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Titrimetric or volumetric analysis- a method of quantitative analysis based on measuring the volume (or mass) of the reagent T spent on the reaction with the analyte X. In other words, titrimetric analysis is an analysis based on titration.

The purpose of laboratory classes on titrimetric methods of analysis is to develop practical skills in the technique of performing titrimetric analysis and master the methods of statistical processing of analysis results using the example of specific quantitative determinations, as well as to consolidate theoretical knowledge by solving typical calculation problems for each topic.

Knowledge of the theory and practice of titrimetric analysis methods is necessary for the subsequent study of instrumental methods of analysis, other chemical and special pharmaceutical disciplines (pharmaceutical, toxicological chemistry, pharmacognosy, pharmaceutical technology). The studied methods of titrimetric analysis are pharmacopoeial and are widely used in the practice of a pharmacist to control the quality of drugs.

Conventions

A, X, T - any substance, analyte and titrant, respectively;

m(A), m(X), t(T)- mass of any substance, analyte and titrant, respectively, g;

M(A), M(X), M(T)- molar mass of any substance, analyte and titrant, respectively, g/mol;

n(A), n(X), n(T) - the amount of any substance, analyte and titrant, respectively, mol;

The amount of the substance of the equivalent of any substance, the substance to be determined and the titrant, respectively, mol;

- the volume of a solution of any substance, analyte and titrant, respectively, l;

- the volume of an aliquot of the analyte, equal to the capacity of the pipette, l;

- the volume of the analyzed solution of the analyte, equal to the capacity of the flask, l.

1. Basic concepts of titrimetric

analysis

1.1. Titration- the process of determining substance X by the gradual addition of small amounts of substance T, in which, in some way, the detection of the point (moment) when all substance X has reacted is provided. Titration allows you to find the amount of substance X from a known amount of substance T added up to this point (moment), taking into account the fact that the ratio in which X and T react is known from stoichiometry or otherwise.

1.2. titrant- a solution containing active reagent T, with which the titration is carried out. Titration is usually carried out by adding titrant from a calibrated burette to the titration flask containing the solution to be analyzed. Into this flask before titration add aliquot analyzed solution.

1.3. Aliquot share (aliquot)- precisely known part of the analyzed solution, taken for analysis. It is often taken with a calibrated pipette and its volume is usually indicated by the symbol V ss .

1.4. Equivalence point (TE)- such a point (moment) of titration at which the amount of added titrant T is equivalent to the amount of titrated substance X. Synonyms for TE: stoichiometric point, theoretical end point.

1.5. End point titration (KTT) - the point (moment) of the titration, at which some property of the solution (for example, its color) shows a noticeable (sharp) change. LTT corresponds more or less to TE, but most often does not coincide with it.

1.6. Indicator- a substance that exhibits a visible change in the TE or near it. Ideally, the indicator is present at a concentration low enough to transition interval not cost-

a significant amount of titrant T was used. A sharp visible change in the indicator (for example, its color) corresponds to CTT.

1.7. Indicator transition interval- the area of ​​concentration of hydrogen, metal or other ions within which the eye is able to detect a change in hue, color intensity, fluorescence or other property of a visual indicator caused by a change in the ratio of two corresponding forms of the indicator. This area is usually expressed as the negative logarithm of the concentration, for example: For a redox indicator, the transition interval is the corresponding region of the redox potential.

1.8. Degree of titration - volume ratio V (T) of the added titrant to the volume V (TE) of the titrant corresponding to the TE. In other words, the degree of titration of a solution is the ratio of the amount of the titrated substance to its initial amount in the analyzed solution:

1.9. Titration level- order the concentration of the titrant solution used, for example, 10 -1 , 10 -2 , 10 -3 , etc.

1.10. Titration curve - graphic representation of the dependence of the change in concentration c (X) of the analyte X or some related property of the system (solution) on the volume V (T) added titrant T. The value of c (X) during the titration changes by several orders of magnitude, so the titration curve is often plotted in the coordinates: The abscissa shows the volume of added titrant V (T) or degree of titration / . If the equilibrium concentration c (X) or the intensity of a property proportional to it is plotted along the y-axis, then we get linear titration curve. If on the y-axis we set aside or the logarithm of the intensity of a property proportional to c(X), then one gets logarithmic (or monologarithmic) titration curve. To more clearly identify the features of the titration process and for applied purposes, sometimes they build differential titration curves, plotting along the abscissa axis the volume of the added titrant V (T), and along the y-axis - the first derivative of the logarithm of the concentration (or the intensity of a property proportional to it) with respect to the volume of the added titrant: Such titration curves are usually used in physicochemical methods of analysis, for example, in potentiometric titrations.

1.11. Standard solution- a solution having a known concentration of the active substance.

1.12. Standardization- the process of finding the concentration of an active reagent in a solution (most often by titrating it with a standard solution of the corresponding substance).

1.13. Titration jump- the interval of a sharp change in any physical or physico-chemical property of the solution near the equivalence point, usually observed when 99.9-100.1% of the titrant is added compared to its stoichiometric amount.

1.14. Blank titration- titration of a solution that is identical to the analyzed solution in terms of volume, acidity, amount of indicator, etc., but does not contain the analyte.

2. Basic operations of titrimetric analysis

2.1. Cleaning, washing, storage of measuring utensils.

2.2. Checking the capacity of measuring utensils.

2.3. Taking a sample with a precisely known mass by the difference between the results of two weighings (usually on an analytical balance).

