The character of the calculated dependences of the magnitude of the anodic peak of the electronegative element on the concentration of the electropositive metal in solution, and the deviation from the calibration curve, was confirmed in the systems \(Cd-Au\); \(Cu-Au\); \(Ag-Au\); \(Ni-Cr\) for commensurate amounts of the components of the electrolytic alloy on a graphite electrode.

In the systems \(Cu-Au\); \(Ag-Au\); \(Ni-Cr\), in the dependence of the current of the electronegative element on the concentration of the electropositive one, at a constant concentration of the former in solution, an increase in the peak of the electronegative element is observed in the region of low concentrations of the electropositive element, much smaller than that of the electronegative one. With a further increase in the concentration of the electropositive element, that is, with commensurate amounts of the components in solution, there is an exponential decrease in the anodic peak of the more electronegative metal, coinciding in character with the calculated data. The character of the deviation of the calibration curve of the electronegative element at a constant concentration of the electropositive one in all the indicated systems coincides with the theoretical one.

References

1. E. M. Roizenblat, Kh. Z. Brainina. Elektrokhimiya, 5, 396, 1969.
2. Kh. Z. Brainina. “Inversion Voltammetry of Solid Phases,” Moscow, 1972.
3. L. F. Trushina, A. A. Kashin. ZhAKh, 25, 1616, 1970.
4. I. Prigogine, R. Defay. “Chemical Thermodynamics,” Novosibirsk, Publishing House “Nauka,” 1966.
5. K. Wagner. “Thermodynamics of Alloys,” GONTI, Moscow, 1957.
6. A. G. Stromberg. Izvestiya SO AN SSSR, 5, 76, 1962.
7. T. F. Ryashentseva, A. A. Kashin. Izv. TPI, present volume, p. 13.

INDIRECT DETERMINATION OF CHROMIUM (III) BY STRIPPING POLAROGRAPHY WITH ACCUMULATION ON A GRAPHITE ELECTRODE

T. F. Ryashentseva, A. A. Kashin

The possibility of determining chromium by polarography with accumulation after concentrating it on the electrode in the form of metal has so far not been experimentally confirmed. Apparently, this is connected with the low solubility of chromium in mercury, as well as with the rapid passivation of metallic chromium in aqueous solutions, which complicates the process of its anodic oxidation.

Several methods are known for the indirect determination of chromium using polarography with accumulation /1–3/. The basis

these methods is based on the concentration of chromium on the electrode in the form of its sparingly soluble compounds during a chemical reaction on the electrode surface.

For the purposes of the indirect determination of a number of elements, including chromium, we propose to use the formation—occurring in the course of preliminary electrolysis—of intermetallic compounds or solid solutions in the bulk of the electrode or on its surface between the element being determined, which is not oxidized in the potential region convenient for recording, and an element that is anodically oxidized in the region under study. The mutual influence of the elements, which may be taken as the basis for their indirect determination, consists in a change in the conditions of electrodeposition during alloy formation and in a change in the activity of the components forming a compound or solid solution on the electrode. This is manifested in a change in the potential, height, and width of the anodic peak of one of the alloy components, the current of whose anodic dissolution is recorded on the polarogram. In addition to the indirect determination of elements, the study of the mutual influence of two metals forming an alloy on a solid electrode opens up the possibility of applying accumulation polarography to the solution of both practical and theoretical problems of alloy formation.

In the present work, data are presented on the use of the proposed principle for the determination of microquantities of chromium. The advantage of the proposed method over those indicated above is that it eliminates the need to convert $\mathrm{Cr}(\mathrm{III})$ into $\mathrm{Cr}(\mathrm{VI})$ and provides greater thermodynamic stability of the compound formed with respect to the electrolyte solution.

