Titration. Indicators in acid-base titration. Titration technique Titration procedure

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

Types of classification

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

Analysis methods:

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

Neutralization

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

Complexation

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

Redoximetry

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

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

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

By titration method

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

Analysis value

All methods of titrimetric analysis assume:

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

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

Research methods in analytical chemistry

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

Analytical Methods Division

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

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

Specifics of titrimetric research

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

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

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

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

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

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

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

Conclusion

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

TITRATION.(from the French titre - quality, characteristic) - one of the methods of quantitative analysis, based on measuring the amount of a reagent that completely reacts with the analyte. For example, if you know exactly how much potassium hydroxide (in grams or moles) was consumed in the reaction with hydrochloric acid, then using the reaction equation KOH + HCl = KCl + H 2 O it is easy to calculate how many grams (or moles) of hydrogen chloride were in the analyzed solution .

Such calculations can only be carried out for stoichiometric reactions. This term was introduced into use in 1792 by the German chemist Jeremiah Richter. He derived it from the Greek words meaning "invisible" and "measurement", which was supposed to mean the ratio of "invisible" chemical elements in reactions. Richter for the first time in the history of chemistry began to use quantitative reaction equations. For example, according to his data, from 2400 grains of CaCO 3 upon strong calcination, 1342 grains of CaO are obtained, i.e. the decomposition reaction CaCO 3 = CaO + CO 2 is characterized by the ratio CaCO 3:CaO = 2400:1342 = 1.788, which is surprisingly in good agreement with modern calculation, which gives a ratio of 1.785.

All reactions used in quantitative analysis must be stoichiometric. For these reactions, the coefficients in front of the reagent formulas show the quantitative ratios of the reagents and products. For example, the ratio of reactants in the oxidation reaction of oxalic acid with potassium permanganate in an acidic medium exactly corresponds to the equation

5H 2 C 2 O 4 + 2KMnO 4 + 3H 2 SO 4 = 2MnSO 4 + K 2 SO 4 + 10CO 2 + 8H 2 O.

Therefore, this reaction can be used to accurately determine the concentration of permanganate in a solution if the amount of oxalic acid consumed is known (and vice versa). But it is impossible to calculate exactly the amount of alkene reacted with potassium permanganate, since this reaction is non-stoichiometric: upon oxidation, a diol is formed from the alkene: R-CH=CH-R + 2[O] ® R-CH(OH)-CH(OH)- R, which can be further oxidized to break the carbon-carbon bond and form two molecules with a carbonyl group (acid or ketone). At the same time, different experiments, even conducted under the same conditions, will give slightly different amounts of products and their ratio; in organic chemistry, the yield of a reaction is very rarely exactly 100.00%.

For the analysis to be accurate, it is necessary, in addition to the complete completion of the reaction, that the reagent is added to the analyte in small portions (for example, one drop of a dilute solution), and also that the moment when the reaction ends can be reliably determined. To fulfill the second condition, various indicators are used.

Indicators are very different. Consider the reaction of baking soda with vinegar: NaHCO 3 + CH 3 COOH = CH 3 COONa + CO 2 + H 2 O. From this equation it follows that 1 mole of soda (84 g) completely reacts with 60 g of acetic acid. This releases bubbles of carbon dioxide, which can serve as an indicator. If vinegar is added dropwise to sodium bicarbonate of a known mass until gas evolution ceases, then by measuring the volume of the added solution and knowing its density, it is easy to calculate the amount of pure acetic acid in the added solution and, therefore, the concentration of vinegar. For example, if a complete reaction with 1.00 g of soda required 10.5 g of vinegar, then this means that the vinegar contained 60/84 = 0.714 g of pure acetic acid, and its strength is (0.714/10.5)100% = 6.8%. For very accurate calculations, chemists use refined values ​​of the atomic masses of elements (in this case, 84.01 for sodium bicarbonate and 60.05 for acetic acid).

Of course, the described titration experiment is considered only as an example. After all, gas is not released in all chemical reactions, and it is not easy to notice the last gas bubble, especially if the gas is partially dissolved and the solution is dark in color. Therefore, special indicators are usually used, the change in color indicates that the end of the reaction has been reached - the so-called equivalence point.

Some of the most common indicators are acid-base indicators. They are used in cases where during titration, i.e. Gradually adding a reagent to the solution being analyzed changes the pH of the medium. This occurs, for example, if an acid solution is added to the alkali solution being analyzed (or vice versa). The analyzed solution is prepared by volume or by sample (it is weighed on a precise analytical balance, usually up to 0.1 mg), which is dissolved in a volumetric flask of a precisely known volume (such flasks can have a volume of 10, 25, 50, 100, 200, 250, 500 or 1000 ml). A small volume of the analyzed solution is taken from a volumetric flask using special volumetric pipettes (their volume is also determined with high accuracy and is usually 10, 20, 25 and 50 ml) and placed in a conical titration flask. The reagent solution from the burette is added dropwise to this flask with continuous stirring until the equivalence point is reached.