2.4. Quantitative transfer of a sample of a substance into a volumetric flask and dissolution of the substance.

2.5. Filling volumetric utensils (flasks, burettes, pipettes) with a solution.

2.6. Emptying pipettes, burettes.

2.7. Selection of an aliquot of the analyzed solution.

2.8. Titration and calculations based on titration results.

3. Calibration of measuring instruments

In titrimetric analysis, the exact volumes of the solution are measured using measuring utensils, which are volumetric flasks with a capacity of 1000, 500, 250, 100, 50 and 25 ml, pipettes and graduated pipettes with a capacity of 10, 5, 3, 2 and 1 ml. The capacity of the flask and pipette at 20 °C is engraved on the neck of the flask or on the side of the pipette (nominal volume). In the mass production of volumetric utensils, the actual (true) capacity of volumetric flasks, burettes, pipettes may differ from the nominal values ​​indicated on the utensil. To achieve the required accuracy of the obtained results of titrimetric analysis

Calibration of volumetric glassware is based on determining the exact mass of distilled water poured in or poured out, which is determined by the results of weighing the glassware before and after pouring in or pouring out water. The volume of water in the calibrated vessel (its capacity) and the mass of water are related by the ratio:

where - density of water at the temperature of the experiment, g/ml.

The density of water depends on temperature, so when making calculations, you should use the data in Table. 2-1.

Table 2-1. Density values ​​of water at the corresponding temperature

Volumetric flasks are calibrated for infusion, and burettes and pipettes are calibrated for pouring, since small amounts of liquid always remain on the walls of the dish during pouring.

3.1. Volumetric flask capacity check

The flask is thoroughly washed, dried and weighed on an analytical balance with an accuracy of ± 0.002 g. Then it is filled with water (hereinafter - distilled) along the lower meniscus, the drops of water in the upper part of the neck of the flask are removed with filter paper and weighed again. Each weighing of an empty flask and a flask with water is carried out at least twice, while the difference between two weighings should not exceed ± 0.005 g. The difference between the mass of a flask with water and the mass of an empty flask is equal to the mass of water contained by the flask at a given temperature. The true capacity of the flask is calculated by dividing the average mass of water by its density at the test temperature (see Table 2-1).

For example, if a volumetric flask with a nominal volume of 100 ml is calibrated, the average mass of water at 18 °C is 99.0350 g. Then the true capacity of the volumetric flask is:

3.2. Burette capacity check

The burette is a glass cylinder, the inner diameter of which can vary slightly along the length of the burette. Equal divisions on the burette in its various parts correspond to unequal volumes of the solution. That is why burette calibration calculates the true volumes for each selected buret site.

A clean and dried burette is filled with water to the zero mark along the lower meniscus and water drops are removed from the inner surface of the upper part of the burette with filter paper. Then, under the burette substitute a bottle, previously weighed with a lid on an analytical balance. A certain volume of water (for example, 5 ml) is slowly poured into the bottle from the burette. After that, the bottle is closed with a lid and weighed again. The difference between the mass of the weighing bottle with water and the empty weighing bottle is equal to the mass of water contained in the burette between divisions of 0 and 5 ml at the temperature of the experiment. Then the burette is again filled with water to the zero mark along the lower meniscus, 10 ml of water is slowly poured into an empty bottle and the mass of water contained in the burette between divisions 0 and 10 ml is determined in a similar way. When calibrating the burette, for example, for 25 ml, this operation is carried out 5 times and the mass of water corresponding to the nominal volumes indicated on the burette of 5, 10, 15, 20 and 25 ml is calculated. Each weighing of an empty bottle and a bottle of water is repeated at least twice, while the difference between two weighings should not exceed ± 0.005 g.

Then according to the table. 2-1 determine the density of water at the temperature of the experiment and calculate the true capacity of the burette for each value of the nominal volume indicated on it.

Based on the data obtained, the correction value is calculated equal to the difference between the calculated value of the true capacity and the corresponding value of the nominal volume of the burette:

and then draw a curve of burette capacity errors in coordinates (Figure 2-1).

For example, let the following experimental data be obtained when calibrating a burette with a capacity of 25 ml at a temperature of 20 °C, which, together with the results of the corresponding calculations, are presented in Table. 2-2.

Based on the obtained tabular data, a capacity correction curve for a given buret is plotted, using which it is possible to refine the results of reading by buret.

Table 2-2. Calibration results for a 25 ml burette

Rice. 2-1. Burette capacity correction curve

For example, let 7.50 ml of titrant be used for titration of an aliquot of the analyte according to the results of counting on a burette. According to the graph (see Fig. 2-1), the correction value corresponding to this nominal volume is 0.025 ml, the true volume of titrant used is: 7.50 - 0.025 = 7.475 ml.

3.3. Checking pipette capacity

A pipette, clean and weighed on an analytical balance, is filled with water to the zero mark along the lower meniscus and then the water is slowly filled.

poured along the wall into a pre-weighed bottle. The bottle is covered with a lid and weighed with water. Each weighing of an empty bottle and a bottle with water is repeated at least two times, while the difference between two weighings should not exceed ± 0.005 g. The difference between the mass of a bottle with water and an empty bottle is equal to the mass of water contained by a pipette. The true capacity of the pipette is calculated by dividing the average mass of water by the density of the water at the test temperature (see Table 2-1).

4. Typical calculations in titrimetric analysis

4.1. Ways of expressing concentrations used for calculations in titrimetric analysis

4.1.1. Molar concentration of substance c (A), mol / l - the amount of substance A in mol contained in 1 liter of solution:

(2.1)

where - the amount of substance A in mol, dissolved in V (A) l

solution.

4.1.2. Molar concentration equivalent of a substance , mol / l - the amount of substance A equivalent in mol contained in 1 liter of solution (the former name is the “normality” of the solution):

(2.2)

where
- the amount of substance equivalent to A in mol,

dissolved in V (A) l of solution; - molar mass of the equivalent of ve-

substances A, g / mol; - the equivalence factor of the substance.

4.1.3. Substance titer T(A), g / ml - the mass of solute A in grams, contained in 1 ml of solution:

4.1.4. Titrimetric conversion factor I, g / ml - mass of the analyte in grams, interacting with 1 ml of titrant:

(2.4)

4.1.5. Correction factor F- a value showing how many times the practical concentrations of the titrant differ from the corresponding theoretical values ​​specified in the method:

(2.5)

4.2. Calculation of the molar mass equivalent of substances in reactions used in titrimetric analysis

An equivalent is a real or conditional particle that can add or donate one hydrogen ion H + (or be otherwise equivalent to it in acid-base reactions) or add or donate one electron in redox reactions.