In order to elucidate the possibility of electroconcentrating chromium in the form of the metal and to select the optimum conditions, studies were carried out on the reduction of $\mathrm{Cr}(\mathrm{III})$ at a dropping mercury electrode. On backgrounds of $\mathrm{KCl}$, $\mathrm{K}_2\mathrm{SO}_4$, $\mathrm{NH}_4\mathrm{F}$, and $\mathrm{KCNS}$, the process $\mathrm{Cr}(\mathrm{III}) \to \mathrm{Cr}(0)$ corresponds to two polarographic waves. From the dependences of the limiting-current magnitude on the chromium concentration and on the height of the mercury column, one may assume the complex character of the limiting current of both waves. From the slope of the polarographic waves, the irreversibility of the process follows. The transfer coefficients were calculated and, using the linearity of the dependence of the limiting-current height on the concentration of $\mathrm{Cr}(\mathrm{III})$, the constants of diffusion-

current for both waves on different supporting electrolytes. The data obtained show that the process \(Cr(III) \rightarrow Cr(0)\) proceeds with the greatest rate and the smallest overvoltage in the presence of \(1\,M\) \(KCNS\), which was used as the supporting electrolyte in our work. The potential for electrodeposition of chromium from a \(1\,M\) \(KCNS\) solution was chosen from the region of the \(Cr(III) \rightarrow Cr(0)\) transition in this medium, \((-1.6 \div -1.8)\) V (vs. SCE).

According to the literature, the most thoroughly studied alloys are those of chromium with nickel, iron, and antimony /4/. The choice of nickel as the indicator element was due to the availability of sufficient data on the polarographic behavior of the latter on a graphite electrode /5/.

The anodic current–voltage curves of nickel were recorded by the usual method on an LP-60 polarograph. Cells with insert cups, a graphite electrode impregnated with epoxy resin, and an SCE as the reference electrode were used in the work. Oxygen was removed from the solution with nitrogen. Potassium thiocyanate prepared from fixanal ampoules was used to prepare the supporting electrolyte. A standard chromium(III) solution was prepared from the salt \(Cr_2(SO_4)_3 \cdot 6H_2O\).

As the concentration of chromium(III) in the solution increases, the height of the nickel anodic peak decreases. This is explained by the formation of a compound between nickel and chromium during their simultaneous discharge on the graphite electrode. Since no new peak is observed in this case, it may be assumed that the product of interaction formed is not oxidized in the potential region under study. The change in the height of the nickel anodic peak with the concentration of chromium(III) is used as the basis for the quantitative determination of the latter (Table I). The decrease in the nickel anodic peak with the concentration of \(Cr(III)\) in solution, in various coordinates, is nonlinear in character. The interval of determinable chromium concentrations is determined by the concentration of nickel in the solution and, consequently, by its amount on the electrode. At a chromium(III) concentration close to the nickel concentration, the anodic peak of the latter disappears, i.e., complete binding of nickel into an alloy occurs. This nickel concentration is the lower limit of the given analyzed range of chromium concentrations. Decreasing the nickel concentration shifts the interval for quantitative determination of chromium into the region of lower concentrations and, conversely, increasing the nickel concentration makes it possible to carry out

determination of a higher chromium(III) content in solution. The absence of precise data on the amount of chromium and nickel on the electrode does not yet make it possible to decide unambiguously the question of the composition of the compound being formed. The proposed method was applied to the estimation of the chromium(III) content in chromium-doped crystals of triglycine sulfate.

Table I

Dependence of the height of the anodic nickel peak
(\(I_a, Ni\)) on the concentration of \(Cr(III)\);

Literature

1. E. M. Roizenblat, Kh. Z. Brainina. ZhAKh, 19, 6, 681, 1964.

2. T. A. Krashenkina, Kh. Z. Brainina. “Zavodskaya laboratoriya,” 33, 4, 400, 1967.

3. T. F. Ryabentseva, A. A. Kaplin, N. P. Sorokina. Technical progress and achievements of science in the chemical industry (abstracts of reports). Barnaul, 1973, p. 114.

4. I. R. Panasyuk. Chromium and Its Alloys, 1959.

5. Kh. Z. Brainina. Inversion Voltammetry of Solid Phases. Metallurgiya, Moscow, 1972.