How is titration carried out? Titration method. Research methods in analytical chemistry

analyte + reagent (titrant) → reaction products

Find all the answers to your titration questions!

What is a titration curve?
Procedure for calculating molarity, molar equation and molar concentration
What is the difference between titration to the end point and to the equivalence point?
What is back titration?
What are the benefits of titration?
What types of chemical reactions are used in titration?
What indication methods are used in titration?
In what areas is titration used?
How to speed up titrant addition (incremental or dynamic)?
Why do titrations to the equivalence point using an automatic titrator produce different results compared to manual titrations using a color indicator?
Which electrodes should be used for anhydrous titration?
How often should the titrant be standardized?
What is an automatic titrator?
How does an automatic titrator work?

What is titration? What is the definition of titration?

Titration is an analytical method designed to quantify a single substance (analyte) dissolved in a sample. The method is based on observing the complete passage of the chemical reaction between the analyzed component and a reagent (titrating substance) of known concentration added to the solution.

analyte + reagent (titrant) = reaction products

A well-known example is the titration of acetic acid (CH3COOH) in vinegar with sodium hydroxide NaOH:

CH 3 COOH + NaOH → CH 3 COO - + Na + + H 2 O

Titrant is added until the reaction is completely completed. A reaction is suitable for analytical purposes if its completion can be easily observed. This means that the completeness of the reaction must be controlled by a suitable method, for example, potentiometric (potential measurement with a special sensor), or using color indicator substances. The concentration of the analyte is calculated from the amount of titrant consumed based on the stoichiometry of the chemical reaction. The reaction used for titration must be rapid, unambiguous, complete and clearly observable.

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What is a titration curve?

Titration curves display the quantitative progress of titration. They allow rapid evaluation of the titration method. A distinction is made between logarithmic and linear titration curves.

The titration curve is based on two variables:

titrant volume as an independent variable; and the solution response, that is, the pH value for the acid-base titration, as a dependent variable depending on the composition of the two solutions.

Titration curves can take four different forms and must be analyzed using appropriate evaluation algorithms. These four forms are: symmetrical curve, asymmetrical curve, curve with min/max And segmented curve.

What is acid-base titration?

What is the history of automatic titrators?

Classic way

Titration is a widely used classical method of analysis. It was originally performed by adding titrant from a graduated glass cylinder (burette). The amount of added titrant was adjusted manually using a tap. The moment of completion of the titration reaction (end point) was determined by the change in the color of the indicator. At first, only those titration reactions were used that were accompanied by a noticeable color change. Later they began to add special indicator substances that change their color. The achievable level of accuracy was determined mainly by the skill of the performer and, in particular, by his ability to distinguish between color shades.

Modern way

Over time, the titration process has improved significantly: manual burettes with a piston were equipped with an electric drive, which ensures accuracy and reproducibility when adding titrant. Instead of color indicators, electrodes are used to measure potential, thereby increasing the accuracy of the results. The potential change curve when titrant is added can be used to determine the end point much more accurately than the color change. Using microprocessors, you can control the titration and the data obtained automatically. This is a big step towards full automation.

Today and in the future

Improvement continues. Modern automatic titrators support programming of the entire analytical procedure, providing maximum flexibility in method development. By combining simple operating functions such as Dosing, Stirring, Titration, Calculation, it is possible to create a specific method for each analysis. The workload on laboratory technicians is reduced thanks to additional devices (autosamplers, pumps). Another innovation is connectivity to computers and laboratory information systems (LIMS).

See what “TITRATION” is in other dictionaries:

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Subtitling, microtitling, subtitling, titrating Dictionary of Russian synonyms. titration noun, number of synonyms: 4 microtitration (1) ... Synonym dictionary

The gradual addition of a controlled amount of a reagent (e.g. acid) to a solution to be analyzed (e.g. alkali) in titrimetric analysis... Big Encyclopedic Dictionary

TITRATION, a method used in volumetric analysis to determine the concentration of a COMPOUND in a SOLUTION. A measurement of the amount required to complete a reaction with another compound. A solution of known concentration is added in measured... ... Scientific and technical encyclopedic dictionary

TITRATION, titration, pl. no, cf. (specialist.). Action under Ch. titrate. “Shirshov opened his laboratory to warm it up for titration.” Papanin. Ushakov's explanatory dictionary. D.N. Ushakov. 1935 1940 … Ushakov's Explanatory Dictionary

The process of adding a solution of known concentration (titrated) to a solution of the analyte until equivalent ratios of reacting substances are established. The equivalent point is fixed using indicators. Applies... ... Geological encyclopedia

titration- - [A.S. Goldberg. English-Russian energy dictionary. 2006] Energy industry topics in general EN titration ... Technical Translator's Guide

Titration- The main technique of titrimetric analysis, which consists in gradually adding a titrated solution from a burette to the test solution until equivalence is achieved Source ... Dictionary-reference book of terms of normative and technical documentation

TITRATION- the main titrimetric technique (see), which consists of gradually adding a solution of a reagent of known concentration from a burette to the solution being analyzed until the end of the reaction (see). Based on the volume of the titrated solution consumed, calculate... ... Big Polytechnic Encyclopedia

The gradual addition of a controlled amount of a reagent (for example, an acid) to a test solution (for example, an alkali) in a titrimetric analysis. * * * TITRATION TITRATION, gradual addition of a controlled amount of reagent... ... encyclopedic Dictionary

Books

Analytical chemistry. Redox titration. Textbook for secondary vocational education, Podkorytov A.L. Category: Miscellaneous Series: Vocational education Publisher: YURAYT, Manufacturer: YURAYT,
Analytical chemistry. Redox titration. Textbook for universities, Podkorytov A.L. , The textbook covers the theory of redox titration methods, the relationship between the theoretical foundations of the methods and their practical application. Much attention is paid... Category: Miscellaneous Series: Universities of Russia Publisher:

Titrimetric analysis is based on the precise measurement of the amount of reagent consumed in the reaction with the substance being determined. Until recently, this type of analysis was usually called volumetric due to the fact that the most common way in practice to measure the amount of a reagent was to measure the volume of solution consumed in the reaction. Nowadays, volumetric analysis is understood as a set of methods based on measuring the volume of liquid, gas or solid phases.