Equivalence factor - a number indicating which

the equivalent fraction is from a real particle of substance A. The equivalence factor is calculated based on the stoichiometry of this reaction:

where Z- the number of protons donated or added by one reacting particle (molecule or ion) in an acid-base reaction, or the number of electrons donated or accepted by one reacting particle (molecule or ion) in an oxidation or reduction half-reaction.

The molar mass of the equivalent of a substance is the mass of one mole of the equivalent of a substance, equal to the product of the equivalence factor by the molar mass of the substance, g / mol. It can be calculated using the formula:

(2.6)

4.3. Preparation of a solution by diluting a more concentrated solution with a known concentration

When carrying out titrimetric analysis, in some cases it is required to prepare a solution of substance A with a volume approximately known concentration by diluting a more concentrated solution.

When the solution is diluted with water, the amount of substance A or the amount of substance A does not change, therefore, in accordance with expressions (2.1) and (2.2), we can write:

(2.7)
(2.8)

where indices 1 and 2 refer to solutions before and after dilution, respectively.

From the ratios obtained, the volume of a more concentrated solution is calculated , which must be measured to prepare a given solution.

4.4. Preparation of a predetermined volume of solution by weighing a precisely known mass

4.4.1. Sample Weight Calculation

The theoretical mass of a sample of a standard substance A, necessary to prepare a given volume of a solution with a known concentration, is calculated from expressions (2.1) and (2.2). It is equal to:

(2.9)

if the molar concentration of a substance in solution is used, and:

(2.10)

if the molar concentration of the equivalent of the substance in solution is used.

4.4.2. Calculation of the exact concentration of the prepared solution

The concentration of a solution of substance A, prepared by an accurate sample of mass m (A), is calculated from the relationships (2.1-2.3), where t(A)- the practical mass of substance A, taken from the difference between two weighings on an analytical balance.

4.5. Calculation of titrant concentration during its standardization

Known volume of standard solution with concentration titrated with a titrant solution of volume V (T)(or vice versa). In this case, for the reaction taking place in the solution during the titration process , the law of equivalents has the form:

and

From here, an expression is obtained for calculating the molar concentration of the titrant equivalent from the results of titration:

(2.12)

4.6. Calculation of the mass of the analyte in the analyzed solution4.6.1. direct titration

The substance to be determined in the analyzed solution is titrated directly with a titrant.

4.6.1.1. Calculation using titrant equivalent molar concentration

An aliquot of the analyte solution titrated

titrant solution with volume V(T). In this case, for the reaction occurring in the solution during the titration process:

the law of equivalents has the form: and

(2.13)

Hence, the molar concentration of the equivalent of the analyte, calculated from the results of titration, is equal to:

(2.14)

The resulting expression is substituted into equation (2.2) and a formula is obtained for calculating the mass of the analyte in a flask with a volume according to the results of direct titration:

(2.15)

If, during titration, part of the titrant is consumed by the reaction with the indicator, a "blank experiment" is carried out and the volume of titrant V "(T) is determined,

used for indicator titration. In calculations, this volume is subtracted from the volume of the titrant, which was used to titrate the solution of the analyte. Such an amendment is made during the "blank experiment" in all calculation formulas used in titrimetric analysis. For example, formula (2.15) for calculating the mass of the analyte, taking into account the “blank experiment”, will look like:

(2.16)

4.6.1.2. Calculation using titrimetric conversion factor

We have an analyzed solution with a volume For titration of alik-

mil's share solution of analyte used volume of titrant V (T) with theoretical titrimetric conversion factor and correction factor F. Then the mass of the analyte in an aliquot is equal to:

(2.17)

and throughout the analyzed volume

(2.18)

4.6.2. substitution titration

a known excess of reagent A is added and substituent B is isolated in an amount equivalent to the analyte:

Substituent B is titrated with a suitable titrant:

The law of equivalents for substitution titration:

using relation (2.8) can be written in the form:

From here, a formula is obtained for calculating the molar concentration of the equivalent of the analyte in solution according to the results of substitution titration:

which has the same form as in direct titration (2.14). That is why all calculations of the mass of the analyte in the analyzed problem during substitution titration are carried out according to formulas (2.15-2.18) for direct titration. 4.6.3. Back titration

To an aliquot of the analyte add famous excess of the first titrant :

Then the excess of the unreacted first titrant is titrated with the second titrant, which consumes the volume :

The law of equivalents in this case can be written as:

From here, the molar concentration of the equivalent of substance X in solution is calculated:

(2.19)

Substitute the resulting expression into equation (2.2) and obtain a formula for calculating the mass of the analyte in the analyzed solution, equal to the volume of the flask, based on the results of back titration:

5. Implementation and provision of practical work on titrimetric analysis

5.1. General provisions

When studying the section "Titrimetric analysis", it is planned to carry out work on the following topics.

Theme I Methods of acid-base titration.

Theme II. Methods of redox titration.

Topic III. Methods of precipitation titration.

Topic IV. Methods of complexometric titration.

Lesson 1. Preparation of hydrochloric acid solution and its standardization.

Lesson 2. Determination of the mass of alkali in solution. Determination of the mass of carbonates in solution. Determination of the mass of alkali and carbonate in solution in the joint presence.

Lesson 3. Determination of the mass of ammonia in solutions of ammonium salts.

a) Test control 1.

b) Determination of the mass of ammonia in solutions of ammonium salts. Lesson 4. Permanganometric titration.

a) Written test 1.

b) Determination of the mass of hydrogen peroxide in solution.

c) Determination of the mass of iron(II) in a salt solution. Determination of the mass fraction of iron(II) in a salt sample.

