STUDY OF THE POLAROGRAPHIC BEHAVIOR OF TRIVALENT CHROMIUM ON A SERIES OF SUPPORTING ELECTROLYTES

T. F. Ryashentseva, A. A. Kaplin

(Presented by the scientific-methodological seminar of the Department of Physical Chemistry)

One of the principal stages of the method of amalgam polarography with accumulation (APA) is the stage of concentrating the metal in the amalgam. Therefore, the choice of conditions for preliminary electrolysis, in particular the electrolysis potential, plays a significant role in the process of quantitative concentration of the metal.

The accumulation potential is taken to be the potential of the limiting current for the reduction of metal ions present in solution to zero valence. Therefore, in order to determine the possibility of determining chromium by the APA method, it was necessary first to study the conditions of cathodic reduction of \(\mathrm{Cr}^{+3}\) to zero valence on a mercury electrode.

The literature contains some data on the electrochemical reduction of trivalent chromium on a dropping mercury electrode. The reduction of \(\mathrm{Cr}^{+3}\) proceeds stepwise. The first step corresponds to the discharge of \(\mathrm{Cr}^{+3}\) to \(\mathrm{Cr}^{+2}\), and the second to the reduction of \(\mathrm{Cr}^{+2}\) to the metal [1, 2]. A number of authors have studied the reduction of \(\mathrm{Cr}^{+3}\) on \(\mathrm{K_2SO_4}\) and \(\mathrm{KCl}\) supporting electrolytes [3–7]. On these supporting electrolytes, \(\mathrm{Cr}^{+3}\) is reduced in the form of complex ions, and the concentration of the supporting electrolyte affects the half-wave potential. The polarization curves for \(\mathrm{Cr}^{+3}\) on a KCNS supporting electrolyte have an analogous character [8]. In order to determine the range of potentials for the transition of \(\mathrm{Cr}^{+3}\) to zero valence, we carried out studies of the cathodic reduction of \(\mathrm{Cr}^{+3}\) on the above-mentioned supporting electrolytes, as well as on an \(\mathrm{NH_4F}\) supporting electrolyte, which is one of the most widely used supporting electrolytes in the APA method. The polarization curves were recorded using a Czech LP-60 polarograph. The work used a U-shaped cell, which is an attachment to this instrument. The dropping mercury electrode used as the indicator electrode had the following characteristics: mass of mercury flowing out in 1 sec on a 0.1 M KCNS supporting electrolyte — 3.07 mg/sec; on a 0.5 M \(\mathrm{K_2SO_4}\) supporting electrolyte — 3.63 mg/sec; on a 0.1 M \(\mathrm{NH_4F}\) supporting electrolyte — 3.68 mg/sec; the dropping period at a mercury-column height of 57 cm in the indicated supporting electrolytes varied from 1.2 to 2.5 sec, depending on the applied voltage (the working potential range). A large surface of mercury at the bottom of the cell (“bottom mercury”) served as the reference electrode. Contact of the “bottom mercury” with the conducting leads of the instrument was achieved by means of a platinum wire. Before recording the polarogram, the solution under analysis was purged for 5–10 min

nitrogen to remove traces of dissolved oxygen. The glassware used in the work was washed by boiling in dilute nitric acid. The mercury used for the dropping mercury electrode and for the reference electrode was purified by the method described in [9]. The work used reagents of “chemically pure” and “especially pure” grades and water distilled in a Pyrex apparatus with \(KMnO_4\) after preliminary purification on a Polish redistiller Re-5.

In this work the reduction of \(Cr(III)\) was studied on backgrounds of \(0.5\,M\) \(K_2SO_4\), \(1\,M\) KCl (freshly prepared), KCNS (of different concentrations), and \(0.1\,M\) \(NH_4F\). On all the indicated backgrounds two polarographic waves are observed, which agrees well with the literature data. The second wave of the discharge of \(Cr^{+3}\) is in some cases complicated by a maximum, and on a \(1\,M\) KCl background it considerably exceeds the height of the first wave. The half-wave potential for the second wave at high concentrations of \(Cr^{+3}\) merges with the hydrogen discharge wave, which makes it difficult to calculate the polarographic characteristics of chromium. The values of the half-wave potentials for the reduction of \(Cr^{+3}\) on the backgrounds we studied are given in Table 1.

