polarography


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polarography

(pō'lərŏg`rəfē), in chemistry, method for analyzing the composition of a dilute electrolytic solutionsolution,
in chemistry, homogeneous mixture of two or more substances. The dissolving medium is called the solvent, and the dissolved material is called the solute. A solution is distinct from a colloid or a suspension.
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 (see electrolyteelectrolyte
, electrical conductor in which current is carried by ions rather than by free electrons (as in a metal). Electrolytes include water solutions of acids, bases, or salts; certain pure liquids; and molten salts.
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). Two electrodes are placed in the solution: One has a fixed potential (voltage) and is called the reference electrode, and the other has a variable potential and is called the polarizable electrode. As voltage is applied to the polarizable electrode, the resulting change in the current through the solution is monitored. By plotting the pairs of values for voltage and current, a series of current-voltage curves (polarograms) can be generated. The general name for this method is voltametry; the term polarography was formerly restricted to those cases where the polarizable electrode is a dropping mercury electrode, though now this distinction is often disregarded. Current-voltage curves, which look like a series of steps called polarographic waves, can be used to determine the reduction potentials of any reducible species present in the solution, e.g., inorganic ions or complex organic intermediates (see oxidation and reductionoxidation and reduction,
complementary chemical reactions characterized by the loss or gain, respectively, of one or more electrons by an atom or molecule. Originally the term oxidation
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; electromotive serieselectromotive series,
list of metals whose order indicates the relative tendency to be oxidized, or to give up electrons (see oxidation and reduction); the list also includes the gas hydrogen. The electromotive series begins with the metal most easily oxidized, i.e.
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). Conversely, unknown substances can be identified by their characteristic reduction potentials. Quantitative titrationstitration
, gradual addition of an acidic solution to a basic solution or vice versa (see acids and bases); titrations are used to determine the concentration of acids or bases in solution.
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 of an oxidizing agent by a reducing agent can be performed using a polarographic cell to determine the equivalence point by monitoring changes in the current.

Polarography

 

an electrochemical method for qualitative analysis, quantitative analysis, and the study of kinetics in chemical processes. Polarography was proposed by J. Hey-rovsky and later developed by A. N. Frumkin and other scientists. It is based on the interpretation of current-voltage curves, called polarograms, which are produced during electrolysis of the solutions under study and express the dependence of current intensity on the DC potential Edir applied to the electrolytic cell. To produce polarograms, which are recorded by means of polarographs, the solution is placed in a cell with a reference microelectrode and an indicating electrode; a dropping mercury electrode (DME) with a renewable surface is most often used as the reference microelectrode. The electrode reaction occurring at the reference microelectrode produces neither significant chemical changes in the solution nor any marked difference in potentials because the reference microelectrode is always considerably smaller than the indicating electrode.

Polarography involves processes of oxidation-reduction, adsorption, and catalysis. If the electrode potential Edir is smoothly varied negatively or positively, then at a specific value of Edir (point a in Figure 1) sufficient for the onset of reduction or oxidation, the ions of the substance under study (the depolarizer) begin to discharge on the microelectrode, and their concentration near the reference microelectrode drops. A difference in concentration is observed in the area surrounding the electrode, which results in diffusion of ions toward the surface of the reference microelectrode. An electrolytic current Ie (the diffusion current Id in Figure 1) appears in the circuit. Upon further change in Edir, the current Ie increases and, with time, reaches (at point c) a limiting value, called the limiting current, proportional to the initial concentration of the depolarizer. The potential corresponding to the mean value of the limiting current (point b) is called the half-wave potential E’1/2 and characterizes the nature of the depolarizer (the E1/2 of various substances is usually given in special tables). If several depolarizers are present in the solution, then the polarogram consists of several waves (a polarographic spectrum), each of which describes qualitatively (according to E1/2, E1/2, …) and quantitatively (according to Ie, represented as Id and Id in Figure 1) the corresponding substance, whose concentration is determined by special formulas. The current I, also depends on the rate of the electrode process, which is used to differentiate reversible (rapid) processes from partially reversible and irreversible (slow) processes.

