

Method of individual samples, as is clear from its name, assumes that several portions of the analyzed material are taken for titration. Their masses should be approximately equal. The sample size is selected taking into account the desired titrant consumption per titration (no more than the volume of the burette) and taking into account the titrant concentration.

Let three weighed portions of oxalic acid be taken, the masses of which are indicated in Table 2. Based on the data of each titration, the KOH concentration is calculated (separately!). Then the concentrations are averaged. The volumes spent on titrating different portions cannot be averaged!

Table 2. An example of calculating analysis results using the method of individual samples

Hang number	Weight weight, mg	Titrant volume, ml	Found concentration of KOH, mol/l
	95,7	14,9	0,102
	106,9	16,2	0,105
	80,8	12,7	0,101
Average analysis result C KOH = 0.103 mol/l

Aliquot titration method (or pipetting method) is based on the titration of several individual aliquots - small volumes of the test solution, selected using pipettes.

The method of individual portions and the aliquot titration method are used not only for direct titration, as shown in the examples given, but also for reverse and substitution titration. When choosing a titration method, it should be taken into account that the method of individual samples gives more accurate results, but it is more labor-intensive and requires a larger volume of calculations. Therefore, it is better to use the method of individual portions to standardize working solutions, and for serial analyzes to use the more rapid method of aliquots.

Shape of titration curves

Logarithmic titration curves represent a graphical dependence of the logarithm of the equilibrium concentration of one of the reagents on the volume of added titrant. Instead of the logarithm of concentration, the pH value of the solution (hydrogen value) is usually plotted on the vertical axis. Other similar indicators are also used (for example, pAg = - log), as well as the value of those physicochemical properties of the titrated solution, which linearly depend on the logarithms of equilibrium concentrations. An example would be electrode potential (E).

If the solution contains only one substance that reacts with the titrant, and the reaction is described by a single chemical equation (that is, it does not occur stepwise), an almost vertical section called titration jump . On the contrary, sections of the curve far from the equivalent. close to horizontal. An example is the dependence of the pH of solutions on the volume V of added titrant, shown in Fig. 1

Fig.1. Type of titration curves

The higher the jump height on the titration curve, the more accurately the equivalence point can be fixed.

Acid-base titration (neutralization method)

Principle of the method

The neutralization method is based on acid-base (protolytic) reactions. During this titration, the pH value of the solution changes. Acid-base reactions are most suitable for titrimetric analysis: they proceed according to strictly defined equations, without side processes and at a very high speed. The interaction of strong acids with strong bases leads to high equilibrium constants. To detect c.t.t. There is a convenient and well-studied method - the use of acid-base indicators. Instrumental methods can also be used; they are especially important when titrating non-aqueous, cloudy or colored solutions.

The neutralization method includes two options - acidimetry(titrant - strong acid solution) and alkalimetry(titrant is a solution of a strong base). These methods are respectively used for the determination of bases and acids, including ionic and multiprotic ones. The ability to titrate strong protolytes is determined by their concentration; titration is possible if C x> 10 - 4 M During this titration, the following reaction occurs in an aqueous solution:

H 3 O + +OH - ® 2H2O

Titration of weak acids and weak bases in aqueous solutions follows the following schemes:

HA+OH - ® H 2 O (alkalimetry)

B+H 3 O + ® NV + + H 2 O (acidimetry)

Examples of practical applications of acid-base titration:

· determination of the acidity of food products, soils and natural waters (alkalimetric titration of aqueous solutions with phenolphthalein indicator);

· determination of the acidity of petroleum products (alkalimetric titration of non-aqueous solutions with instrumental control of c.t.t.);

· determination of carbonates and bicarbonates in minerals and building materials (acidimetric titration of aqueous solutions with two indicators);

· determination of nitrogen in ammonium salts and organic substances (Kjeldahl method). In this case, organic nitrogen-containing substances are decomposed by boiling with concentrated sulfuric acid in the presence of mercury salts, ammonia nitrogen is distilled off by the action of an alkali when heated, ammonia is absorbed with a standard solution of HCl, taken in excess. Then the unreacted part of HCl is titrated with alkali in the presence of the methyl orange indicator. This technique uses both the substitution principle and the back titration method.