Lesson 5. Iodometric titration.

a) Determination of the mass of hydrogen peroxide in solution.

b) Determination of the mass of copper(II) in solution. Lesson 6. iodimetric titration.

Lesson 7. Bromatometric titration. Determination of the mass of arsenic (III) in solution.

Lesson 8. bromometric titration. Determination of the mass fraction of sodium salicylate in the preparation.

Lesson 9. Nitritometric titration.

a) Test control 2.

b) Determination of the mass fraction of novocaine in the preparation. Lesson 10. Argentometric titration and hexacyanoferratom-

tric titration.

a) Written test 2.

b) Determination of the mass of potassium bromide and potassium iodide in solution by argentometric titration.

c) Determination of the mass of zinc in solution by hexacyanoferratometric titration.

Lesson 11. Complexometric determination of the mass of zinc and lead in solution.

a) Test control 3.

b) Determination of the mass of zinc and lead in solution.

Lesson 12. Complexometric determination of iron(III) and calcium in solution.

a) Written test 3.

b) Determination of the mass of iron(III) and calcium in solution.

Depending on the specific situation, it is allowed to carry out some work during not one, but two lessons. It is also possible to shift the timing of test controls and written tests.

At the end of each topic, examples of test items for intermediate control of students' knowledge, the content of the final written test, an example of a ticket for a written test are given.

At the end of each lesson, the student draws up a protocol that includes the date and name of the work performed, the essence of the methodology, the order of work, the experimental data obtained, calculations, tables, conclusions. All calculations of the results of the analysis (concentration of the solution, mass of the analyte) are performed by students with an accuracy of the fourth significant figure, except for cases specifically specified in the text.

Intermediate control of practical skills and theoretical knowledge is carried out with the help of test control and written tests.

5.2. Material support for classes in titrimetric analysis

Glassware: 5 ml burettes, 2 and 5 ml volumetric pipettes, 25, 50, 100 and 250 ml volumetric flasks, 10-25 ml conical flasks, glass bottles, 20-30 mm glass funnels, plain or dark glass flasks with a capacity of 100, 200 and 500 ml, measuring cylinders with a capacity of 10, 100 ml.

Reagents: Reagents of "chemically pure" qualification are used in the work and "ch.d.a.", indicator paper.

Devices: analytical balances with weights, technical balances with weights, oven, laboratory thermometer with a scale of 20-100 °C, stands with burette clamps and rings for asbestos nets, gas burners, water baths.

Auxiliary materials and accessories: detergents (soda, washing powders, chromium mixture), dishwashing brushes, rubber bulbs, asbestos nets, stationery glue, glass pencils, filter paper.

Bibliography

1. Lectures for students on the section "Titrimetric analysis".

2.Kharitonov Yu.Ya. Analytical chemistry (analytics): In 2 volumes - ed. 5th - M .: Higher School, 2010 (hereinafter referred to as the "Textbook").

3.Lurie Yu.Yu. Handbook of Analytical Chemistry.- M.: Chemistry, 1989 (hereinafter referred to as the "Handbook").

4.Dzhabarov D.N. Collection of exercises and tasks in analytical chemistry.- Moscow: Russian doctor, 2007.

In titrimetric analysis, the quantitative determination of a substance is carried out on the basis of the volume of a solution of a 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 analysis is called titration. This most important operation of titrimetric analysis consists in the fact that another solution of 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 quantitatively react 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 provision underlies titrimetric analysis. In order to determine the concentration of one of the solutions, one must know exactly the volumes of the reacting solutions, the exact concentration of the other solution, and the moment when the two substances react in equivalent quantities. The conditions for titrimetric determination are:

a) accurate measurement of volumes of reactants;

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) determination of the end of the reaction.

Titrimetric determination takes much less time than gravimetric determination. Instead of many lengthy operations of gravimetric analysis (precipitation, filtration, weighing, etc.), only one operation is carried out in titrimetric determination - 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 by 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 pass quantitatively according to a certain equation without side reactions. You need to be sure. That the added reagent is consumed exclusively for 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 analyte. The equivalence of the reactants is the basis for the calculation of the results of the analysis.

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

TITRATION METHODS

According to the method of performing titration, direct, reverse or indirect titration (substitution method) is distinguished.

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

In a back titration, a known volume of the working solution, taken in excess, is added to the solution of the analyte. After that, the residue of the first working solution is titrated with another working solution and the amount of the reagent that has 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. There is a reaction

Aq + +Cl = AqCl↓.

The excess of 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 analyzed solution, which reacts with the substance to be determined. Then one of the reaction products is determined by titration. For example, to determine hydrocyanic acid, a solution of AqNO 3 is added in excess. There is a reaction

HCN + AqNO 3 = AqCN↓ + HNO 3

Then nitric acid is easily determined using a working alkali solution of 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 TITRIMETRIC METHODS

ANALYSIS

Titrimetric analysis uses reactions of various types (acid-base interaction, complex formation, etc.) that meet the requirements that apply to titrimetric reactions. Separate titrimetric methods are named after the type of the main reaction that occurs during titration or by 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 with color indicators, methods of potentiometric titration, conductometric, photometric, etc. When classifying according to the type of the main reaction occurring during titration, the following methods of titrimetric analysis are usually distinguished:

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

H + + OH - \u003d H 2 O, CH 3 COOH + OH - \u003d CH 3 COO - + H 2 O,

CO 3 2- + H + \u003d HCO - 3;

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

Mg 2+ + H 2 V 2- \u003d MgV 2_ + 2H +

Where V 2 \u003d CH 2 - N /

׀ / CH 2 – COO-

3. Precipitation methods based on the formation of sparingly soluble

connections:

Aq + + Cl - + AqCl↓ (argentometry),

Hg 2 2+ + 2Cl - \u003d Hg 2 Cl 2 ↓ (mercury);

4.methods of redox titration. founded

on redox reactions (oxydimetry):

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

2S 2 O 3 2- + l 2 \u003d S 4 O 6 2- + 2l - (iodine);

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).