The volume of consumed reagent solution is measured by divisions of the burette; its volume can be 10, 25 or 50 ml, and the division price can be 0.1 ml. There are also microburettes with a capacity of 1 to 5 ml with a division value of 0.01 ml. The solution from the burette is added dropwise to the solution of the analyte using a stopcock. In this case, titration is always repeated several times and the average result is taken - this increases the accuracy and reliability of the analysis. If the concentration of solutions is measured in units of mol/l, then the unknown concentration of the substance can be immediately determined from the volume of solutions of the analyte and reagent. For example, if 12.55 ml of HCl solution with a concentration of 0.0865 mol/l was used to titrate 25.00 ml of KOH solution (it is determined in advance), then the alkali concentration is 0.0865(12.55/25.00) = 0 .0432 mol/l. It is clear that if a sulfuric acid solution was used for titration, then it is necessary to take into account the stoichiometric coefficient 2 in the reaction equation 2KOH + H 2 SO 4 = K 2 SO 4 + 2H 2 O. To take into account stoichiometric coefficients, analytical chemists usually use normality instead of molarity solution. So, 1 n. a solution of H 2 SO 4 corresponds to a molar concentration of 0.05 mol/l. Then the product of the volume and the normality of the solution will always be the same for both the analyte and the reagent.

There are many known acid-base indicators (about 100), and each of them has its own area of ​​application. This can be shown in the following examples. When titrating a strong acid (HCl) with a strong alkali (NaOH), complete neutralization is achieved when the solution containing NaCl is neutral (pH = 7). In this case, you can use indicators such as nitrazine yellow (the color changes from yellow to blue-violet in the pH range 6.0 - 7.0) or bromothymol blue, which has similar characteristics. When titrating a strong acid with a strong alkali (or vice versa), the change in pH at the equivalence point is so dramatic that many other indicators can be used. For example, in the above example, at reagent concentrations of 0.1 mol/l, after adding 99.9% of all alkali, the pH of the solution is 4, and after adding 0.1% excess alkali, pH = 10. Such a sharp change in pH during titration can be caused by only 1–2 drops of reagent. Therefore, it would not be a mistake in this case to use such indicators as methyl orange (the color changes from red at pH 3.1 to orange-yellow at pH 4) or the well-known phenolphthalein (the color changes from colorless at pH 8.2 to crimson-red at pH 10.0).

If you titrate a weak acid, for example, acetic acid, with a NaOH solution, then at the end of the titration, after complete neutralization of the acid, the solution contains sodium acetate CH 3 COONa, which, due to hydrolysis, has an alkaline reaction (pH about 9). In this case, you can no longer use methyl orange, but you can use phenolphthalein. On the other hand, when titrating a weak alkali (for example, ammonia solution) with a strong acid (HCl) at the equivalence point, NH 4 Cl is present in the solution, which due to hydrolysis has an acidic reaction (pH about 5), and here methyl orange and not allowed - phenolphthalein.

A special case is the choice of indicator when titrating polybasic acids (for example, H 3 PO 4), as well as mixtures of substances. Thus, NaOH solutions usually contain carbonate impurities due to reaction with carbon dioxide in the air. If a NaOH solution containing Na 2 CO 3 is titrated with a strong acid in the presence of phenolphthalein, the solution will become colorless when all the alkali and part of the carbonate are neutralized (this will happen at a pH of about 8.5) in accordance with the equation

NaOH + Na 2 CO 3 + 2HCl = 2NaCl + NaHCO 3 + H 2 O.

If you add methyl orange to such a solution and continue titration, the yellow color will turn pink when all the bicarbonate reacts (this corresponds to a pH of about 3.5): NaHCO 3 + HCl = NaCl + H 2 CO 3.

Thus, using two acid-base indicators, the alkali and carbonate content of a sample can be calculated separately.

If an oxidation-reduction reaction occurs during titration, special indicators are used that change their color depending on the redox potential of the solution. Often the colored reagent itself can serve as an indicator. for example, when quantitatively analyzing reducing agents by titrating them with a KMnO 4 solution, the equivalence point is determined by the disappearance of the pink color of the permanganate. In this way, for example, you can determine the iron(II) content in a solution in accordance with the equation 10FeSO 4 + 2KMnO 4 + 8H 2 SO 4 = 5Fe 2 (SO 4) 3 + 2MnSO 4 + K 2 SO 4 + 8H 2 O. So far Fe2+ ​​ions are present in the solution, the added KMnO 4 becomes discolored. As soon as the slightest excess of permanganate appears, the solution turns pink. In this way, various reducing agents can be analyzed.