Working solutions.For acidimetric titration of aqueous solutions, the following are used as titrants: solutions of strong acids (HCl, less often HNO 3 or H 2 SO 4). IN alkalimetry titrants - solutions of NaOH or KOH. However, the listed reagents do not have properties that would make it possible to prepare standard solutions from them simply by accurately weighing them. Thus, solid alkalis are hygroscopic and always contain carbonate impurities. In the case of HCl and other strong acids, the starting reagent is not a pure substance, but a solution with an imprecisely known concentration. Therefore, in the neutralization method, a solution with an approximately known concentration is first prepared, and then it is standardized. Acid solutions are standardized using anhydrous sodium carbonate Na 2 CO 3 (soda) or sodium tetraborate Na 2 B 4 O 7 . 10H 2 O (borax). Borax reacts with water when dissolved:

B 4 O 7 2– +3H 2 O=2H 3 VO 3 + 2VO 2 –

The resulting metaborate is a fairly strong base. It is titrated with acid:

VO 2 – + H 3 O + = H 3 VO 3.

Obviously, the molar mass of borax equivalent is M(½Na 2 B 4 O 7 . 10H 2 O) = 190.71 g/mol. High molar mass equivalent is an advantage of borax as a primary standard. Alkali solutions are standardized using potassium hydrophthalate. The hydrophthalate molecule contains a mobile proton and has the properties of a weak acid:

Benzoic acid C 6 H 5 COOH and oxalic acid H 2 C 2 O 4 are often used as standards . 2H 2 O and other weak organic acids (solid, pure, stable substances). Standard 0.1000 M solutions of acids and bases in laboratories are usually prepared from fixanals. The prepared acid solution can be used to standardize the alkali solution, and vice versa. Standardized acid solutions are stable and can be stored without change for an indefinitely long time. Alkali solutions are less stable; it is recommended to store them in waxed or fluoroplastic containers to prevent interaction with glass. It must be taken into account that alkali solutions absorb CO 2 from the air; during storage they are protected using a tube filled with quicklime or soda lime.

Rice. 2. Strong acid neutralization curves.

1 - 0.1 M, 2 - 0.01 M, 3 - 0.001 M.

To detect c.t.t. with a color indicator, it is necessary that the jump height be greater than the width of the indicator transition interval. The latter is usually about two pH units.

The height of the jump in the neutralization curve of weak acids depends on the strength of the acid (the value of its acid constant, or pK a). Namely, the weaker the acid (the larger the pK a value), the smaller, other things being equal, should be the height of the jump.

1 - hydrochloric acid, 2 - acetic acid (pK a = 4.8), 3 - hydrocyanic acid (pK a = 9.2).

The jump height should be greater than the width of the indicator transition zone, which is usually 2 pH units. Therefore, to As in the case of strong electrolytes, titration criterion weak protolith with a 1% error can be derived from the condition ∆p Н ±1% ≥ 2. For an aqueous solution of a weak acid, we obtain the required criterion in the following form:

R TOa+ p WITH≤ 8

When p C = 2 critical value p K a equals 6. In other words, if the acid is very weak, and its pK A more than 6, then it is impossible to accurately titrate it with color indicators.

Titration of mixtures of protoliths and multiproton protoliths. In mixed solutions, strong acids suppress the protolysis of weaker ones. The same is observed in solutions containing a mixture of bases of different strengths. When a titrant is added to such a mixture, the stronger protolyte is first titrated, and only then the weaker one reacts with the titrant. However, the number of jumps observed in the titration curve of a mixture depends not only on the number of protolytes present, but also on the absolute values ​​of the corresponding acidity (basicity) constants, as well as on their ratio. The acidity (or basicity) constants of the components of the mixture must differ by more than 10 4 times, only in this case clearly pronounced titration jumps will be separately observed on the titration curve, and the relative error in determining each component will not exceed 1%. The criterion for the possibility of separate titration of protolytes is the so-called “rule of four units”:

(6)

Multiproton protolytes react with titrants stepwise, first in the first step, then in the second, etc., if the corresponding acidity constants differ in accordance with condition (6). When calculating neutralization curves, multiproton protolytes can be considered as mixtures of different electrolytes.

As an example, let us analyze the possibility

Fig.5. Titration curve of a mixture of carbonate and bicarbonate ions with a solution HCl.

The pH values ​​at which color transitions of the indicators are observed are indicated.

