TITRIMETRIC ANALYSIS

From Big Medical Encyclopedia

TITRIMETRIC ANALYSIS (synonym measure analysis) — the method of quantification based on measurement of volume or mass of the reagent which is required for reaction with the studied substance. T. and. is the section of analytical chemistry (see) and it is widely applied in biochemical, clinical, sanitary and hygienic and other laboratories in pilot studies and for a wedge, analyses, napr, at establishment of an acid-base state (see Acid-base equilibrium), definition of gastric acidity (see), acidities and alkalinities of urine (see) etc. The titrimetric analysis serves one of the main methods of the chemical analysis in kontrolnoanalitichesky pharmaceutical laboratories.

Amount of the studied substance at T. and. define by titration: to precisely measured volume of solution of the studied substance gradually flow solution of other substance of precisely known concentration until its quantity does not become equivalent to amount of the studied substance. The condition of equivalence is called a point of equivalence of titration. The solution of a reactant of the known concentration applied to titration is called the titrated solution (standard solution, a titrant); exact concentration of the titrated solution can be expressed by a caption (g/ml), a normality (ekv/l), etc.

To the reactions used at T. and., a number of requirements is imposed: substances shall react in strictly certain quantitative (stoichiometric) relations (see. Stekhiometriya ) without side reactions, reactions shall proceed quickly and practically up to the end, it is necessary to apply rather reliable ways to establishment of a point of equivalence, influence of foreign substances on the course of reaction shall be excluded, it is desirable that reactions proceeded at the room temperature.

Methods T. and. depending on the reactions which are their cornerstone are subdivided into the following basic groups.

1. Methods of neutralization, or methods of acid-base titration (see Neutralizations a method). Among methods of neutralization usually distinguish an atsidometriya (quantitative definition of the bases by means of standartized solutions to - t), an alkalimetriya (definition to - t by means of the titrated solutions of the bases), a galometriya (quantitative definition of salts by means of the bases or to - t if they react with salts in stoichiometric relationships, napr, salts of ammonium react with the bases, carbonates — with to-tami, etc.). Methods of neutralization are based on neutralization tests, i.e. on interaction of acids and the bases (see). The constitutive equation of process of neutralization in water solutions is the interaction of hydrogen ions with ions of a hydroxyl which is followed by formation of the low-dissociated water molecules: N + + 0H - = N 2 O.

2. The methods of sedimentation (see Sedimentation methods) based on titration of the substances forming insoluble compounds in a certain environment [salts of barium, silver, lead, zinc, cadmium, mercury (II), copper (III), etc.]. Carry to these methods, e.g., an argentometriya (titration by solution of silver nitrate), a merkurometriya (titration by solution of nitrate of protoxidic mercury), etc. Methods of sedimentation give the chance to quantitatively define cations of barium (see), mercury (see), silver (see), lead (see), zinc (see), etc., and also the chlorides besieged by them, bromides, iodides, cyanides, rhodanates, sulfates, chromates, phosphates, ferricyanides, etc. Using special methods of titration, it is possible to define by these methods not only separate cations and anions, as well as their mixes.

3. Methods of a complex formation, or a complexometry (a merkurimetriya, a ftoro-metriya, etc.), are based on use of reactions, at to-rykh complex connections, e.g. Ag are formed - + 2CN - <-> [Ag(CN) 2 ] - . Methods of a complex formation are closely connected with methods of sedimentation since many precipitation reactions are followed by a complex formation, and formation of complexes — sedimentation of slightly soluble connections. It is possible to define by methods of a complex formation quantitatively different cations (Ag +, Hg2+, Al3 +, etc.) and the anions (CN-, F-, Cl-, etc.) entering the corresponding reactions. Holds a specific place among methods of a complex formation kompleksonometriya (see), based on use of organic reactants — complexons.

4. Methods of redox, or oxidimetry (see), include a permanganatometriya, a hromatometriya (bikhromatometriya), an iodometry, a bromatometriya, a tserimetriya, a vanadometriya, etc.

The point of equivalence in the titrimetric analysis is determined by decolourization of the titrable solution or the indicator entered at the beginning or in the course of titration on change of conductivity of solution, potential of the electrode shipped in titrable solution, to change of size of current (see. Potentiometric titration ), optical density, etc.