Moreover, permanganatometry can also be used to analyze oxidizing agents! For this purpose, the so-called back titration is used. To do this, a known amount of a reducing agent, iron(II), is added in excess to a known volume of an oxidizing agent (for example, potassium dichromate). The reaction K 2 Cr 2 O7 + 6FeSO 4 + 7H 2 SO 4 = Cr 2 (SO 4) 3 + 3Fe 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O occurs very quickly. Then, using titration with permanganate, determine how much iron(II) remains and, by simple subtraction, calculate how much was consumed in the first reaction with the dichromate.

Another common method using redox reactions is iodometry ( cm. IOD) It is used, for example, to determine oxidizing agents that, reacting with potassium iodide, oxidize it to free iodine, for example: 10KI + 2KMnO 4 + 8H 2 SO 4 = 2MnSO 4 + 5I 2 + 6K 2 SO 4 + 8H 2 O The principle of reverse titration is also used here: the amount of iodine released in the first reaction can be determined using its reaction with sodium thiosulfate: I 2 + 2Na 2 S 2 O 3 = Na 2 S 4 O 6 + 2NaI. The end of this reaction is determined by the disappearance of the iodine color. However, when there is little iodine left in the solution, its pale yellow color is almost invisible and it is difficult to notice the moment when the solution becomes completely discolored. To increase the accuracy of the titration, a little starch solution is added to the solution towards the end: the slightest traces of iodine turn it blue. Therefore, the disappearance of the blue color indicates that the reaction has completed completely. Chemists typically use very dilute solutions of thiosulfate (e.g., 0.01 mol/L), which greatly improves the accuracy of the analysis because larger volumes of solution are measured more accurately.

Iodine very easily and quickly oxidizes ascorbic acid (vitamin C). Therefore, using iodometric analysis, you can even determine at home (of course, without much accuracy) the content of this vitamin, for example, in orange juice. (Acid-base titration cannot be used here, since the juice contains other organic acids in much larger quantities - citric, malic, tartaric and others.) The analysis is based on the fact that 1 mole of ascorbic acid (176 g) reacts with 1 mole of iodine (254 g). For titration, you can use a pharmacy iodine tincture, assuming that the iodine is not exhausted and it is exactly 5% (this corresponds to a concentration of about 0.2 mol/l). The amount of iodine consumed can be estimated using a regular pipette - by the number of drops of tincture used in the reaction. Since there is usually not very much ascorbic acid in juice, only 1-2 drops of tincture can be used to titrate its portion (for example, 20 ml), which will lead to a very large analysis error. To make the result more accurate, you must either take a lot of juice or dilute the iodine tincture; in both cases, the number of iodine drops used for titration will increase, which will make the analysis more accurate. Chemists prefer the second way.

If the tincture is diluted 40 times with boiled water (chemists use distilled water), then the concentration of such a solution will be about 0.005 mol/l; 1.0 ml of such a solution corresponds to 0.88 mg of ascorbic acid. It is also necessary to determine the volume of the drop (it depends on the type of pipette, as well as on the specific solution). To do this, use a 1 or 2 ml medical syringe to measure 1 ml of a diluted iodine solution, and then count how many drops there are in this volume (this will only take a few minutes). It is advisable to first test the technique on a standard solution of ascorbic acid. It can be prepared from a tablet with a known content of ascorbic acid - for example, 0.1 or 0.5 g. The tablet should be dissolved in 0.5 liters of boiled water and 25 ml of this solution should be taken using a pharmacy beaker (the vitamin in it will be 20 times less than in a tablet). To this solution add diluted iodine tincture, not forgetting to add a little liquid starch paste at the end. And if, for example, 6.0 ml of iodine solution was used to titrate 25 ml of a solution, then there was 0.88.6 = 5.28 mg of ascorbic acid in the solution, and 20 times more in the original tablet, i.e. 105.6 mg. Such a small error indicates the correctness of the “home” analysis.

Chemists, of course, do not drop from a pipette, but use precise burettes with graduations. In addition, they often do not even prepare standard solutions themselves, but use factory ones; such solutions in sealed ampoules are called fixanals - they contain a fixed amount of reagent (usually 0.1 mol) to accurately determine the concentration of the working solution. For example, solutions of KMnO 4, K 2 Cr 2 O 7, NaCl, H 2 C 2 O 4, HCl, AgNO 3, NaOH, etc. serve as fixanals.

