Volumetric analysis. Titration. General chemistry.

The analysis of a gas, liquid or solid sample or mixture to determine the precise percentage composition of the sample in terms of elements, radicals, or compounds.

Quantitative analysis the aggregate of chemical, physicochemical methods of determining the quantitative ratios of constituents in the substance being analyzed. Together with qualitative analysis quantitative analysis is one of the major branches of analytical chemistry.

Physicochemical analysis Quantitative analysis can be divided into three methods: Quantitative analysis. Chemical analysis Physical analysis Physicochemical analysis

The classical chemical methods of quantitative analysis include gravimetric analysis and volumetric analysis. The gravimetric analysis is based on the accurate measurement of the weight of the substances. Volumetric analysis(titrimetric volumetric analysis), comprising methods of measuring the volume of the reagent solution expended in the reaction with the substance under analysis.

The physical and physicochemical (instrumental) methods are based on the measurement of optical, electrical, adsorption, catalytic, and other characteristics of the substances that are depend on their amount (concentration).

These methods are usually divided into following groups: electrochemical methods(conductometry, polarography, potentiometry);

spectral, or optical methods(emission and adsorption spectral analysis, photometric analysis, colorimetric analysis);

chromatographic methods;

X-ray phase analysis; radiometric methods and mass spectrometric methods.

Classification of Titrimetric Analysis methods Titrimetric methods are classified into four groups based on the type of reaction involved. These groups are Acid – base titration, in which an acidic or basic titrants reacts with an analyte that is a base or an acid. 𝐻 + + 𝑂𝐻 − → 𝐻 2 O

Complexometric titration involving a metal-ligand complexation reaction;

Redox titrations, where the titrant is an oxidizing or reducting agent;

Precipitation titrations, in which the analyte and titrant react to form a precipitate.

Table 1 Method Technique Titrant Analyte Acid – base titration (neutralization) Alalimetry Acidimetry 𝑁𝑎𝑂𝐻, 𝐾𝑂𝐻, 𝐵𝑎 (𝑂𝐻) 2 0.1÷0.01N 𝐻𝐶𝑙, 𝐻 2 𝑆 𝑂 4 0.1÷0.01N Acids, bases, salts. Redoximetry Permanganatometry Iodometry 𝐾𝑀𝑛 𝑂 4 , 𝐹𝑒𝑆 𝑂 4 , 𝐼 2 , 𝑁𝑎 2 𝑆 2 𝑂 3 Oxidizing and reducting agents. Precipitation titration Argentometry Mercurometry 𝐴𝑔𝑁 𝑂 3 , 𝐻𝑔 2 (𝑁 𝑂 3 ) 2 𝐶𝑙 − , 𝐵𝑟 − Complexometry Complexonometry Metal ions

Requirements to chemical reaction used in titrimetric analysis Reaction must be stoichiometric Titrant must react rapidly with the analyte so that the time required between additions of reagent is minimized. Reaction between reagent and analyte must be specific.

Titrant must react more or less completely with the analyte so that satisfactory and points are realized. Undergo a selective reaction with the analyte that can be described by simple balanced equation. Equilibrium constant must have high value.

Types of titration. Direct titration – titrant add to an analyte solution and react with determining substance;

Back titration – is a process in which the excess of a standard solution used to react with an analyte is determining by titration with a second standard solution. Back titration are required when the reagent is slow or when the standard solution lacks stability. For example: 𝐶𝑎𝐶 𝑂 3 +𝐻𝐶𝑙→𝐶𝑎𝐶 𝑙 2 + 𝐻 2 𝑂+𝐶 𝑂 2 (titrant 1) 𝐻𝐶𝑙+𝑁𝑎𝑂𝐻→𝑁𝑎𝐶𝑙+ 𝐻 2 𝑂 (titrant 2)

Substitute titration – is a process in which a standard solution used to react with an additional(substitute) substance, amount of which is equivalent an analyte amount. Substitute titrations are required when the analytes are unstable substance or when is impossible to indicate the end-point in direct titration. For example 6𝐾𝐼+ 𝐾 2 𝐶𝑟 2 𝑂 7 +7 𝐻 2 𝑆 𝑂 4 → 𝐶𝑟 2 (𝑆 𝑂 4 ) 3 +3 𝐼 2 +4 𝐾 2 𝑆 𝑂 4 +7 𝐻 2 𝑂 (reagent) (analyte) (substitute) 𝐼 2 +2 𝑁𝑎 2 𝑆 2 𝑂 3 →2𝑁𝑎𝐼+ 𝑁𝑎 2 𝑆 4 𝑂 6 (titrant)

Acid-base titrations: When the strength of an acid is determined with the help of a standard solution of base, it is known as acidimetry. Similarly, when the strength of a base (alkali) is determined with the help of a standard solution of an acid, it is known as alkalimetry. Both these titrations involve neutralization of an acid with an alkali. In these titrations H+ ions of the acid combine with OH- ions of the alkali to form ionized molecules of water.