The name titrimetric is associated with the word titer, indicating the concentration of the solution. The titer shows the number of grams of solute in 1 ml of solution.

A titrated or standard solution is a solution whose concentration is known with high accuracy. Titration is the addition of a titrated solution to the test solution to determine an exactly equivalent amount. The titrating solution is often called the working solution or titrant. For example, if an acid is titrated with an alkali, the alkali solution is called a titrant. The point of titration when the amount of added titrant is chemically equivalent to the amount of the titrated substance is called the equivalence point.

Reactions used in titrimetry must satisfy the following basic requirements:

1) the reaction must proceed quantitatively, i.e. the equilibrium constant of the reaction must be large enough;

2) the reaction must proceed at high speed;

3) the reaction should not be complicated by adverse reactions;

4) there must be a way to determine the end of the reaction.

If a reaction does not satisfy at least one of these requirements, it cannot be used in titrimetric analysis.

In titrimetry, there are direct, reverse and indirect titrations.

In direct titration methods, the analyte reacts directly with the titrant. To carry out analysis using this method, one working solution is sufficient.

Back titration methods (or, as they are also called, residue titration methods) use two titrated working solutions: a main and an auxiliary solution. For example, back titration of chloride ion in acidic solutions is widely known. First, a known excess of a titrated solution of silver nitrate (the main working solution) is added to the analyzed chloride solution. In this case, a reaction occurs to form slightly soluble silver chloride.

The excess amount of AgNO 3 that has not reacted is titrated with a solution of ammonium thiocyanate (auxiliary working solution).

The third main type of titrimetric determination is titration of a substituent, or titration by substitution (indirect titration). In this method, a special reagent is added to the substance being determined, which reacts with it. One of the reaction products is then titrated with the working solution. For example, during the iodometric determination of copper, a deliberate excess of KI is added to the analyzed solution. The reaction 2Cu 2+ +4I - =2CuI+ I 2 occurs. The released iodine is titrated with sodium thiosulfate.

There is also the so-called reverse titration, in which a standard reagent solution is titrated with the analyzed solution.

The calculation of titrimetric analysis results is based on the principle of equivalence, according to which substances react with each other in equivalent quantities.

To avoid any contradictions, it is recommended that all acid-base reactions be reduced to a single common base, which can be a hydrogen ion. In redox reactions, it is convenient to relate the amount of reactant to the number of electrons accepted or donated by the substance in a given half-reaction. This allows us to give the following definition.

An equivalent is a real or fictitious particle that can attach, release, or be otherwise equivalent to one hydrogen ion in acid-base reactions or one electron in redox reactions.

When using the term "equivalent", it is always necessary to indicate which specific reaction it refers to. The equivalent of a given substance is not a constant value, but depends on the stoichiometry of the reaction in which they take part.

In titrimetric analysis, reactions of various types are used: - acid-base interaction, complexation, etc., satisfying the requirements for titrimetric reactions. The type of reaction that occurs during titration forms the basis for the classification of titrimetric methods of analysis. Typically, the following titrimetric analysis methods are distinguished.

1. Methods of acid-base interaction are associated with the process of proton transfer:

2. Complexation methods use reactions of the formation of coordination compounds:

3. Precipitation methods are based on the formation reactions of poorly soluble compounds:

4. Oxidation-reduction methods combine a large group of redox reactions:

Some titrimetric methods are named by the type of main reaction that occurs during titration or by the name of the titrant (for example, in argentometric methods the titrant is an AgNO 3 solution, in permanganatometric methods - a KMn0 4 solution, etc.).

Titration methods are characterized by high accuracy: the determination error is 0.1 - 0.3%. Working solutions are stable. To indicate the equivalence point, there is a set of various indicators. Among titrimetric methods based on complexation reactions, reactions using complexones are of greatest importance. Almost all cations form stable coordination compounds with complexons; therefore, complexometry methods are universal and applicable to the analysis of a wide range of different objects.

The acid-base titration method is based on reaction reactions between acids and bases, that is, neutralization reactions:

H + + OH - ↔ H 2 O

The working solutions of the method are solutions of strong acids (HCl, H 2 S, HNO3, etc.) or strong bases (NaOH, KOH, Ba(OH) 2, etc.). Depending on the titrant, the acid-base titration method is divided into acidimetry , if the titrant is an acid solution, and alkalimetry , if the titrant is a solution of a base.