Complexometric indicators are widely used - substances that form colored complex compounds with ions of certain metals (many of which are colorless). An example is eriochrome black T; a solution of this complex organic compound is blue in color, and in the presence of magnesium, calcium and some other ions, complexes are formed that are colored intensely wine-red. The analysis is carried out as follows: a complexing agent, most often Trilon B, which is stronger than the indicator, is added dropwise to a solution containing the analyzed cations and an indicator. As soon as Trilon completely binds all metal cations, a distinct transition from red to blue will occur. Based on the amount of added trilon, it is easy to calculate the content of metal cations in the solution. Using complexometric analysis, for example, the total hardness of water is determined.

There are titration methods based on the formation of sediment. Thus, using argentometry, it is possible to determine the content of chlorides and bromides in a solution. To do this, the solution is titrated with a solution of AgNO 3. To more clearly establish the equivalence point, add 1-2 drops of K 2 Cr 2 O 4 solution to the analyzed solution. As long as there is an excess of halide ions in the solution, less soluble AgCl or AgBr are formed. After complete precipitation of these ions, a reddish precipitate of Ag 2 CrO 4 will immediately appear. If it is necessary to determine silver ions in a solution, it is titrated with a NaCl solution.

The described techniques are far from exhausting the existing titration methods. Methods in which the titration progress is monitored using instruments have also become widespread. For example, in conductometric analysis (from the English conductivity - electrical conductivity), the electrical conductivity of a solution is measured, which changes during titration. With the potentiometric method, the potential of an electrode immersed in the solution being analyzed is measured. Photometric analysis is based on measuring light absorption as the color intensity of a solution changes. Instruments have been developed that not only determine the equivalence point, but also automatically add the working solution drop by drop to the sample being analyzed and produce a ready-made analysis result.

Titration as an analysis method is distinguished by its simplicity of methodology and equipment, as well as high accuracy: using titration it is not difficult to determine the concentration of a substance in a solution with an accuracy of 0.1%. Therefore, titrimetric methods are widely used in scientific research and in the control of technological processes in production. Thus, when studying the kinetics of a reaction, a decrease in the concentration of the initial reagent over time or an increase in the concentration of the reaction product is determined; In this way, for example, classical work on the mechanism of substitution reactions in organic compounds was carried out. Titrators, devices for automatically performing titrimetric analyses, are widely used in industry. They are very convenient for carrying out mass analyzes of the same type (for example, to determine the composition of an alloy during its smelting process or the content of harmful impurities in it). Such devices are able to work for a long time in the absence of a laboratory assistant, automatically take samples and provide analysis results. This is especially important when it is necessary to work with radioactive, toxic or explosive substances, in dusty rooms, etc.

Ilya Leenson

titrimetric analysis

Titrimetric (volumetric) analysis combines a group of methods for quantitative chemical analysis based on the titration process. It consists in measuring the volume of a reagent solution consumed for equivalent interaction with the substance being determined. Based on the concentration and volume of the reagent solution, the content of the substance being determined is calculated. The titrimetric method of analysis is applicable for determining medium and high contents of substances (over 1%).

Reactions used in titrimetry must satisfy the following basic requirements:

– the reaction must proceed quantitatively, i.e. the equilibrium constant of the reaction must be sufficiently large;

– the reaction must take place quickly;

– the reaction should not be complicated by adverse reactions;

– 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.

Depending on the type of reaction that underlies the determination, the following methods of titrimetric analysis are distinguished: acid-base, redox, precipitation and compleximetric.

According to the method of indicating the end point, there are visual, potentiometric, photometric, conductometric, amperometric titration and etc.

Depending on the method used, titration can be direct, inverse, indirect (by substituent).

Titration can be done from individual portions and pipetting. In the first case, the entire amount of the analyte is titrated. When pipetting, the test solution (or a sample of the substance) is quantitatively transferred into a volumetric flask, adjusted to the mark with water and mixed thoroughly. Next, several samples of the solution (aliquots) are taken from the volumetric flask with a pipette for parallel titrations.

Basic terms used in titrimetric analysis

Titration- the process of gradual, controlled addition of a solution with a precisely known concentration to a certain volume of another solution.

Titrant (titrated, working solution)– the solution that is poured has a precisely known concentration.

Titrated solution- a solution to which the titrant is added.

Titrimetric system– a mixture of substances formed by the interaction of the titrant and the titrated substance.

Equivalence point (i.e.)– the moment of titration when the number of equivalents of the titrant is equal to the number of equivalents of the analyte.

Indicator- a substance or device used to establish a titration end point, which usually differs little from the equivalence point.

Degree of titration ( f) – the ratio of the number of equivalents of the titrant used for titration at any point in the titration to the initial number of equivalents of the substance being determined:

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The burette is graduated in cm3 with divisions of one or two tenths of cm3. According to the SI system, it is recommended to express volumes in dm3 and cm3, however, the old units: liters and milliliters are also acceptable. 1 liter occupies a volume of 1 dm3, 1 milliliter - 1 cm3. Conventional burettes have a capacity of 10, 25 and 50 cm3 (ml), and the volume of solution in them is measured in three digital digits - tens, units and tenths of a milliliter. Hundredths of a milliliter are determined approximately.