One of widely applied ways of fixing of a point of equivalence is the indicator method. Indicators (see) — substances, to-rye give the chance to establish a final point of titration (the moment of jump of coloring of titrable solution). Most often the indicator is added to titrable solution (the internal indicator). During the work with external indicators periodically take a drop of titrable solution and mix with a drop of solution of the indicator or place on indicator paper that leads to losses of the analyzed substance.

Process of titration is represented graphically in the form of curves of titration. In a method of neutralization on ordinate axis postpone pH values from 0 to 14, on abscissa axis — amount of the added titrating solution. In methods of sedimentation and a complex formation on ordinate axis postpone values of the ion concentrations forming a deposit or complex connection in oxidation-reduction methods — oxidation potential. Curve titration allows to present visually all course of titration and to choose the indicator, the most suitable for obtaining exact results since the curve of titration can be compared with an interval of decolourization of the indicator.

Distinguish three main receptions of titration: direct, the return (titration on the rest) and indirect.

At direct titration the studied solution is directly titrated standard solution (or on the contrary). Having measured the volume of standard solution and knowing oho concentration and volume of the studied solution, it is possible to calculate easily concentration) the last:

N x = (N1*V1)/V x

where N1 and Nx — normal concentrations of the standard and studied solutions, a V1 and Vx — volumes of these solutions.

At back titration when definition of a point of equivalence is complicated, to precisely measured volume of the studied substance flow precisely measured volume of standard solution taken much. Surplus is ottitrovyvat other standard solution. E.g., for definition of concentration of hydrochloric acid (see) it is possible to add to it precisely measured amount of standard solution of AgNO silver nitrate 3 , taken much and then to ottitrovat not reacted surplus of AgNO3 standard solution of ammonium rhodanide, to-ry reacts with silver with formation of slightly soluble AgSCN. In the presence of ions of iron (III) the excess drop of titrating solution causes emergence of rose-red coloring. By results of titration it is possible to calculate the excess of AgNO3 solution which did not react with salt to - that, then to calculate the quantity of AgNO3 which reacted with it (a difference between added and unspent quantities), and thus to define concentration of the studied substance, as well as at direct titration.

Indirect titration (titration of the deputy) is that to solution of the studied substance add any aid reacting with the studied substance with formation of equivalent amount of new substance, a cut ottitrovyvat standard solution. E.g., during the definition of oxidizers titrate solution of Na2S2O3 sodium thiosulphate the iodine emitted with an oxidizer at interaction with KI potassium iodide. Concentration of the studied solution is calculated as well as at direct titration.

Mistakes in T. and. can be the methodical, connected with features of a method of titration, and specific, caused features of this reaction. Methodical systematic errors of titration depend on errors of measuring devices, calibration of measured ware, pipettes, burettes, incomplete plaint of liquids on piles of measured ware. To avoid random errors, it is necessary to choose correctly volumes of the reacting reactants, measured ware, to carefully carry out titration, to meet strictly identical conditions.

Specific mistakes depend on an equilibrium constant of reaction (see. Kinetics chemical ) and from the accuracy of detection of a point of equivalence. Theoretically in a point of equivalence there should not be neither titrant, nor the analyzed substance since reaction of their interaction proceeds quantitatively. However often the reactions applied in T. and., are reversible and in a point of equivalence practically do not reach the end. It is one of the reasons that the point of equivalence and a final point of titration not always match. The indicator error of titration is connected with the fact that discoloration of the indicator occurs not absolutely precisely in a point of equivalence, and sooner or later (a deviation rk the indicator from pH in a point of equivalence), it leads to a nedotitrovyvaniye or a peretitrovyvaniye of solution. When the point of equivalence completely or almost completely matches a final point of titration, calculation of amount of the studied substance is carried out under the law of equivalence (see the Example of direct titration). When these points do not match, enter a correction factor, to-ry calculate on the basis of the data obtained at titration in simulated condition of solutions with the known content of the defined substance.

See also Amperometrichesky titration , Conductometry .


Bibliography: Kreshkov A. P. Fundamentals of analytical chemistry, t. 2, page 38, M., 1970; Seleznyov K. A. Analytical chemistry, Qualitative polumikroana-liz and quantification, page 164, M., 1973; Shemyakin F. M., Karpov A. N. and Brusnetsov A. N. Analytical chemistry, page 325, M., 1973; Eshvort M. R. F. Titrimetric methods of the analysis of organic compounds, the lane with English, M., 1968, bibliogr.


L. M. Pimenova.

Яндекс.Метрика