In titrimetry, a wide variety of reactions are used. Depending on which reaction underlies the titration, the following methods of titrimetric analysis are distinguished.

Acid-base methods, based on the neutralization reaction:

H + + OH - → H 2 O

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

Oxidation - reduction methods(oxydimetry). 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 complex formation methods are based on the precipitation of ions in the form of sparingly soluble compounds and on the binding of ions into a poorly dissociated complex.

There are the following titration methods:

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

the opposite, to when a deliberately excessive, but accurately measured volume of a solution of a known concentration is added to the solution to be determined, and the excess of the 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 a solution is called, with the help of which a titrimetric determination is made, i.e. solution to be titrated. To carry out the determination using a titrant, you need to know its exact concentration. There are two methods for preparing titrated solutions, i.e. solutions of 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 the solute and the volume of the solution are known. In this case, the solutions are called solutions with prepared titers.

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

Titrants are usually prepared at 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 regular intervals (from 1 to 3 weeks, depending on the substance from which the solution is prepared). Therefore, if the titrant is prepared according to 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: the titres of the titrants should be set under the same conditions under which the analysis will be performed.

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

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

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

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

Stable in solution (does not oxidize or decompose).

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

Good solubility in water.

The ability to react with a solution whose titer is set according to a strictly defined equation and at high speed.

To set the titer of the titrant from the adjusting agent prepare an exact solution according to a precisely taken sample. The solution is prepared in a volumetric flask. The volumetric flask should be washed with chromium mixture until it "runs down", rinsed many times with tap water and then 3-4 times with distilled water. The funnel must be clean, dry and free to enter the neck of the flask.

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

The sample taken is carefully transferred through the funnel into a volumetric flask. The remains of the weighing bottle are thoroughly washed into the funnel with distilled water from the wash bottle. Then they wash the inner walls of the funnel and, slightly raising it, the outer part of the tube. It is necessary to ensure that the total amount of water used to wash the weighing bottle and funnel does not take more than half of the flask. Stir the contents of the flask with a gentle twisting motion until the sample is completely dissolved. Then, the contents of the flask are brought to the mark with distilled water from the wash bottle. To do this, pour water about 1 cm below the mark. Place the flask so that the mark is at eye level and carefully, drop by drop, add water until the lower part of the meniscus touches the mark on the neck of the flask (Fig. 1). Close the flask carefully with a stopper and, inverting the flask, mix the solution 12-15 times. Solutions for setting the titer should be freshly prepared.

Often used to prepare titrated solutions. fixed channels, which are sealed glass ampoules with accurate 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 must be dry. Then a special glass head is inserted into the funnel (usually attached to the box with fixes), 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, lifts it up and lightly hits the end of the striker. In this case, the contents of the ampoule enter the flask through the funnel (Fig. 2). On the side or top of the ampoule there is a recess 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 washer. You need to rinse many times in small portions. After that, the outer walls of the ampoule are rinsed and the ampoule is discarded. Rinse the funnel and the head, then raise the funnel and wash the outer

Part of the 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 by the end of all operations does not exceed 2∕3 of the volume of the flask. Gently swirl the contents of the flask. If fixanal contained a dry substance, stir it until complete dissolution. Then dilute the contents of the flask to the mark with distilled water. Close the flask carefully and stir the solution 12-15 times.

To set the titer 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 material and, dissolving 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 laborious. Therefore, in the laboratory, practically when performing analyzes, they use the first method.

5. DETERMINATION OF THE EQUIVALENCE POINT AND END

REACTIONS

During titration, not an excess of the reagent is used, but an amount equivalent to the amount of the analyte. A necessary condition for determining the content of a substance titrimetrically is the exact establishment of the moment when the reaction between the titratable substance and the titrant ends, that is, fixing the point equivalence. The more precisely the end of the reaction is determined, the more accurate the result of the analysis will be.

To determine the end of the reaction, special reagents, the so-called indicators, are used. The action of indicators is usually reduced 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 is added from the burette that a noticeable change in the color of the titrated solution is observed, it is said that end point of the titration.

In most cases, indicators are added to a solution of the analyte and the titration takes place in the presence of the indicator. These are the so-called internal indicators. In some cases, they act differently: as the titration proceeds, a drop of the solution is taken from the titrated solution with a capillary, to which a drop of indicator is added over a porcelain plate. Thus, the reaction with the indicator takes place outside the titrated solution. The indicators used in this case are called external.

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

Lab #8

TITRIMETRIC ANALYSIS

The purpose of the work: to get acquainted with the basics of titrimetric analysis, to study the main 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 possibility of using it to determine 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 to be determined according to the reaction equation. The reaction that proceeds in this case must be stoichiometric, i.e., the substances must react strictly quantitatively, according to the coefficients in the equation. Only under this condition can the reaction be used for quantitative analysis.

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

Equivalence point called the moment that occurs during the titration, when the reactants have completely reacted. At this point they are in equivalent quantities , i.e. sufficient for complete, without residue, the reaction.

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

primary standard called a solution with a precisely known concentration, prepared by an accurate sample of 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 the established norms. Often, the substance is subjected to additional purification in order to prepare standard solutions. Before weighing, the substance is dried in a desiccator over a drying agent or kept at an 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. Store standard solutions 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 the preparation of a substance for the preparation of a standard solution is a long and laborious process, the chemical industry produces so-called. fixed channels. Fixanal is a glass ampoule in which a certain amount of a substance is sealed. The ampoule is broken, and the substance is quantitatively transferred into a volumetric flask, then bringing the volume of liquid to the mark. The use of fixanals greatly simplifies the process and reduces the preparation time of the standard solution.

Some substances are difficult to obtain in a chemically pure form (for example, KMnO4). Due to the content of impurities, 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 called a solution of a substance with a precisely known concentration, which is established according to the primary standard. Secondary standards (eg solutions of KMnO4, NaOH, etc.) are stored under the same conditions as primary standards, but their concentration is periodically checked against standard solutions of so-called reference substances.