Volumetric flasks usually have capacities of 25, 50, 100, 200, 250, 500 and 1000 cm3 (ml). Pipettes are usually made with a volume of 5, 10, 15, 20, 25, 50 cm3 (ml).

When using measuring utensils, you should remember that their capacity often does not exactly correspond to the indicated one. Class 1 dishes with a capacity of more than 10 ml are suitable for working with an accuracy of 0.1%; for class 2 dishes, the permissible deviations are twice as large.

Filling burettes with solution

Fill a clean burette 1/3 with titrant, make sure that the shutter is in good working order and that there is no air bubble in it. To do this, lift the tip of the burette and open the clamp slightly. If the liquid flows in a smooth stream, without air bubbles, the burette is filled correctly. By tilting and turning the burette, wet the walls with the solution, after which almost all of the solution is drained through the spout. Before starting the titration, the burette is placed strictly vertically and filled with titrant to zero. In this case, the level of the liquid meniscus with the concave part should coincide with the zero division of the scale (the zero division should be at eye level) for colorless solutions. For colored solutions, the zero is set at the upper edge of the meniscus.

Measuring solutions with a pipette

Using a rubber bulb, fill a clean pipette with the titrated solution until expansion begins. Covering the upper end with your index finger, turn the pipette several times, trying to wet the entire inner surface with the solution slightly above the mark. Drain the solution.

Now fill the pipette using a rubber bulb slightly above the mark. Remove the bulb, lightly close the hole with your finger, “holding” the pipette mark at eye level, carefully pour off the excess solution so that the concave part of the liquid meniscus coincides with the mark. After this, the pipette hole is clamped and transferred to another vessel. The upper part of the pipette is opened and the liquid is allowed to flow out quietly. After the liquid has drained from the pipette, the last drops are poured off by touching the wall of the vessel into which the liquid is poured. Then the pipette is removed, not paying attention to the liquid that remains in it. Do not blow liquid out of the pipette.

Titration rules

The place where titration is carried out must be well prepared and illuminated. Place a sheet of white paper on the base of the burette stand. The burette is strengthened parallel to the tripod rod.

Titrate in small portions - drop by drop. Open the burette clamp with your left hand, and hold the titration flask with your right, constantly stirring its contents with rotational movements. After the solution has flowed out, the divisions on the burette are counted after 20-30 s to allow the liquid remaining on the walls of the burette to drain.

The reading is taken along the lower (colorless solutions) or upper (colored solutions) edge of the meniscus. The meniscus should be at eye level. To obtain reliable results, repeat the titration at least three times. Each repeated titration begins with a zero reading on the burette.

Titration errors

During titration, random and systematic errors are possible. Random errors are associated with the measurement of the volume and mass of a sample; systematic (indicative) errors appear when the titration end point does not correspond to the equivalence point.

Measurement errorssolutions arise due to inaccuracy in measuring solutions of the substance and titrant. They consist of the volume of one drop (V ~ 0.05 ml), with which the solution is usually titrated, and the calibration errors of meters (burette, pipette, volumetric flask), which allow deviations of ± (0.01 - 0.02) ml. The relative error of titration depends on the volume of titrant or solution being titrated and is equal to:

where v is the sum of the drop volume (~ 0.05 ml) and deviations in volume

burettes (~0.02 ml) and pipettes (~0.02 ml);

V is the volume of the titrated solution or titrant, ml.

Technique for performing titrimetric analysis

Measuring utensils.Graduated cylinders used for approximate, with an accuracy of 1-2 ml, measurement of liquids.

Volumetric flasks used to prepare solutions with precisely known concentrations. Typically, a sample of the substance is transferred quantitatively into a volumetric flask, dissolved and diluted with water to a certain volume (for example, 100 ml), limited by a circular mark (line) on the neck (until the lower edge of the liquid meniscus will not touch the line).

Pipettes used to select and transfer an exact volume of solution from one vessel to another. Before use, wash the pipette, rinse with distilled water and be sure to rinse with the same solution that will be measured. Otherwise, the water remaining in the pipette will dilute the solution measured for analysis and its concentration will change. Rules for working with pipettes: The lower end of the pipette is immersed in the solution and the solution is sucked with a rubber bulb through the upper hole. When the fluid level rises above the line, quickly close the upper hole with the index finger of the right hand and remove the pipette from the solution. Next, the excess solution is carefully released until the lower edge of the meniscus does not coincide with the line applied to the pipette. At the moment when the meniscus touches the line, the finger is pressed tightly to the upper hole of the pipette and the liquid stops flowing out. The filled pipette is transferred to the titration flask. To do this, the flask is held in an inclined position, the pipette is placed with its lower end against the wall of the flask, holding the pipette vertically. Lightly releasing the index finger, allow the solution to drain, wait another 15 seconds and remove the last drop by touching the tip of the pipette to the wall of the flask. Do not blow or shake out the last drops of liquid from the pipette., since when calibrating the pipette, the mark is applied taking into account the fact that when the liquid flows freely, a little of it remains on the walls.