𝐻𝐴+𝐵𝑂𝐻↔𝐵𝐴+ 𝐻 2 𝑂 acid base salt or 𝐻 + + 𝐴 − + 𝐵 + + 𝑂𝐻 − ↔ 𝐵 + + 𝐴 − + 𝐻 2 𝑂 𝐻 + + 𝑂𝐻 − ↔ 𝐻 2 𝑂

The end point in these titrations is determined by the use of organic dyes which are either weak acids or weak bases. These change their colors within a limited range of hydrogen ion concentrations, i.e., pH of the solution. Phenolphthalein is a suitable indicator in the titrations of strong alkalies (free from carbonate) against strong acids or weak acids. Methyl orange is used as an indicator in the titrations of strong acids against strong and weak alkalies.

Calculations in acid-base titration Equivalence law At the end-point 𝑛 𝑒𝑞 (𝑎𝑛𝑎𝑙𝑦𝑡𝑒)= 𝑛 𝑒𝑞 (𝑡𝑖𝑡𝑟𝑎𝑛𝑡) Since 𝐶 𝑁 = 𝑛 𝑒𝑞 𝑉 𝑛 𝑒𝑞 = 𝐶 𝑁 ∙𝑉 𝐶 𝑁 (𝑎𝑛𝑎𝑙𝑦𝑡𝑒)∙𝑉(𝑎𝑛𝑎𝑙𝑦𝑡𝑒)= 𝐶 𝑁 (𝑡𝑖𝑡𝑟𝑎𝑛𝑡)∙𝑉(𝑡𝑖𝑡𝑟𝑎𝑛𝑡) 𝐶 𝑁 (𝑎𝑛𝑎𝑙𝑦𝑡𝑒)= 𝐶 𝑁 (𝑡𝑖𝑡𝑟𝑎𝑛𝑡)∙𝑉(𝑡𝑖𝑡𝑟𝑎𝑛𝑡) 𝑉(𝑎𝑛𝑎𝑙𝑦𝑡𝑒) 𝑚(𝑎𝑛𝑎𝑙𝑦𝑡𝑒)= 𝐶 𝑁 (𝑎𝑛𝑎𝑙𝑦𝑡𝑒)∙ 𝑀 𝑒𝑞 (𝑎𝑛𝑎𝑙𝑦𝑡𝑒)∙𝑉(𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛)

Practical problem 1 Calculate the molar mass of an equivalent for following substances: 𝐻𝐶𝑙 𝐶𝑎(𝑂𝐻 ) 2 𝐻 3 𝑃 𝑂 4 𝑁𝑎𝐻𝐶 𝑂 3 𝐻 2 𝑆 𝑂 4 𝐵𝑎(𝑂𝐻 ) 2 𝐶 𝐻 3 𝐶𝑂𝑂𝐻 𝐻𝑁 𝑂 2 𝐻 2 𝐶 𝑂 3 𝑁𝑎 2 𝐶 𝑂 3

Practical problem 2 For titration 5 ml of 𝐻 2 𝑆 𝑂 4 are spent 10 ml 0.1N 𝑁𝑎𝑂𝐻. Calculate mass of 𝐻 2 𝑆 𝑂 4 in 150 ml of solution.

Practical problem 3 For titration 15 ml of 𝐻 3 𝑃 𝑂 4 are spent 20 ml 0.01N 𝑁𝑎𝑂𝐻. Calculate mass of 𝐻 3 𝑃 𝑂 4 in 50 ml of solution.

Practical problem 4 For titration 5 ml of 𝐶𝑎(𝑂𝐻 ) 2 are spent 10 ml 0.05N 𝐻𝐶𝑙. Calculate mass of 𝐶𝑎(𝑂𝐻 ) 2 in 250 ml of solution.