Working solutions are mainly prepared as secondary standard solutions, since the starting materials for their preparation are not standard, and then they are standardized against standard substances or standard solutions. For example: acid solutions can be standardized according to standard substances- sodium tetraborate Na 2 B 4 O 7 ∙10H 2 O, sodium carbonate Na 2 CO 3 ∙10H 2 O or standard solutions of NaOH, KOH; and base solutions - using oxalic acid H 2 C 2 O 4 ∙H 2 O, succinic acid H 2 C 4 H 4 O 4 or standard solutions of HCl, H 2 SO 4, HNO 3.

Equivalence point and titration end point. According to the equivalence rule, titration must be continued until the amount of added reagent becomes equivalent to the content of the substance being determined. The moment during the titration process when the amount of a standard reagent solution (titrant) becomes theoretically strictly equivalent to the amount of the substance being determined according to a certain chemical reaction equation is called equivalence point .

The equivalence point is determined in various ways, for example, by changing the color of the indicator added to the titrated solution. The moment at which an observed change in the color of the indicator occurs is called titration end point. Very often the end point of the titration does not exactly coincide with the equivalence point. As a rule, they differ from each other by no more than 0.02-0.04 ml (1-2 drops) of titrant. This is the amount of titrant that is necessary to interact with the indicator.

TITRATION

TITRATION, a method used in volumetric analysis to determine the concentration of a COMPOUND in a SOLUTION. A measurement of the amount required to complete a reaction with another compound. A solution of known concentration is added in measured quantities from a burette (graduated glass tube) to a measured volume of liquid of unknown concentration until the reaction is complete (as indicated by an indicator or electrochemical device). The added volume allows the unknown concentration to be calculated.

Scientific and technical encyclopedic dictionary.

Synonyms:

See what “TITRATION” is in other dictionaries:

Subtitling, microtitling, subtitling, titrating Dictionary of Russian synonyms. titration noun, number of synonyms: 4 microtitration (1) ... Synonym dictionary

TITRATION, determination of the content of a substance by gradually mixing the analyzed solution (for example, acid) with a controlled amount of a reagent (for example, alkali). End point of titration (completion of chemical reaction)… … Modern encyclopedia

The gradual addition of a controlled amount of a reagent (e.g. acid) to a solution to be analyzed (e.g. alkali) in titrimetric analysis... Big Encyclopedic Dictionary

titration- - [A.S. Goldberg. English-Russian energy dictionary. 2006] Energy industry topics in general EN titration ... Technical Translator's Guide

Books

Analytical chemistry. Redox titration. Textbook for secondary vocational education, Podkorytov A.L. Category: Miscellaneous Series: Vocational education Publisher: YURAYT, Manufacturer: YURAYT,
Analytical chemistry. Redox titration. Textbook for universities, Podkorytov A.L. , The textbook covers the theory of redox titration methods, the relationship between the theoretical foundations of the methods and their practical application. Much attention is paid... Category: Miscellaneous Series: Universities of Russia Publisher: YURAYT, Manufacturer:

Introduction

The laboratory workshop is carried out after studying the theoretical course “Analytical chemistry and physical chemical analysis” and serves to consolidate and deepen the acquired knowledge.

The task of quantitative analysis is to determine the amount (content) of elements (ions), radicals, functional groups, compounds or phases in the analyzed object. This course covers the basic methods of titrimetric (volumetric) analysis, titration methods and their practical applications.

Before starting laboratory work, students undergo safety instructions. Before completing each work, the student must pass a colloquium on the sections specified by the teacher, as well as on the analysis methodology. To do this you need:

1) repeat the relevant section of the course;

2) become familiar with the work methodology in detail;

3) draw up equations of chemical reactions that form the basis of the chemical analysis being carried out;

4) study the features of the analysis from a safety point of view.

Based on the results of their work, students draw up a report, which should indicate:

· job title;

· Objective;

· theoretical foundations of the method: essence of the method, basic equation, calculations and construction of titration curves, choice of indicator;

· reagents and equipment used during the work;

· analysis technique:

Preparation of primary standards;

Preparation and standardization of working solution;

Determination of the content of the test substance in solution;

· experimental data;

· statistical processing of analysis results;

· conclusions.

TITRIMETRIC ANALYSIS METHODS

Titrimetric method of analysis is based on measuring the volume of a reagent of precisely known concentration (titrant) spent on a chemical reaction with the substance being determined.

The determination procedure (titration) consists of adding a titrant dropwise to a precisely known volume of a solution of the analyte with an unknown concentration from a burette until the equivalence point is reached.

Where X– analyte; R– titrant, P– reaction product.

Equivalence point (i.e.)- this is the theoretical state of the solution that occurs at the moment of adding an equivalent amount of titrant R to the analyte X. In practice, the titrant is added to the analyte until it reaches the end point of titration (e.t.t.), which is understood in the visual indication of the equivalence point as the moment the color of the indicator added to the solution changes. In addition to visual indication, the equivalence point can be registered by instrumental means. In this case, the end point of titration (end point of titration) is understood as the moment of a sharp change in a physical quantity measured during the titration process (current strength, potential, electrical conductivity, etc.).

The titrimetric method of analysis uses the following types of chemical reactions: neutralization reactions, oxidation-reduction reactions, precipitation reactions and complexation reactions.

Depending on the type of chemical reaction used, the following are distinguished: titrimetric analysis methods:

– acid-base titration;

– precipitation titration;

– complexometric titration or complexometry;

– redox titration or redoximetry.

