CONDUCTOMETRY

From Big Medical Encyclopedia

CONDUCTOMETRY (English conductivity conductivity + grech, metreo to measure, measure) — the electrochemical method of the analysis based on measurement of conductivity of any fluid mediums (including biological). Conductivity of fluid mediums is one of characteristics their physical. state and chemical properties. Registration of conductivity can be made in the automatic mode with high precision (errors of measurement do not exceed the 100-th shares of percent). All this allows to use To. at qualitative and quantification of various chemical substances, at gematol, researches, during the studying of a water salt metabolism, acid-base balance in an organism etc. (see. Conductivity of biological systems ).

At the fixed concentration of c temperature conductivity (L) nappe located between two electrodes is directly proportional to the surface area of electrodes (S) and is inversely proportional to distance (d) between them: L = k (S/d). The coefficient of proportionality of k is called specific conductivity: k = L, if S = 1 and d = 1. In the SI system conductivity is expressed in the return ohms (mo) or siemenses (Cm), 1 Ohm is taken for unit of specific conductivity - 1 - m - 1 . Often use concepts molar (μ), equivalent (λ) and ionic (λ-λ+) conductivity which represent conductivity of the solutions containing respectively 1 g-mol, 1 g-ekv of substance and 1 g ion of ions: λ = kVN where VN — the volume of solution, in Krom contains 1 g-ekv of electrolyte; VN = 1/C, where CN — normal concentration.

For the first time (1879) measurement of conductivity of a number of solutions was carried out by F. Kohlrausch who suggested to use for this purpose Uitston's bridge with food from alternating current and an electrolytic cell with plane-parallel electrodes. However broad use of konduktometrichesky methods began only in 20 century in connection with the general progress in area of electronics, automatic equipment, electrochemistry etc. The modern industry releases many types of konduktometrichesky devices of different function. These devices depending on the principle underlain in a basis of operation of their sensors can be combined in two groups: devices with contact and contactless konduktometrichesky converters.

Konduktometrichesky contact converters (R-cells, contact sensors etc.), differing on design features, have one general basic element — the electrodes immersed in the studied liquid through which in operating duty pass electric current. At the same time the processes proceeding on border an electrode — liquid are registered. The electric generator of low frequency is a source of working tension (to 1000 Hz). The advantage of these methods is the possibility of measurement with high precision and a possibility of receiving direct counting of the measured size at the corresponding calibration of the device. A shortcoming — existence of the polarizing phenomena which are especially shown in the concentrated solutions.

Fig. 1. The curves characterizing dependence of conductivity of a number of substances on concentration of solutions: each curve has the form, characteristic of this substance (electrolyte), depending on its electrochemical properties (ordinate axis — specific conductivity, in mo/m; abscissa axis — solute concentration, in %).
Fig. 2. Curve of titration of 0,1 N of solution of the strong basis strong acid: the site of a curve to the left of a point of equivalence (the place of a break of a curve corresponding to the moment of neutralization) reflects dynamics of binding of ions of OH-of the basis ions of H+ of acid; sharp increase of conductivity after neutralization of the basis is caused by emergence of excess quantity of H+ (ordinate axis — specific conductivity, in mo/m; abscissa axis — amount of the added acid, in ekv/l).

Konduktometrichesky contactless converters register not processes on limits of the section an electrode — liquid, and electrochemical properties of all system in general concluded between electrodes. Currents low (to 20 kHz), high (to 100 MHz) and ultrahigh (>100 MHz) frequencies are applied. Therefore high-frequency methods are applied as indirect, e.g., to conductometric titration. The advantage of such methods is the possibility of measurement of conductivity of the concentrated, aggressive and volatile liquids, and also lack of interaction of electrodes with the studied substance. At high-frequency titration use two types of cells (capacity and inductive). In capacity cells the studied solution is connected with a chain of comparison of the device electric field. Capacity cells are applied to a research of electrolytes with concentration to 0,4 M. At higher concentration apply inductive cells in which link is magnetic field. Distinguish methods of a straight line To. and conductometric titration. At a straight line To. concentration of any substances in solution is determined by the size of conductivity. Direct calculations of concentration of electrolyte carry out by means of the standard curve constructed for each of the defined substances (fig. 1). At conductometric titration (see. Titrimetric analysis ) the amount of substance (titrant) added to the studied liquid is determined by a break of a curve of change of conductivity in the course of titration (fig. 2). The bias at conductometric titration does not exceed usually 0,02%. A kind of conductometric titration is hronokonduktometrichesky titration, at Krom quantitative definition of substances is made on time spent under certain conditions for titration.

By means of various methods K. determine the content of salts in mineral waters, control refining processes and qualities of water, define harmful impurity in milk, wines, fruit juice etc. By means of conductometric titration make the analysis of alkaloids, barbiturates and other medicinal substances in chemical - pharm, the industries, watch the course of enzymatic reactions and define on the basis of it concentration of various connections and elementary ions which are substrates or cofactors of enzymes. By means of a straight line To. conduct researches biol, liquids for definition of disturbances water salt metabolism (see) in an organism, control the course of processes of dialysis, salting-out of proteins, adsorptions of various ions on firm precipitators (sorbents) etc. To. it is the basis for carrying out special tests (e.g., C=N-gidrolazny the test for pregnancy, the test of «an index of destruction» at differential diagnosis of some diseases, the heparin test for rheumatism etc.). By methods K. it is possible to study the mechanism of a blood coagulation, hemagglutination and other processes proceeding in fabrics, vessels and various bodies.



Bibliography: Andreyev V. S. Kondukto-metrichesky methods and devices in biology and medicine, M., 1973, bibliogr.; At-to about in Yu. P. and To at l and to about in M. V. High-frequency electrodeless conductometry, M., 1968; Lopatin B. A. Conductometry, Novosibirsk, 1964; Khudyakova T. A. and To r e sh to about in A. P. Theory and practice of the conductometric and hronokonduktometrichesky analysis, M., 1976, bibliogr.; Purdy W. Page of Electro-analytical methods in biochemistry, N. Y., 1965.


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