2. Methods and types of titration.

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

direct titration is that the solution of the analyte BUT titrated with standard titrant solution AT. The direct titration method is used to titrate solutions of acids, bases, carbonates, etc.

At reverse titration an aliquot of the standard solution AT titrated with a solution of the analyte BUT. Reverse titration is used when 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, with a slow reaction, etc. During back titration to an aliquot of the analyte BUT add an accurately measured volume of a standard solution of a substance AT taken in excess. Unreacted excess of a substance AT determined by titration with a standard solution of the excipient FROM. By the difference in the initial amount of the substance AT and its amount remaining after the reaction, determine the amount of substance AT that has reacted with a substance BUT, on the basis of which the content of the substance is calculated BUT.

Indirect titration or substituent titration. It is based on the fact that it is not the substance itself that is being titrated, but the product of its reaction with an auxiliary substance FROM.

Substance D must be formed strictly quantitatively with respect to the substance BUT. Determining the content of the reaction product D titration with a standard solution of a substance AT, according to the reaction equation, the content of the analyte is calculated BUT.

Reactions that are used in titrimetric analysis should be strictly stoichiometric, proceed quickly enough and, if possible, at room temperature. Depending on the type of reaction taking place, there are:

Acid-base titration, based on the neutralization reaction.

redox titration, based on redox reactions.

complexometric titration, based on complex formation reactions.

3. Acid-base titration.

Acid-base titration is based on a 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 proceeds almost instantaneously at room temperature. Acid-base titration is used to determine acids, bases, as well as 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, there is a gradual change in the acidity of the medium, 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 medium, 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 the water. When an acid is added, the concentration of hydrogen ions increases, which leads to a shift in the equilibrium of water dissociation to the left, as a result of which the concentration of hydroxide ions decreases. For example, if = 10-3m/l, then = 10-11m/l. The ionic product of water will remain constant.

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

pH pH is called the negative decimal logarithm of the concentration of hydrogen ions.

pH \u003d - lg. (2)

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

pH \u003d - lg 10-7 \u003d 7.

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

pOH \u003d - lg \u003d 14 - pH. (3)

In the course of acid-base titration, the pH of the solution changes with each portion of the 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 should be measured. To determine the pH at the equivalence point, build titration curve- graph of the dependence of the pH of the solution on the volume of the added titrant. The titration curve can be built experimentally by measuring the pH at different points in the titration, or theoretically calculated using formulas (2) or (3). As an example, consider the titration of the strong acid HCl with the strong base NaOH.

Table 1. Titration of 100 ml of 0.1M HCl solution with 0.1M 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, the excess of 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 mol and 1 10-3 mol, respectively

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

The titration curve shows that at the beginning of the 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 slightly changes from the volume of the added titrant, as in 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 neutral point of the solution.

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

HAn ® H+ + An-.

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

(4)

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

When an alkali solution is added to a solution of a weak acid, a salt of a 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 a weak electrolyte, but also on the salt concentration. Formula (5) can be used to calculate the pH of buffer solutions.

СKtAn is the salt concentration in the buffer solution.

KD is the dissociation constant of a weak electrolyte

CHAn 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). The addition of a strong acid to a buffer solution causes the weak acid to be displaced from its salt and, consequently, to 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 required 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.

CH3COOH + NaOH ® CH3COOHa + H2O

It can be seen from the reaction equation that CH3COOH and NaOH react in a ratio of 1:1, therefore, the amount of acid that reacted is equal to the amount of alkali contained in the poured titrant. The amount of sodium acetate CH3COOHa formed is also equal to the amount of alkali that entered the solution during titration.

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

CH3COOH - + 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 from the formula.

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CH3COOH reacted.

CH3COOH remaining in solution

1,00 10-2

1,00 10-2

0 ,100

Based on the data obtained, we construct a titration curve for a weak acid with a strong base.

It can be seen from the titration curve that the equivalence point in the titration of a weak acid with a strong base does not coincide with the neutral point and lies in the region of the alkaline reaction of the solution.

Titration curves allow you to accurately determine the pH of the solution at the equivalence point, which is important for determining the end point of the titration. The equivalence point can be determined by the instrumental method, 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 medium. By themselves, indicators are weak acids or bases that reversibly dissociate according to the equation:

HInd ® H+ + Ind-

The molecular and ionic forms of the indicator have different colors and change into each other at a certain pH value. The pH range in which the indicator changes color is called the indicator transition range. 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; 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 transition interval of the indicator should, if possible, coincide with the pH jump. For example, when a strong acid is titrated with a strong base, a pH jump is observed in the range of 4-10. This interval includes the transition intervals of such indicators 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. Such indicators 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 the most contrasting and sharp. For this purpose, mixtures of various indicators or mixtures of indicators with dyes are sometimes used.

3. Redox titration.

(redoximetry, oxidimetry.)

Redox methods include an extensive group of titrimetric analysis methods based on the occurrence of redox reactions. Redox titrations use a variety of 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, the determination of 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 a wide variety of 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 are fast enough to allow titrations to be carried out in just a few minutes. Many of them proceed in acidic, neutral and alkaline environments, which greatly 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 able to react in acidic, neutral and alkaline environments. about 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 permanganometric method can be used to 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 set without the use of indicators.

3. Potassium permanganate is a strong oxidizing agent and 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 a chemically pure form. Therefore, it is difficult to prepare a standard solution for an exact sample of a substance. For titration, secondary permanganate standards are used, the concentration of which is set according to standard solutions of other substances: (NH4) 2C2O4, K4, H2C2O4, etc., which are called setting substances.

2. Permanganate solutions are unstable and change their concentration during long-term storage, which must be periodically checked using solutions of adjusting substances.

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

Iodometry.

