STUDY OF THE ELECTROCHEMICAL BEHAVIOR OF IRIDIUM (+4) BY STRIPPING POLAROGRAPHY

N. A. KOLPAKOVA, M. S. CHEMERIS, A. G. STROMBERG

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

The aim of our work was to investigate the possibility of determining iridium (+4) at concentrations of \(10^{-3}\)—\(10^{-7}\) mol/l. Similar studies have not been described in the literature.

A graphite rod was chosen as the indifferent electrode; its treatment has been described previously [1]. The apparatus and experimental procedure were analogous to those described in [2]. Standard solutions were prepared from the salt \(K_2IrCl_6\), synthesized according to the procedure in [3].

For our studies, as a result of examining various supporting electrolytes, we selected a hydrochloric acid solution.

In hydrochloric acid solutions containing \(10^{-3}\) mol/l or more of iridium (+4), we obtained an anodic peak of electrooxidation of iridium after its preliminary electrodeposition.

However, the magnitude of the anodic peak obtained under the specified conditions increases with increasing concentration of iridium (+4) ions in the solution nonproportionally and is poorly reproducible. We investigated the factors affecting the character of the voltammetric curves and established optimal conditions that make it possible to obtain clear and reproducible polarograms. It was found that the shape of the anodic iridium peak, as well as its reproducibility, is substantially improved when mercury (+2) ions are added to the solution being analyzed. The magnitude of the dissolution current reaches a maximum at a twofold excess of mercury ions. In this case mercury exerts a catalytic effect at the stage of iridium reduction. To prove this assumption, electrodeposition of iridium (+4) was carried out in the absence of mercury ions. The electrode was then transferred into a solution containing mercury ions. Additional electrodeposition of mercury from the solution was carried out, and then the anodic polarogram of electrooxidation of the deposit was recorded. With this arrangement of the experiment, it was not possible to detect iridium at a concentration of \(10^{-5}\)—\(10^{-3}\) mol/l. When the experiment is carried out in the reverse order, well-reproducible iridium peaks are obtained at its concentration in solution of \(5 \cdot 10^{-7}\)—\(10^{-3}\) mol/l. The height of the anodic peak depends on the concentration of hydrochloric acid. The maximum value of the anodic iridium peak is observed in 1 M hydrochloric acid.

The peaks obtained against a background of 1 M HCl with the addition of mercury ions have a shape convenient for measurements and increase linearly with increasing concentration (Fig. 1).

The maximum current of anodic electrooxidation of iridium is observed at potentials of \(+0.75\)—\(+0.85\) V. Study of the dependence of the current height

electrodissolution of iridium on the electrolysis potential showed that at a potential of about \(-0.6\) V the anodic peak approaches its maximum value (Fig. 2).

Fig. 1. Dependence of the electrodissolution current (b) of iridium (+4) on its concentration in solution

Thus it has been established that the stripping polarography method can be used for the analytical determination of iridium (+4) ions. The optimal conditions

Fig. 2. Dependence of the electrodissolution current of iridium (+4) on the accumulation potential

for the determination are: background—1 M HCl, \(\varphi_{\mathrm{el}} = -0.6\) V, and a two-hundredfold excess of divalent mercury ions.

REFERENCES

1. Kh. Z. Brainina, V. B. Belyavskaya. In the collection Methods of Analysis of Chemical Reagents and Preparations, issue 5–6, Moscow, IREA, 1963, p. 129.

2. A. I. Kartushinskaya, A. G. Stromberg, N. A. Kolpakova, Electrochemistry, 7, 9, 1971.

3. I. I. Chernyaev et al. Synthesis of Complexes of Platinum-Group Metals. Handbook, Moscow, “Nauka,” 1964, p. 251.