When titrating a mixture of two strong acids, a mixture of two equally weak acids, or a mixture of two bases with similar p TOb There are no two separate jumps in the titration curve. However, it is still quite possible to determine the concentration of the components of such mixtures separately. These problems are successfully solved using differentiating non-aqueous solvents.

Acid-base indicators and their selection

To detect c.t.t. in the neutralization method, acid-base indicators are traditionally used - synthetic organic dyes, which are weak acids or bases and change visible color depending on the pH of the solution. Examples of some (most often used in laboratories) acid-base indicators are given in Table 3. Structure and properties indicators are given in reference books. The most important characteristics of each acid-base indicator are transition interval And titration index (pT). The transition interval is the zone between two pH values, corresponding to the boundaries of the zone within which a mixed color of the indicator is observed. Thus, an observer will characterize an aqueous solution of methyl orange as pure yellow - at pH< 3,1 и как чисто красный при рН >4.4, and between these boundary values ​​a mixed pink-orange color of different shades is observed. The width of the transition interval is usually 2 pH units. Experimentally determined indicator transition intervals are in some cases less than or more than two pH units. This, in particular, is explained by the different sensitivity of the eye to different parts of the visible spectrum. For single-color indicators, the width of the interval also depends on the concentration of the indicator.

Table 3

The most important acid-base indicators

Indicator	Transition interval ΔрН Ind		R TOa(HInd)	Color change
Methyl orange				Red - yellow
Bromocresol green				Yellow - blue
Methyl red				Red - yellow
Bromocresol purple				Yellow - purple
Bromothymol blue				Yellow - blue
Phenol red				Yellow - red
Thymol blue
Phenolphthalein				Colorless - red

Knowing the characteristics of different indicators, you can theoretically select them in a sound manner in order to obtain correct analysis results. Adhere to the following rule: the transition interval of the indicator should lie in the jump region on the titration curve.

When choosing indicators for titration of weak protolytes, it is necessary to take into account that t.eq. and the titration jump are shifted to a slightly alkaline environment when titrating an acid and to a slightly acidic environment when titrating a base. Hence, For the titration of weak acids, indicators that change color in a weakly alkaline environment (for example, phenolphthalein) are suitable, and for the titration of a weak base, indicators that change color in a slightly acidic environment (for example, methyl orange

There is another characteristic of each acid-base indicator - this is titration index ( pT ). This is the name of the pH value at which the observer most clearly notices a change in the color of the indicator and it is at this moment that the titration is considered complete. Obviously, pT = pH K.T.T. . When choosing a suitable indicator, we must strive to ensure that the pT value is as close as possible to the theoretically calculated value pH T.EKV .. Typically the pT value is close to the middle of the transition interval. But pT is a poorly reproducible value. Different people performing the same titration with the same indicator will obtain significantly different pT values. In addition, the pT value depends on the order of the titration, that is, on the direction of the color change. When titrating acids and bases with the same indicator pT values ​​will vary slightly. For single-color indicators (phenolphthalein, etc.), the pT value also depends on the concentration of the indicator.

Filled with titrant to the zero mark. It is not recommended to titrate starting from other marks, since the burette scale may be uneven. The burettes are filled with the working solution through a funnel or using special devices if the burette is semi-automatic. The end point of titration (equivalence point) is determined by indicators or physicochemical methods (electrical conductivity, light transmission, indicator electrode potential, etc.). The analysis results are calculated based on the amount of working solution used for titration.

Types of titrimetric analysis

Titrimetric analysis can be based on different types of chemical reactions:

• acid-base titration - neutralization reactions;
• redox titration (permanganatometry, iodometry, chromatometry) - redox reactions;
• precipitation titration (argentometry) - reactions that occur with the formation of a slightly soluble compound, while the concentrations of precipitated ions in the solution change;
• complexometric titration - reactions based on the formation of strong complex compounds of metal ions with a complexone (usually EDTA), while the concentrations of metal ions in the titrated solution change.

Types of titration

There are direct, reverse and substituent titrations.

• At direct titration A titrant solution (working solution) is added in small portions to the solution of the substance being determined (an aliquot or sample, the substance being titrated).

• At back titration First, a known excess of a special reagent is added to the solution of the substance being determined, and then its remainder that has not entered into the reaction is titrated.

• At substitution titration A known excess of a special reagent is first added to the solution of the analyte, and then one of the reaction products between the analyte and the added reagent is titrated.

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