The reactions used in the titrimetric method of analysis require the following: requirements:

· the reaction must proceed in stoichiometric ratios, without side reactions;

· the reaction must proceed almost irreversibly (≥ 99.9%), the equilibrium constant of the reaction K p >10 6, the resulting precipitates must have solubility S < 10 -5 моль/дм 3 , а образующиеся комплексы – К уст > 10 -6 ;

· the reaction must proceed at a sufficiently high speed;

· the reaction must take place at room temperature;

· the equivalence point must be fixed clearly and reliably in some way.

Titration methods

In any titrimetric analysis method, there are several titration methods. Distinguish forward titration, back titration and displacement titration .

Direct titration– the titrant is added dropwise to the solution of the analyte until the equivalence point is reached.

Titration scheme: X + R = P.

Law of equivalents for direct titration:

C (1/ z) X V X = C (1/ z) R V R . (2)

The amount (mass) of the analyte contained in the test solution is calculated using the law of equivalents (for direct titration)

m X = C (1/z)R V R M (1/z) X٠10 -3 , (3)

Where C (1/ z) R– molar concentration of titrant equivalent, mol/dm 3 ;

V R– titrant volume, cm3;

M ( 1/ z) X– molar mass of the equivalent of the substance being determined;

C (1/ z) X– molar concentration of the equivalent of the analyte, mol/dm 3 ;

V X– volume of the substance being determined, cm3.

Back titration– two titrants are used. At first
The exact volume of the first titrant is added to the solution being analyzed ( R 1), taken in excess. The remainder of the unreacted titrant R1 is titrated with a second titrant ( R 2). Titrant quantity R 1, spent
for interaction with the analyte ( X) is determined by the difference between the added volume of titrant R 1 (V 1) and titrant volume R 2 (V 2) spent on titration of the remaining titrant R 1.

Titration scheme: X + R 1 fixed excess = P 1 (R 1 remainder).

R 1 remainder + R 2 = P2.

When using back titration, the law of equivalents is written as follows:

The mass of the analyte in the case of back titration is calculated using the formula

The reverse titration method is used in cases where it is impossible to select a suitable indicator for a direct reaction or it proceeds with kinetic difficulties (low rate of chemical reaction).

Titration by substitution (indirect titration)– used in cases where direct or reverse titration of the analyte is impossible or difficult, or when a suitable indicator is not available.

To the analyte X add some reagent A in excess, upon interaction with which an equivalent amount of the substance is released R. Then the reaction product R titrate with a suitable titrant R.

Titration scheme: X + A excess = P1.

P 1 + R = P2.

The law of equivalents for titration by substitution is written as follows:

Since the number of equivalents of the analyte is X and reaction product R are the same, the calculation of the mass of the analyte in the case of indirect titration is calculated using the formula

m X = C (1/z) R V R M (1/z) X٠10 -3 . (7)

Reagents

1. Succinic acid H 2 C 4 H 4 O 4 (reagent grade) – primary standard.

2. Sodium hydroxide NaOH solution with molar concentration
~2.5 mol/dm 3

3. H 2 O distilled.

Equipment students describe on their own.

Work progress:

1. Preparation of the primary standard of succinic acid HOOCCH 2 CH 2 COOH.

Succinic acid is prepared in a volume of 200.00 cm 3 with a molar concentration of the equivalent mol/dm 3 .

g/mol.

Reaction equation:

Taking a sample (weighing):

Hitch weight

Weighed quantitatively transferred to a volumetric flask ( cm 3), add 50 - 70 cm 3 of distilled water, stir until succinic acid is completely dissolved, adjust to the mark with distilled water
and mix thoroughly.

count on
according to the formula

Reagents

1. Sodium carbonate Na 2 CO 3 (reagent grade) – primary standard.

2. H 2 O distilled.

3. Hydrochloric acid HCl concentration 1:1 (r=1.095 g/cm3).

4. Acid-base indicator (selected according to the titration curve).

5. Mixed indicator - methyl orange and methylene blue.

Work progress:

1. Preparation of primary standard sodium carbonate (Na 2 CO 3).

A sodium carbonate solution is prepared with a volume of 200.00 cm 3 with a molar concentration of the equivalent mol/dm 3 .

Calculation of sample mass, g: (mass is taken accurate to the fourth decimal place).

Reaction equations:

1) Na 2 CO 3 + HCl = NaHCO 3 + NaCl

2) NaHCO 3 + HCl = NaCl + H 2 O + CO 2

_____________________________________

Na 2 CO 3 + 2HCl = 2NaCl + H 2 O + CO 2

H 2 CO 3 – weak acid (K a1= 10 -6.35 , K a2 = 10 -10,32).

Taking a sample (weighing):

Weight of watch glass (glass)

Weight of watch glass (glass) with weight

Hitch weight

Weighed quantitatively transferred to a volumetric flask ( cm 3), add 50 - 70 cm 3 of distilled water, mix until sodium carbonate is completely dissolved, adjust to the mark with distilled water
and mix thoroughly.

Actual concentration of the primary standard count on
according to the formula

2. Preparation and standardization of titrant (HCl solution)

A solution of hydrochloric acid is prepared with a volume of approximately 500 cm3
with a molar concentration equivalent of approximately 0.05÷0.06 mol/dm 3)

Titrant - a solution of hydrochloric acid with an approximate concentration of 0.05 mol/dm 3 is prepared from hydrochloric acid diluted 1:1 (r = 1.095 g/cm 3).

Standardization of the solution HCl is carried out according to the primary standard Na 2 CO 3 by direct titration, using the pipetting method.

The indicator is selected according to the titration curve of sodium carbonate with hydrochloric acid (Fig. 4).