Burettes They are cylindrical graduated vessels with a tap or rubber stopper. Large divisions are applied every milliliter, and small divisions are applied every 0.1 ml. Burettes are used to measure the volume of solution used for titration. Before use, the burette is washed and then rinsed with the solution that will be used for titration. Then, placing the clamp on the rubber part of the burette, fill it with the solution for titration above the “0” division, fill the drawn tube, making sure that there is no air left in it. After this, set the lower meniscus at the “0” division, releasing excess solution from the burette. Readings on the burette are made with an accuracy of 0.05 ml. The reading is made difficult by the fact that the liquid in the burette has a concave meniscus. For this reason When counting, the eye should be kept exactly at the level of the liquid. Otherwise, the count will be done incorrectly. Each titration begins with a “0” division, since this best compensates for errors in burette calibration. Do not release the solution from the burette very quickly (no faster than 3-4 drops per second), otherwise it will not drain from the walls in time and the reading will be incorrect.

Preparation of standard solutions:

1. Create an equation for the reaction between the standard substance and the substance whose concentration should be determined. Using the reaction equation, calculate the molar mass of the equivalent (E) of the standard substance. Next, calculate the mass of the standard substance required to prepare a given volume of a solution of a given concentration, using the formula:

where C is the molar concentration of the equivalent (normality) of the solution; V – required volume of solution in ml.

2. Weigh the empty bottle on a technical chemical scale.

3. Weigh the weighing bottle with a sample on a technical and chemical scale.

4. Weigh the bottle with the sample on an analytical balance.

5. Quantitatively transfer the sample from the beaker into a volumetric flask through a dry funnel without loss. (after transferring the substance, do not remove the funnel from the flask!). Weigh the empty bottle on an analytical balance.

6. Prepare the solution.
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To do this, first wash off the remaining substance from the funnel into the flask, first slightly raise the funnel so that there is a gap between it and the walls of the flask. Add distilled water to the flask by 1/3 - 1/2 of its volume and mix the contents of the flask thoroughly with rotational movements until the sample is completely dissolved. Bring the volume of the solution to the calibration mark (along the lower meniscus), close the flask with a stopper and, holding it with your index finger, mix thoroughly, turning the flask upside down at least 8 times.

Sampling and titration:

1. Prepare the burette for use. To do this, rinse the burette with a small amount of titrant solution, discard the used solution. After this, fill the burette with titrant solution almost to the top; then, placing a glass under it and slightly opening the clamp, fill the “spout” of the burette (the extended tube of the burette) so that there are no air bubbles left in it. Set the titrant level to “0” division along the lower meniscus of the solution.

2. Using a measuring pipette, remove a separate portion of the titrated solution (aliquot) into the titration flask, having first rinsed the pipette with the sampled solution to remove the remaining water from it. Add the reagents and indicator necessary for titration to the flask.

3. Carry out titration. To do this, the flask with the titrated solution is placed on a stand under the burette so that the “spout” of the burette is in the flask. Hold the clamp with your left hand, and hold the flask by its upper part with your right hand, so as not to cover the solution in the flask. Squeezing the clamp and using circular motions stirring constantly the contents of the flask are titrated. In this case, the titrant is released from the burette no faster than 3-4 drops per second, otherwise it will not drain from the walls in time and the reading will be incorrect. Upon reaching the equivalence point (this is externally manifested by a change in the color of the solution), the titration is stopped. Take titration readings on the burette with an accuracy of 0.05 ml and record the titrant volume in the laboratory notebook. Titration is carried out at least three times. In this case, the titration results should be convergent, ᴛ.ᴇ. the discrepancy should not exceed 0.1 ml. When three converging results are obtained, the average value is found and the concentration of the analyzed solution is calculated. If, as a result of three titrations, convergent results are not obtained, carry out the 4th, 5th titration until three converging results.

Calculation of titration results:

Calculation of the average titrant volume carried out according to the formula:

Calculation of the molar concentration of the equivalent (normality) of the titrant from a solution of the standard substance. According to the law of equivalents:

where C st.r-ra is the normality of the standard solution; C t – titrant normality; V st. solution – volume of standard solution equal to the volume of the pipette; V t - volume of titrant ͵ equal to the average value of readings on the burette (V avg).