In iodometric titrations, iodine is the oxidizing agent. Iodine oxidizes many reducing agents: SO32-, S2O32-, S2-, N2O4, Cr2+, etc. But the oxidizing power 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 the standard iodine solution is adjusted with a standard solution of sodium thiosulfate Na2S2O3.

2S2O32- + I2 ® S4O62- + 2I-

In the iodometric determination, various titration methods are used. Substances readily oxidized with iodine are titrated directly with standard iodine solution. This is how they determine: 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 analyte. After completion of the reaction, excess iodine is titrated with a standard solution of thiosulfate. The indicator in iodometric titration is usually starch, which gives a characteristic blue color with iodine, the appearance of which can be used to judge the presence of free iodine in the solution.

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

Advantages of the iodometric method.

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

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 makes it possible to use it for titration of non-aqueous solutions.

Iodometry also has some disadvantages.

1. Iodine is a volatile substance and during titration, its losses due to evaporation are possible. 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 are the ways to establish it, and which of them were used in this laboratory work?

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

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

The name titrimetric is related to the word titer, which denotes the concentration of the solution. The titer indicates the number of grams of a solute in 1 ml of a solution.

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

Reactions used in titrimetry must meet 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 a high rate;

3) the reaction should not be complicated by side 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, back and indirect titrations.

In direct titration methods, the analyte reacts directly with the titrant. For analysis by this method, one working solution is sufficient.

In back titration methods (or, as they are also called, residue titration methods), two titrated working solutions are used: the main and auxiliary. It is widely known, for example, the back titration of the chloride ion in acidic solutions. To the analyzed solution of chloride, first add a deliberate excess of a titrated solution of silver nitrate (basic working solution). In this case, the reaction of the formation of sparingly soluble silver chloride occurs.

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

The third main type of titrimetric determinations is substituent titration, or substitution titration (indirect titration). In this method, a special reagent is added to the substance to be determined, which reacts with it. One of the reaction products is then titrated with the working solution. For example, in the iodometric determination of copper, a deliberate excess of KI is added to the analyzed solution. The reaction 2Cu 2+ +4I - \u003d 2CuI+ I 2 occurs. The liberated iodine is titrated with sodium thiosulfate.

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

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

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

An equivalent is a certain real or conditional particle that can attach, release, or be any other sample of the equivalent of one hydrogen ion in acid-base reactions or one electron in redox reactions.

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

In titrimetric analysis, reactions of various types are used: - acid-base interaction, complex formation, etc., which meet the requirements that apply to titrimetric reactions. The type of reaction that occurs during titration is the basis for the classification of titrimetric methods of analysis. Usually, the following methods of titrimetric analysis are distinguished.

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

2. Methods of complex formation use reactions of formation of coordination compounds:

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

4. Methods of oxidation - reduction combine a large group of redox reactions:

Separate titrimetric methods are named after the type of the 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 permanganometric 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. A variety of indicators are available to indicate the equivalence point. Among the titrimetric methods based on complex formation reactions, the most important are reactions using complexones. Almost all cations form stable coordination compounds with complexones; therefore, complexometric methods are universal and applicable to the analysis of a wide range of various objects.

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

H + + OH - ↔ H 2 O

The working solutions of the method are solutions of strong acids (HCl, H 2 S, HNOz, 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 base solution.

Working solutions are mainly prepared as secondary standard solutions, since the substances used 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 solutions of bases - according to 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 end point of titration. According to the equivalence rule, titration must be continued until the amount of added reagent becomes equivalent to the content of the analyte. The moment that occurs in the process of titration, when the amount of the standard solution of the reagent (titrant) becomes theoretically strictly equivalent to the amount of the analyte according to a certain chemical reaction equation, is called equivalence point .

The equivalence point is set in various ways, for example, by changing the color of the indicator added to the titrated solution. The moment at which the observed change in the color of the indicator occurs is called titration end point. Very often, the end point of a titration does not exactly match 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 required to interact with the indicator.

The titrimetric method of analysis (titration) allows for volumetric quantitative analysis and is widely used in chemistry. Its main advantage is the variety of ways and methods, due to which it can be used to solve various analytical problems.

Principle of analysis

The titrimetric method of analysis is based on measuring the volume of a solution of known concentration (titrant) that has reacted with the test substance.

For analysis, you will need special equipment, namely, a burette - a thin glass tube with applied graduations. The upper end of this tube is open, and at the lower end there is a stopcock. The calibrated burette is filled with the titrant to the zero mark using a funnel. The analysis is carried out to the end point of the titration (CTT) by adding a small amount of solution from the buret to the substance under study. The end point of the titration is identified by a change in the color of the indicator or some physical-chemical property.

The final result is calculated from the amount of titrant used and is expressed in titer (T) - the mass of the substance per 1 ml of solution (g / ml).

Process Justification

The titrimetric method of quantitative analysis gives accurate results because the substances react with each other in equivalent quantities. This means that the product of their volume and quantity are identical to each other: C 1 V 1 = C 2 V 2 . From this equation, it is easy to find the unknown value of C 2 if the remaining parameters are set independently (C 1 , V 2) and are established during the analysis (V 1).

Endpoint titration detection

Since the timely fixation of the end of the titration is the most important part of the analysis, it is necessary to choose its methods correctly. The most convenient is the use of colored or fluorescent indicators, but instrumental methods can also be used - potentiometry, amperometry, photometry.

The final choice of the LTT detection method depends on the required accuracy and selectivity of the determination, as well as its speed and the possibility of automation. This is especially true for cloudy and colored solutions, as well as aggressive environments.

Requirements for the titration reaction

In order for the titrimetric method of analysis to give the correct result, it is necessary to choose the right reaction that will underlie it. Its requirements are as follows:

• stoichiometry;
• high flow rate;
• high equilibrium constant;
• the presence of a reliable method of fixing the experimental end of the titration.

Suitable reactions may be of any type.

Types of analysis

The classification of titrimetric analysis methods is based on the type of reaction. On this basis, the following titration methods are distinguished:

• acid-base;
• redox;
• complexometric;
• precipitation.