Rice. 4. Titration curve of 100.00 cm 3 Na 2 CO 3 solution with WITH= 0.1000 mol/dm 3 HCl solution with C 1/ z= 0.1000 mol/dm 3

When titrating to the second equivalence point, use the indicator methyl orange, 0.1% aqueous solution (pT = 4.0). Change in color from yellow to orange (tea rose color). Transition interval
(pH = 3.1 – 4.4).

Scheme 3. Standardization of HCl solution

Place a 25.00 cm 3 aliquot of a standard Na 2 CO 3 solution (with a pipette) into a conical titration flask with a capacity of 250 cm 3, add 2–3 drops of methyl orange, dilute with water to 50–75 cm 3 and titrate with a solution of hydrochloric acid until the color changes. from yellow to “tea rose” color with one drop of titrant. Titration is carried out in the presence of a “witness” (a stock solution of Na 2 CO 3 with an indicator). The titration results are recorded in the table. 4. The concentration of hydrochloric acid is determined according to the law of equivalents: .

Table 4

Results of standardization of hydrochloric acid solution

Tasks

1. Formulate the concept of equivalent in acid-base reactions. Calculate the equivalents of soda and phosphoric acid in the following reactions:

Na 2 CO 3 + HCl = NaHCO 3 + NaCl

Na 2 CO 3 + 2HCl = 2NaCl + CO 2 + H 2 O

H 3 PO 4 + NaOH = NaH 2 PO 4 + H 2 O

H 3 PO 4 + 2NaOH = Na 2 HPO 4 + H 2 O

H 3 PO 4 + 3NaOH = Na 3 PO 4 + 3H 2 O

2. Write the reaction equations between hydrochloric acid, sulfuric acid, sodium hydroxide, aluminum hydroxide, sodium carbonate, potassium bicarbonate and calculate the equivalent mass of these substances.

3. Plot a titration curve for 100.00 cm 3 of hydrochloric acid with a molar concentration equivalent to 0.1 mol/dm 3 with sodium hydroxide with a molar concentration equivalent to 0.1 mol/dm 3. Select possible indicators

4. Plot a titration curve for 100.00 cm 3 acrylic acid (CH 2 =CHCOOH, pK a= 4.26) with molar concentration equivalent
0.1 mol/dm 3 sodium hydroxide with molar concentration equivalent
0.1 mol/dm3. How does the composition of a solution change during titration? Select possible indicators and calculate the indicator error of the titration.

5. Plot a titration curve for hydrazine (N 2 H 4 + H 2 O, pK b= 6,03)
with a molar concentration equivalent to 0.1 mol/dm 3 hydrochloric acid
with a molar concentration equivalent of 0.1 mol/dm 3 . What are the similarities
and the difference in pH calculations and titration curve compared to the titration curve of a weak acid with alkali? Select possible indicators
and calculate the indicator error of titration.

6. Calculate activity coefficients and active ion concentrations
in 0.001 M solution of aluminum sulfate, 0.05 M sodium carbonate, 0.1 M potassium chloride.

7. Calculate the pH of a 0.20 M solution of methylamine if its ionization in an aqueous solution is described by the equation

B + H 2 O = BH + + OH - , K b= 4.6 ×10 - 3, where B is the base.

8. Calculate the dissociation constant of hypochlorous acid HOCl if a 1.99 × 10 - 2 M solution has pH = 4.5.

9. Calculate the pH of a solution containing 6.1 g/mol glycolic acid (CH 2 (OH)COOH, K A= 1.5 × 10 - 4).

10. Calculate the pH of the solution obtained by mixing 40 ml of 0.015 M hydrochloric acid solution with:

a) 40 ml of water;

b) 20 ml of 0.02 M sodium hydroxide solution;

c) 20 ml of 0.02 M barium hydroxide solution;

d) 40 ml of 0.01 M solution of hypochlorous acid, K A=5.0 × 10 - 8.

11. Calculate the concentration of acetate ion in a solution of acetic acid
with a mass fraction of 0.1%.

12. Calculate the concentration of ammonium ion in an ammonia solution with a mass fraction of 0.1%.

13. Calculate the mass of a sample of sodium carbonate required to prepare 250.00 ml of a 0.5000 M solution.

14. Calculate the volume of a solution of hydrochloric acid with a molar concentration equivalent to 11 mol/l and the volume of water that must be taken to prepare 500 ml of a 0.5 M solution of hydrochloric acid.

15. 0.15 g of metallic magnesium was dissolved in 300 ml of a 0.3% solution of hydrochloric acid. Calculate the molar concentration of hydrogen, magnesium and chlorine ions in the resulting solution.

16. When 25.00 ml of sulfuric acid solution is mixed with a barium chloride solution, 0.2917 g of barium sulfate is obtained. Determine the titer of the sulfuric acid solution.

17. Calculate the mass of calcium carbonate that reacted
with 80.5 mmol hydrochloric acid.

18. How many grams of monosodium phosphate should be added?
to 25.0 ml of 0.15 M sodium hydroxide solution to obtain a solution with pH = 7? For phosphoric acid pK a1= 2.15; pK a2= 7.21; pK a3 = 12,36.

19. To titrate 1.0000 g of fuming sulfuric acid, thoroughly diluted with water, 43.70 ml of 0.4982 M sodium hydroxide solution is consumed. Fuming sulfuric acid is known to contain sulfuric anhydride dissolved in anhydrous sulfuric acid. Calculate the mass fraction of sulfuric anhydride in fuming sulfuric acid.