Table 1

Values of the half-wave potentials and calculated diffusion-current constants of the Ilković equation and transfer coefficients \(\alpha\) for the reduction process of Cr(III) at a dropping mercury electrode

In studying the effect of the concentration of the background in the case of reduction of \(Cr^{+3}\) on a KCNS background, it was established that with increasing concentration of \(CNS^{-}\) ions the first reduction wave of \(Cr^{+3}\) becomes more sloping, which indicates a change in the reversibility of the process. At the same time, a shift of the discharge potential \(Cr^{+2}\to Cr^0\) into the negative region is observed, and at a KCNS concentration greater than \(0.1\,M\) the second wave merges with the hydrogen discharge wave. This may be explained by a change in the composition of the rhodanide complex of chromium discharging at the electrode. One of the criteria of reversibility of the process is the slope of the polarographic curve in the coordinates \(\lg \dfrac{I}{I_g-I},\ \varphi\).

The study of this dependence for the reduction of \(Cr^{+3}\) on the indicated backgrounds made it possible to conclude that the process is irreversible for both the first and the second wave. Using the expression

for the potential of the irreversible cathodic wave and from analysis of the dependences

$\lg \dfrac{I}{I_g-I}, \varphi$

the transfer coefficients $\alpha$ were calculated for the reduction process of $\mathrm{Cr}^{+3}$ in these supporting electrolytes. The calculated values of the transfer coefficients are given in Table 1.

The diffusion character of the limiting current of the first and second waves was checked from the dependence of the limiting current on the height of the mercury column, which was linear. Therefore, for the processes studied, the Ilković equation for the diffusion current is applicable:

\[ I_d = 605 nD^{1/2} m^{2/3} \tau^{1/6} C, \]

where $I_d$ is the diffusion current, $\mu\mathrm{A}$;
$D$ is the diffusion coefficient of the metal ions in solution, $\mathrm{cm}^2/\mathrm{sec}$;
$m$ is the mass of mercury flowing out in 1 sec, $\mathrm{mg}/\mathrm{sec}$;
$\tau$ is the drop time, sec;
$C$ is the concentration of ions in solution, $\mathrm{mmol}/\mathrm{l}$.

We calculated the diffusion-current constants, $K_g = 605\, nD^{1/2}$, for the process of reduction of chromium (III) in a series of supporting electrolytes. The data are presented in Table 1. From comparison of the constants given, it is seen that in most cases the value of the diffusion constant for the first wave is approximately two times smaller than for the 2nd wave. This confirms the assumption that the first wave corresponds to the transition $\mathrm{Cr}^{+3} \to \mathrm{Cr}^{+2}$; at a more negative potential, a process involving two electrons takes place.

The investigations carried out showed that the reduction of $\mathrm{Cr}^{+3}$ to zero valence occurs at negative potentials close to the hydrogen discharge potential. It follows from this that the stage of preliminary concentration of chromium in the amalgam must be carried out at potentials equal to $-1.65$ V; $-1.95$ V and $-2.0$ V (relative to the mercury pool) for the supporting electrolytes 0.1 M KCNS, 0.1 M NH$_4$F, and 0.5 M K$_2$SO$_4$, respectively.

Conclusions

1. The polarographic behavior of trivalent chromium has been studied in supporting electrolytes of 0.1 M KCNS, 0.5 M K$_2$SO$_4$, 0.1 M NH$_4$F, and 1 M KCl.

2. The diffusion-current constants and transfer coefficients $\alpha$ were calculated for the process studied in a series of supporting electrolytes.

3. The region of limiting-current potentials for the transition of trivalent chromium ions to the metal at a mercury electrode was determined.

LITERATURE

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8. D. P. Malyuga. Transactions on Analytical Chemistry, vol. IV (VII), Publishing House of the Academy of Sciences of the USSR, 1952, p. 96.

9. T. A. Kryukova, S. I. Sinyakova, T. V. Aref’eva. Polarographic Analysis. Moscow, Goskhimizdat, 1959, pp. 183–184.