Each type is based on its own type of reaction, specific titrants are selected, depending on which subgroups of methods are distinguished in the analysis.

Acid-base titration

The titrimetric method of analysis using the reaction of interaction of hydroxonium with hydroxide ion (H 3 O + + OH - \u003d H 2 O) is called acid-base. If a known substance in solution forms a proton, which is typical for acids, the method belongs to the acidimetry subgroup. Here, stable hydrochloric acid HCl is usually used as the titrant.

If the titrant forms a hydroxide ion, the method is called alkalimetry. The substances used are alkalis such as NaOH, or salts obtained by reacting a strong base with a weak acid such as Na 2 CO 3 .

In this case, color indicators are used. They are weak organic compounds - acids and bases, which differ in the structure and color of protonated and non-protonated forms. The most common indicators used in acid-base titrations are phenolphthalein, a single color indicator (a clear solution turns crimson in an alkaline environment) and a two-color methyl orange indicator (a red substance becomes yellow in an acidic environment).

Their widespread use is associated with high light absorption, due to which their color is clearly visible to the naked eye, and contrast and a narrow color transition region.

Redox Titration

Redox titrimetric analysis is a quantitative analysis method based on changing the ratio of the concentrations of the oxidized and reduced forms: aOx 1 + bRed 2 = aRed 1 + bOx 2.

The method is divided into the following subgroups:

• permanganatometry (titrant - KMnO 4);
• iodometry (I 2);
• dichromatometry (K 2 Cr 2 O 7);
• bromatometry (KBrO 3);
• iodatometry (KIO 3);
• cerimetry (Ce(SO 4) 2);
• vanadatometry (NH 4 VO 3);
• titanometry (TiCl 3);
• chromometry (CrCl 2);
• ascorbinometry (C 6 H 8 OH).

In some cases, the role of an indicator can be played by a reagent participating in the reaction and changing its color with the acquisition of an oxidized or reduced form. But they also use specific indicators, for example:

• when determining iodine, starch is used, which forms a dark blue compound with I 3 - ions;
• in the titration of ferric iron, thiocyanate ions are used, which form bright red complexes with the metal.

In addition, there are special redox indicators - organic compounds that have different colors of oxidized and reduced forms.

Complexometric titration

In short, the titrimetric method of analysis, called complexometric, is based on the interaction of two substances with the formation of a complex: M + L = ML. If mercury salts are used, for example, Hg(NO 3) 2, the method is called mercurymetry, if ethylenediaminetetraacetic acid (EDTA) - complexometry. In particular, with the help of the latter method, a titrimetric method for analyzing water is carried out, namely, its hardness.

In complexometry, transparent metal indicators are used, which acquire color when complexes are formed with metal ions. For example, when titrating ferric salts with EDTA, transparent sulfosalicylic acid is used as an indicator. It turns the solution red when complexed with iron.

However, more often metal indicators have their own color, which changes depending on the concentration of the metal ion. As such indicators, polybasic acids are used, which form fairly stable complexes with metals, which are rapidly destroyed when exposed to EDTA with a contrasting color change.

Precipitation titration

The titrimetric method of analysis, which is based on the reaction of the interaction of two substances with the formation of a solid compound that precipitates (M + X = MX ↓), is precipitation. It is of limited value, since usually the deposition processes proceed non-quantitatively and non-stoichiometrically. But sometimes it is still used and has two subgroups. If the method uses silver salts, for example, AgNO 3, it is called argentometry, if mercury salts, Hg 2 (NO 3) 2, then mercurymetry.

The following methods are used to detect the end point of the titration:

• Mohr's method, in which the indicator is a chromate ion, which forms a red-brick precipitate with silver;
• the Folhard method, based on the titration of a solution of silver ions with potassium thiocyanate in the presence of ferric iron, which forms a red complex with the titrant in an acidic medium;
• the Faience method, which involves titration with adsorption indicators;
• the Gay-Lussac method, in which the CTT is determined by the enlightenment or turbidity of the solution.

The latter method has not been practically used recently.

Titration methods

Titrations are classified not only by the underlying reaction, but also by the way they are performed. On this basis, the following types are distinguished:

• direct;
• reverse;
• substituent titration.

The first case is used only under ideal reaction conditions. The titrant is added directly to the analyte. So with the help of EDTA, magnesium, calcium, copper, iron and about 25 other metals are determined. But in other cases, more complex methods are more often used.

Back titration

It is not always possible to find the ideal response. Most often, it proceeds slowly, or it is difficult to find a way to fix the end point of the titration for it, or volatile compounds are formed among the products, due to which the analyte is partially lost. These shortcomings can be overcome by using the back titration method. To do this, a large amount of titrant is added to the substance to be determined so that the reaction goes to completion, and then it is determined how much of the solution remains unreacted. For this, the titrant residues from the first reaction (T 1) are titrated with another solution (T 2), and its amount is determined by the difference in the products of volumes and concentrations in two reactions: C T1 V T 1 -C T 2 V T 2.

The use of the reverse titrimetric method of analysis underlies the determination of manganese dioxide. Its interaction with ferrous sulfate proceeds very slowly, so the salt is taken in excess and the reaction is accelerated by heating. The unreacted amount of iron ion is titrated with potassium dichromate.

Substituent titration

Substituent titration is used in case of non-stoichiometric or slow reactions. Its essence is that for the substance to be determined, a stoichiometric reaction with an auxiliary compound is selected, after which the interaction product is subjected to titration.

This is exactly what is done when determining dichromate. Potassium iodide is added to it, as a result of which an amount of iodine equivalent to the analyte is released, which is then titrated with sodium thiosulfate.

Thus, titrimetric analysis makes it possible to determine the quantitative content of a wide range of substances. Knowing their properties and features of the course of reactions, it is possible to choose the optimal method and method of titration, which will give a result with a high degree of accuracy.

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