20. The absolute error in measuring volume using a burette is 0.05 ml. Calculate the relative error of measuring volumes in 1; 10 and 20 ml.

21. A solution is prepared in a volumetric flask with a capacity of 500.00 ml
from a sample of 2.5000 g of sodium carbonate. Calculate:

a) molar concentration of the solution;

b) molar concentration of the equivalent (½ Na 2 CO 3);

c) solution titer;

d) titer for hydrochloric acid.

22. What is the volume of 10% sodium carbonate solution with the density
1.105 g/cm 3 needs to be taken for preparation:

a) 1 liter of solution with a titer of TNa 2 CO 3 = 0.005000 g/cm 3 ;

b) 1 liter of solution with TNa 2 CO 3 /HCl = 0.003000 g/cm 3?

23. What volume of hydrochloric acid with a mass fraction of 38.32% and a density of 1.19 g/cm3 should be taken to prepare 1500 ml of a 0.2 M solution?

24. What volume of water must be added to 1.2 L of 0.25 M HCl to prepare a 0.2 M solution?

25. From 100 g of technical sodium hydroxide containing 3% sodium carbonate and 7% indifferent impurities, 1 liter of solution was prepared. Calculate the molar concentration and hydrochloric acid titer of the resulting alkaline solution, assuming that sodium carbonate is titrated to carbonic acid.

26. There is a sample that may contain NaOH, Na 2 CO 3, NaHCO 3 or a mixture of these compounds weighing 0.2800 g. The sample was dissolved in water.
To titrate the resulting solution in the presence of phenolphthalein, 5.15 ml is consumed, and in the presence of methyl orange - 21.45 ml of hydrochloric acid with a molar concentration equivalent of 0.1520 mol/l. Determine the composition of the sample and the mass fractions of components in the sample.

27. Plot a titration curve for a 100.00 cm 3 0.1000 M ammonia solution with a 0.1000 M hydrochloric acid solution, justify the choice of indicator.

28. Calculate the pH of the equivalence point, beginning and end of the titration of 100.00 cm 3 0.1000 M malonic acid solution (HOOCCH 2 COOH) with 0.1000 M sodium hydroxide solution (pK a 1=1.38; rK a 2=5,68).

29. The titration of 25.00 cm 3 of sodium carbonate solution with a molar concentration equivalent of 0.05123 mol/dm 3 required 32.10 cm 3 of hydrochloric acid. Calculate the molar concentration of hydrochloric acid equivalent.

30. How many ml of 0.1 M ammonium chloride solution must be added
to 50.00 ml of 0.1 M ammonia solution to form a buffer solution
with pH=9.3.

31. A mixture of sulfuric and phosphoric acids was transferred to a 250.00 cm 3 volumetric flask. For titration, two samples of 20.00 cm 3 were taken, one was titrated with a solution of sodium hydroxide with a molar concentration of the equivalent
0.09940 mol/dm 3 with methyl orange indicator, and the second with phenolphthalein. The sodium hydroxide consumption in the first case was 20.50 cm 3 , and in the second case 36.85 cm 3 . Determine the masses of sulfuric and phosphoric acids in the mixture.

In complexometry

Up to the equivalence point =( C M V M – C EDTA V EDTA)/( V M+ V EDTA). (21)

At the equivalence point = . (22)

After the equivalence point = . (23)

In Fig. Figure 9 shows the titration curves of calcium ion in buffer solutions with different pH values. It can be seen that titration of Ca 2+ is possible only at pH ³ 8.

Reagents

2. H 2 O distilled.

3. Standard solution of Mg(II) with molar concentration
0.0250 mol/dm3.

4. Ammonia buffer with pH = 9.5.

5. Solution of potassium hydroxide KOH with a mass fraction of 5%.

6. Eriochrome black T, indicator mixture.

7. Kalcon, indicator mixture.

Theoretical foundations of the method:

The method is based on the interaction of Ca 2+ and Mg 2+ ions with the disodium salt of ethylenediaminetetraacetic acid (Na 2 H 2 Y 2 or Na-EDTA) with the formation of stable complexes in the molar ratio M:L=1:1 in a certain pH range.

To fix the equivalence point when determining Ca 2+ and Mg 2+, calcon and eriochrome black T are used.

Determination of Ca 2+ is carried out at pH ≈ 12, while Mg 2+ is
in solution in the form of a precipitate of magnesium hydroxide and is not titrated with EDTA.

Mg 2+ + 2OH - = Mg(OH) 2 ↓

Ca 2+ + Y 4- « CaY 2-

At pH ≈ 10 (ammonia buffer solution), Mg 2+ and Ca 2+ are
in solution in the form of ions and upon addition of EDTA are titrated together.

Ca 2+ + HY 3- « CaY 2- + H +

Mg 2+ + HY 3- « MgY 2- +H +

To determine the volume of EDTA spent on the titration of Mg 2+,
from the total volume used for titrating the mixture at pH ≈ 10, subtract the volume used for titration of Ca 2+ at pH ≈ 12.

To create a pH ≈ 12, use a 5% KOH solution to create
pH ≈ 10 use an ammonia buffer solution (NH 3 × H 2 O + NH 4 Cl).

Work progress:

1. Standardization of titrant - EDTA solution (Na 2 H 2 Y)

An EDTA solution is prepared with an approximate concentration of 0.025 M
from ≈ 0.05 M solution, diluting it with distilled water 2 times. To standardize EDTA, use a standard solution of MgSO 4
with a concentration of 0.02500 mol/dm3.