Figure 1. Direct-current, or classical, polarogram (absolute values of E are given)

To exclude the component of the current generated by ion transfer because of the forces of the electric field arising between the reference microelectrode and the indicating electrode (the current is not proportional to the concentration of the depolarizer), a more than 50-fold excess of a supporting electrolyte, whose ions are polarographically passive within the range of polarization voltages, is added to the solution being studied. The application of a voltage to the electrode-solution boundary induces the formation of a double layer, which in turn causes the appearance of a primary interference, the capacitive current Ic

Various types of polarography are evaluated according to their sensitivity (the minimum concentration that can be determined) and resolution (the permissible ratio of concentrations of the supporting component to the component being determined) and depend on the shape and rate of change of the polarizing voltage.

In direct-current (classical) polarography, which is based on the dependence of Ie on the slowly varying polarizing Edir, Ie is proportional to the number of electrons n that take part in the reaction. The sensitivity in determining reversibly reactive substances is 10-5 mole per liter (M), and resolution is about 10. In alternating-current polarography, based on the dependence on Edir of the alternating current Ialt that arises upon superimposition of various forms of a voltage Ealt ((low-amplitude rectangular, trapezoidal, and sinusoidal), Ialt is proportional ton2. The high sensitivity of alternating-current polarography (10-7M) results from the possibility of separating the effective signal Ialt from Ic, and its high resolution (up to several thousand) results from the bell shape of the polarogram (the ordinate rapidly tends toward zero upon a deviation of Edir from peak potential) and by the possibility of determining reversibly reactive substances in the presence of components with irreversible reactivity (sensitivity in determining the latter is low).

High-frequency polarography involves the superimposition of Edir and a high-frequency E modulated by a low-frequency E. In this case Imf, the component of the current for the modulated frequency, depends on Edir and is proportional to n3. The difference in variation between Imf and Ic upon application of a high frequency is used to separate the effective signal Imf from Ic. High-frequency polarography makes it possible to determine the rate constant of fast reactions.

Pulse polarography is based on the measurement of the current Ip, which arises upon application of an 0.04-sec voltage pulse at the moment when the surface of the mercury drop is maximal. The current Ip is separated from Ic by measuring Ip at the moment of damping of Ic. Pulse polarography has a sensitivity of 1–5 × 10-8M and resolution ~5 × 103.

Oscillographic polarography is based on measurement of the dependence of I, on the rapidly varying Edir (0.1–100 volts per sec). The polarograms produced in oscillographic polarography, which are recorded by means of a cathode-ray tube, have a distinct maximum. In this type of polarography Ie is proportional to n2/3, sensitivity is 10-6M, and resolution is – 400.

In addition to the DME, stationary mercury and solid electrodes are also used in polarography.

A distinction is made between direct and inversion polarography, depending on the nature of the current being measured. In inversion polarography, the accumulation method is used to increase sensitivity (up to 10-9M) and resolution (5 × 105 and higher). In this case electrodes with a constant surface are used: at limiting current potentials or upon formation of an insoluble compound, the substance being analyzed accumulates on the electrode surface (the pre-electrolysis stage), and the accumulated solid compound is subsequently dissolved upon a change in Edir. Electrodes made of mercury, graphite, and noble metals are used.

Polarography is very widely used in monitoring the production of especially pure substances; in metallurgy, geology, and pharmacology; in the preparation of organic compounds and polymers; in medicine (for early diagnosis of diseases and for determining the presence of oxygen and trace elements in tissue and in products of vital activity); and in studying the mechanism of electrode reactions.

REFERENCES

Heyrovský, J., and J. Kuta. Osnovy poliarografii, Moscow, 1965. (Translated from Czech.)
Kriukova, T. A., S. I. Siniakova, and T. V. Arefeva. Poliarograficheskii analiz. Moscow, 1959.
Tsfasman, S. B. Elektronnye poliarografy. Moscow, 1960.
Pats, R. G., and L. N. Vasil’eva. Metody analiza s ispolzovaniem poliarografii peremennogo toka. Moscow, 1967.
Bruk, B. S. Poliarograficheskie metody, 2nd ed. Moscow, 1972.

R. G. PATS

polarography

[‚pō·lə′räg·rə·fē]
(analytical chemistry)
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