From formula (31) we express the molar concentration of the titrant equivalent:

Calculation of the mass of the analyte in a certain volume of solution carried out according to the formula:

where C is the normality of the titrant; E – molar mass of the equivalent of the analyte; V av is the average volume of three converging titration results.

Technique for performing titrimetric analysis - concept and types. Classification and features of the category “Technique for performing titrimetric analysis” 2017, 2018.

During the titration process, it is necessary to accurately determine the moment of equivalence, i.e. state of the system in which the number of moles of equivalent of the analyte is equal to the number of moles of equivalent of the reagent in the added volume of titrant. This moment is called equivalence point . When the equivalence point is reached, the titration is usually completed and the volume of titrant consumed is counted. The equivalence point is determined by a sharp change in any property of the solution. The most widely used method is observation of the color of the solution. If one of the reactants is colored, and the reaction products are colorless or have a different color, then a color change occurs at the equivalence point. Very often titrimetric reactions are carried out in the presence indicators - substances that can change their color depending on the acidity of the environment.

Currently, instrumental methods for determining the equivalence point are widely used, in which it is determined by changes in some physical properties of solutions during titration. Electrotitrimetric methods of analysis are based on this principle. For example, in the conductometric method, the electrical conductivity of a solution is measured, and the potentiometric method is based on measuring the redox potential. In such methods, as the titrant is added to the analyzed solution, a continuous record is made of changes in the physical properties of the solution: temperature, electrical conductivity, electrode potential. In the resulting volume-property relationship, called the titration curve, a jump or inflection is detected at the equivalence point.

Only those reactions that satisfy the following conditions can be used as titrimetric ones.

1. During the reaction, changes must occur that can be observed visually or using appropriate instruments, fixing the equivalence point.

2. The reaction must proceed fairly quickly, since with slow reactions it is extremely difficult or even impossible to fix the equivalence point.

3. The titrimetric reaction must be irreversible, otherwise accurate titration becomes impossible.

4. During titration, no side reactions should occur that make it impossible to accurately calculate the results of the analysis.

The main equipment in conventional titrimetric (volumetric) analysis are burettes, volumetric pipettes, conical flasks, volumetric flasks and graduated cylinders.

The first operation in volumetric analysis - preparation of a solution with a known concentration (titer) or a titrated solution. Titrated solutions are called solutions with a prepared titer or standard. There are the following methods for preparing titrated solutions.

1. Titrated solutions are prepared by dissolving an accurately weighed amount of the reagent (sample) in water, followed by dilution with water to a certain volume, obtaining a solution of a precisely known concentration. The cooking technique is as follows. The volumetric flask is thoroughly washed and rinsed with distilled water. There should be no drops of liquid on the inner surface of the flask; the walls of the flask should be moistened with water in an even layer. Insert a clean, dry funnel into the neck of the flask. On an analytical balance (with an accuracy of 0.0001g), the calculated amount of the standard substance is weighed in a clean weighing cup. Standard substances must satisfy a number of requirements : 1) be chemically pure; 2) strictly comply with the chemical formula; 3) be stable both in solid form and in solution. These include many salts: sodium tetraborate (borax), sodium oxalate, sodium chloride, potassium dichromate, as well as oxalic acid and a number of others.

There is no need to ensure that the sample taken exactly corresponds to the calculated one; it is important to know its true value, from which it is easy to determine the concentration of the resulting solution. Carefully, without spraying, pour a sample of the substance through a funnel into a volumetric flask and thoroughly, repeatedly rinse with distilled water over the funnel. The walls of the weighing cup and the funnel are also rinsed. Then add water to the flask (so that the flask is filled to 2/3 of the volume) and completely dissolve the substance, mixing the contents of the flask with a smooth circular motion. When all the substance goes into solution, adjust the volume of the solution to the mark. The last millimeter of water must be added drop by drop, holding the flask so that the mark and the eye are at the same level, focusing on the lower edge of the meniscus of the solution in the flask. After this, the flask is tightly closed with a stopper and the solution is thoroughly mixed, turning and shaking the flask repeatedly (the stopper should be held with your index finger).

2. However, not all substances are standard and meet the above requirements. For example, solutions of commonly used acids: hydrochloric, sulfuric, nitric cannot be prepared from precise samples, since the initial acid solutions contain a variable amount of water. Alkalis always contain an indefinite amount of water and carbonate, therefore, no matter how accurately the alkali is weighed, it is impossible to obtain a solution with a known concentration. The exact concentrations of such solutions are determined by titration with a suitable standard solution. This process is called standardization of the solution , A titrated solutions, the concentration of which is found as a result of titration, are called standardized, solutions with a set titer, and sometimes - working solutions.