Scheme 5. Standardization of titrant - EDTA solution

In a conical titration flask with a capacity of 250 cm 3, place 20.00 cm 3 of a standard MgSO 4 solution with a concentration of 0.02500 mol/dm 3, add ~ 70 cm 3 of distilled water, ~ 10 cm 3 of ammonia buffer solution with pH ~ 9.5 – 10 and add the indicator eriochrome black T about 0.05 g
(at the tip of the spatula). In this case, the solution turns wine red. The solution in the flask is slowly titrated with EDTA solution until the color changes from wine red to green. The titration results are recorded in the table. 6. The concentration of EDTA is determined according to the law of equivalents: .

Table 6

Results of standardization of EDTA solution

2. Determination of Ca 2+ content

Titration curves of Ca 2+ with EDTA solution at pH=10 and pH=12 are constructed independently.

The solution of the problem in a volumetric flask is brought to the mark with distilled water and mixed thoroughly.

Scheme 6. Determination of Ca 2+ content in solution

An aliquot of the test solution 25.00 cm 3 containing calcium and magnesium is placed in a conical titration flask with a capacity of 250 cm 3, ~ 60 cm 3 of water, ~ 10 cm 3 of a 5% KOH solution are added. After an amorphous precipitate of Mg(OH) 2 ↓ has formed, a calcone indicator of about 0.05 g is added to the solution (at the tip of a spatula) and slowly titrated with an EDTA solution until the color changes from pink to pale blue. Titration results ( V 1) are entered in Table 7.

Table 7

Experience no.		Volume of EDTA, cm 3	Ca 2+ content in solution, g
	25,00
	25,00
	25,00
	25,00
	25,00

3. Determination of Mg 2+ content

The titration curve of Mg 2+ with EDTA solution at pH=10 is constructed independently.

Scheme 7. Determination of Mg 2+ content in solution

An aliquot of 25.00 cm 3 of the test solution containing calcium and magnesium is placed in a conical titration flask with a capacity of 250 cm 3, ~ 60 cm 3 of distilled water, ~ 10 cm 3 of ammonia buffer solution with pH ~ 9.5–10 are added, and an indicator is added. eriochrome black T about 0.05 g
(at the tip of the spatula). In this case, the solution turns wine red. The solution in the flask is slowly titrated with EDTA solution until the color changes from wine red to green. Titration results ( V 2) entered into the table. 8.

Table 8

Results of titration of a solution containing calcium and magnesium

Experience no.	Volume of the test solution, cm 3	Volume of EDTA, V∑, cm 3	Mg 2+ content in solution, g
	25,00
	25,00
	25,00
	25,00
	25,00

Reagents

1. EDTA solution with a molar concentration of ~ 0.05 mol/dm 3.

2. Standard solution of Cu(II) with a titer of 2.00×10 -3 g/dm 3 .

3. H 2 O distilled.

4. Ammonia buffer with pH ~ 8 – 8.5.

5. Murexide, indicator mixture.

Tasks

1. Calculate α 4 for EDTA at pH=5, if the ionization constants of EDTA are as follows: K 1 =1.0·10 -2, K 2 =2.1·10 -3, K 3 =6.9·10 -7 , K 4 =5.5·10 -11.

2. Plot a titration curve for 25.00 ml of 0.020 M nickel solution with 0.010 M EDTA solution at pH = 10, if the stability constant
K NiY = 10 18.62. Calculate p after adding 0.00; 10.00; 25.00; 40.00; 50.00 and 55.00 ml titrant.

3. For titration of 50.00 ml of solution containing calcium ions
and magnesium, it took 13.70 ml of 0.12 M EDTA solution at pH=12 and 29.60 ml at pH=10. Express the concentrations of calcium and magnesium in solution in mg/ml.

4. When analyzing 1 liter of water, 0.2173 g of calcium oxide and 0.0927 g of magnesium oxide were found. Calculate what volume of EDTA with a concentration of 0.0500 mol/l was spent on titration.

5. To titrate 25.00 ml of a standard solution containing 0.3840 g of magnesium sulfate, 21.40 ml of Trilon B solution was consumed. Calculate the titer of this solution for calcium carbonate and its molar concentration.

6. Based on the formation constants (stability) of metal complexonates given below, evaluate the possibility of complexometric titration of metal ions at pH = 2; 5; 10; 12.

7. When titrating a 0.01 M solution of Ca 2+ with a 0.01 M solution of EDTA at pH = 10, the stability constant K CaY = 10 10.6. Calculate what the conditional stability constant of the metal complex with the indicator should be at pH=10 if = at the end point of titration.

8. The acid ionization constant of the indicator used in complexometric titration is 4.8·10 -6. Calculate the content of acidic and alkaline forms of the indicator at pH = 4.9, if its total concentration in the solution is 8.0·10 -5 mol/l. Determine the possibility of using this indicator when titrating a solution
with pH=4.9, if the color of its acid form matches the color of the complex.

9. To determine the aluminum content in the sample, a 550 mg sample was dissolved and 50.00 ml of a 0.05100 M solution of complexone III was added. The excess of the latter was titrated with 14.40 ml of 0.04800 M zinc(II) solution. Calculate the mass fraction of aluminum in the sample.

10. When destroying a complex containing bismuth and iodide ions, the latter are titrated with a solution of Ag(I), and bismuth with complexone III.
To titrate a solution containing 550 mg of sample, 14.50 ml of 0.05000 M solution of complexone III is required, and to titrate the iodide ion contained in 440 mg of sample, 23.25 ml of 0.1000 M Ag(I) solution is required. Calculate the coordination number of bismuth in the complex if iodide ions are the ligand.

11. A sample weighing 0.3280 g containing Pb, Zn, Cu was dissolved
and transferred to a 500.00 cm 3 volumetric flask. The determination was carried out in three stages:
a) for the titration of the first portion of a solution with a volume of 10.00 cm 3 containing Pb, Zn, Cu, 37.50 cm 3 of 0.0025 M EDTA solution was spent; b) in the second portion with a volume of 25.00 cm 3, Cu was masked, and 27.60 cm 3 EDTA was used for titration of Pb and Zn; c) in the third portion with a volume of 100.00 cm 3 Zn was masked
and Cu, 10.80 cm 3 EDTA was spent on the titration of Pb. Determine the mass fraction of Pb, Zn, Cu in the sample.

Titration curves

In redoxmetry, titration curves are plotted in coordinates E = f(C R),
they illustrate graphically the change in system potential during the titration process. Before the equivalence point, the potential of the system is calculated by the ratio of the concentrations of the oxidized and reduced forms of the analyte (because before the equivalence point, one of the titrant forms is practically absent), after the equivalence point - by the ratio of the concentrations of the oxidized and reduced forms of the titrant (because after the equivalence point, the analyte is titrated almost completely).

The potential at the equivalence point is determined by the formula

, (26)

where is the number of electrons participating in half-reactions;

– standard electrode potentials of half-reactions.

In Fig. Figure 10 shows the titration curve of a solution of oxalic acid H 2 C 2 O 4 with a solution of potassium permanganate KMnO 4 in an acidic medium
( = 1 mol/dm3).

Rice. 10. Titration curve for 100.00 cm 3 oxalic solution

acids H 2 C 2 O 4 s C 1/ z= 0.1000 mol/dm 3 permanganate solution

potassium KMnO 4 s C 1/ z= 0.1000 mol/dm 3 at = 1 mol/dm 3

Half-reaction potential MnO 4 - + 5 e+ 8H + → Mn 2+ + 4H 2 O depends on the pH of the medium, since hydrogen ions participate in the half-reaction.

Permanganatometry

The titrant is a solution of potassium permanganate KMnO 4, which is a strong oxidizing agent. Basic equation:

MnO 4 - +8H + + 5e = Mn 2+ + 4H 2 O, =+1.51 V.

M 1/ z (KMnO 4)= g/mol.

In slightly acidic, neutral and slightly alkaline environments, due to the lower redox potential, the permanganate ion is reduced to Mn +4.

MnO 4 - +2H 2 O + 3e = MnO 2 ¯ + 4OH - , = +0.60 V.

M 1/ z (KMnO 4) = 158.03/3 = 52.68 g/mol.

In an alkaline environment, a solution of potassium permanganate is reduced
up to Mn +6.

MnO 4 - + 1e = MnO 4 2-, = +0.558 V.

M 1/ z (KMnO 4) = 158.03 g/mol.

To eliminate side reactions, titration with potassium permanganate is carried out in an acidic environment, which is created with sulfuric acid. It is not recommended to use hydrochloric acid to create a medium, since potassium permanganate can oxidize the chloride ion.

2Cl - – 2e = Cl 2 , = +1.359 V.

Potassium permanganate is most often used in the form of a solution
with a molar equivalent concentration of ~ 0.05 – 0.1 mol/dm 3 . It is not a primary standard due to the fact that aqueous solutions of potassium permanganate are capable of oxidizing water and organic impurities in it:

4MnO 4- + 2H 2 O = 4MnО 2 ¯+ 3O 2 + 4OH -

The decomposition of potassium permanganate solutions is accelerated in the presence of manganese dioxide. Since manganese dioxide is a product of the decomposition of permanganate, this precipitate has autocatalytic effect to the decomposition process.

Solid potassium permanganate used to prepare solutions is contaminated with manganese dioxide, so it is impossible to prepare a solution from an accurate sample. In order to obtain a sufficiently stable solution of potassium permanganate, after dissolving a sample of KMnO 4 in water, it is left in a dark bottle for several days (or boiled), and then the MnO 2 is separated by filtration through glass filter (a paper filter cannot be used, as it reacts with potassium permanganate to form manganese dioxide).

The color of the potassium permanganate solution is so intense that
that an indicator is not required in this method. In order to give a noticeable pink color to 100 cm 3 of water, 0.02 - 0.05 cm 3 of KMnO 4 solution is sufficient
with a molar concentration equivalent of 0.1 mol/dm 3 (0.02 M). The color of potassium permanganate at the end point of titration is unstable and gradually discolors as a result of the interaction of excess permanganate
with manganese(II) ions present at the end point in relatively large quantities:

2MnO 4 - + 3Mn 2+ + 2H 2 O « 5MnО 2 ¯ + 4H +

Standardization of working solution KMnO 4 is carried out with sodium oxalate or oxalic acid (freshly recrystallized and dried at 105°C).

Use solutions of primary standards with a molar concentration equivalent WITH(½ Na 2 C 2 O 4) = 0.1000 or 0.05000 mol/l.

C 2 O 4 2- – 2e ® 2CO 2 , = -0.49 V

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