3. Standard solutions can also be prepared from commercially available standard titers (“fixanals”). These are glass ampoules containing precisely defined amounts of various solids or dissolved substances. Having opened the ampoule, the substance is transferred without any residue into a volumetric flask, after which it is dissolved and the water in the flask is brought to the mark. Titrated solutions are stored under conditions that prevent changes in concentration due to evaporation, decomposition of the substance, or the ingress of impurities from the environment. Their concentration is periodically checked.

The second operation during titration is preparing the test sample for volumetric analysis. If the sample being analyzed is a solid, then a sample of it is dissolved in a volumetric flask, brought to the mark with water and the solution is thoroughly mixed, repeatedly inverting the stoppered volumetric flask. Using a pipette, aliquots of the titration solution are measured. An aliquot fraction of a solution is a portion of a solution measured with a pipette, the volume of which exactly corresponds to the volume of the pipette. The pipette is filled by plunging its lower end deeply into the solution and sucking the solution with a rubber bulb attached to its upper end. Fill the pipette with the solution so that the liquid level in it is approximately 2 cm above the mark. After this, quickly clamp the upper hole of the pipette with a slightly damp index finger and slightly open the hole so that excess liquid drains out and the lower edge of the meniscus touches the mark (the mark should be at eye level). Without removing your finger, bring the pipette to the titration flask, remove your finger and allow the solution to flow freely into the flask. When all the liquid has flowed out, touch the tip of the pipette to the wall of the vessel and wait 20-30 seconds until the remaining liquid flows out. Blowing out the remaining drop of solution in the pipette is unacceptable!

If the substance being determined is in solution, then the exact volume is diluted in a volumetric flask with distilled water, and then proceed in the same way. This way of working is called pipetting.

Another method of transferring samples into solution is called using the method of individual samples. Take separate, similar-sized portions of the original or analyzed substance, and, having dissolved each of them in an arbitrary volume of water, titrate the resulting solutions in their entirety.

The third operation of volumetric analysis is filling the burette with titrant and performing the titration. Burettes allow you to measure the required volumes of liquid, and are calibrated for pouring. Conventional laboratory macroburettes are graduated cylindrical tubes with a tapered end, which is equipped with a special stopcock, or connected by a rubber tube to an extended glass tube. A small glass ball is inserted into the rubber tube; If you lightly press the rubber band in the place where the ball is placed, the liquid will flow out of the burette. Burettes vary in capacity (usually from 10 to 100 ml). They are calibrated in milliliters and tenths of a milliliter (i.e., each small division of the burette corresponds to 0.1-0.2 ml). For semi-microanalysis, burettes with a capacity of 1–5 ml, with a division value of 0.01 ml, are convenient. The zero division is located at the top of the burette. Before titration, a thoroughly washed burette is filled with titrant. It is necessary to ensure that the entire burette is filled with solution to the very tip; There should be no air bubbles, especially in its narrow part. To remove air bubbles from the burette, some of the liquid is released with a strong stream. After this, the solution is again poured above the zero mark and the initial zero level of the solution is set, which is measured along the lower edge of the meniscus at eye level.

Carrying out titration. The conical flask with the solution prepared for titration is placed under the tip of the burette on a sheet of white paper so that the change in color of the titrated solution is clearly visible. By slightly opening the tap or pressing the ball in the rubber tip, add the titrant in small portions to the titration flask. Simultaneously with the addition of the titrant, the contents of the flask are moved in a smooth circular motion. Near the equivalence point, the solution at the site of contact with a drop of titrant for a short time acquires a color characteristic of the equivalence point. This indicates that the end of the titration is approaching. The titrant is added one drop at a time, constantly stirring the solution. A change in color should occur from the addition of one drop of titrant. As soon as a permanent change in the color of the solution has occurred, the titration is stopped and the burette readings are recorded. The first titration serves to establish an approximate volume, and its result is not included in the control measurements. Then titration is carried out at least three more times. The second and subsequent experiments are accelerated by the one-time addition of titrant in a volume that is only a fraction of a milliliter less than the result of the first titration. The titrant is then added dropwise to the equivalence point. The discrepancy between titrations should be no more than 0.1 ml (or 0.01 ml, depending on the burette division value). Having received the results of several titrations, the arithmetic mean of the three volumes is determined, which is used in further calculations. Obviously erroneous results (misses) are not taken into account when determining the average.

The source of random errors in titrimetric analysis is the inevitable deviations in setting the zero level, inaccuracy in reading the volume on the scale, and the uncertainty of the excess of the reagent after adding the last drop of the titrant. Systematic errors can arise due to incorrect determination of the concentration of standard solutions, changes in the concentration of standard solutions, changes in concentration during storage, inaccuracy of measuring containers, incorrect choice of indicator, subjective characteristics of the perception of the color of indicators, reading volumes, etc. Carrying out several parallel determinations allows us to eliminate mistakes and perform statistical processing of the results.