STUDY OF THE ELECTROCHEMICAL BEHAVIOR OF RHODIUM(III) BY THE PPN METHOD ON A GRAPHITE ELECTRODE

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 determine the possibility of applying the method of film polarography with accumulation for the determination of microgram amounts of rhodium.

As the indifferent electrode we chose a graphite electrode, the preliminary preparation of which has been described previously [1]. The advantages of this electrode for the determination of noble metals in comparison with mercury-film and platinum electrodes have already been discussed [1—3]. All measurements were carried out on a Hungarian OH—101 polarograph. A cell of the design of V. I. Kuleshov [4] was used. Standard solutions were prepared from a commercial preparation of rhodium(III) chloride. The solution in the cell was purged and stirred with nitrogen, which had been previously purified of oxygen.

Most studies of the electrochemical reduction of rhodium(III) have been carried out on a dropping mercury electrode [5—7]. Voltammetric curves for the reduction of rhodium(III) have also been obtained on a platinum electrode, on a rotating platinum microdisk electrode [8], and on a solid silver electrode [9]. A detailed review is given in works [10—11]. As the authors of the reviews note, at present there is no selective, highly sensitive method, simple in its instrumental design, for the determination of rhodium. The minimum concentration determined by the method of oscillographic polarography is \(1 \cdot 10^{-5}\) mole/l [12].

It is known [13] that salts of rhodium(III), present in chloride and sulfate solutions even in trace amounts, cause the formation of a catalytic hydrogen wave, which is observed at more positive potentials than the potential of hydrogen evolution in the absence of rhodium salts. On a dropping mercury electrode, the reduction of rhodium proceeds at more positive potentials than the catalytic evolution of hydrogen. On a graphite electrode these processes coincide; therefore it is not possible to record the process of either discharge or ionization of rhodium. Under these conditions we obtained a cathodic peak for the reduction of rhodium against a background of 1 M KCl solution in the presence of mercury(II) ions at a potential of \(-0.05\) V relative to the saturated calomel electrode. This peak increases both with the concentration of mercury(II) ions and with the concentration of rhodium(III) ions. At the same time the catalytic hydrogen wave shifts into the cathodic region of potentials.

Reactions of chemical oxidation of rhodium are already used in analytical practice [14], but the electrochemical oxidation of rhodium has been studied

insufficient. We obtained an anodic peak for the dissolution of radium in a 1 M KCl solution at a potential of +0.4 V relative to the SCE.

The peaks are well reproduced, have a shape convenient for measurements, and increase linearly with increasing concentration of radium(III) ions in solution over the range \(5 \cdot 10^{-9}\)—\(1 \cdot 10^{-5}\) mol/L (Fig. 1).

Fig. 1. Dependence of the electro-dissolution current of radium on its concentration in solution

Study of the dependence of the height of the anodic peak of radium on the concentration of mercury(II) ions showed that, at a concentration ratio Rh : Hg = 1 : 100 in solution, the height of the anodic peak of radium does not depend on the concentration of mercury(II) ions. Study of the height of the anodic peak of radium as a function of the electrolysis potential showed that, with an increase in the electrolysis potential up to −0.3 V relative to the SCE, the anodic peak of radium increases. A further increase in the electrodeposition potential causes a sharp decrease in the anodic current, and at a potential of −0.4 V relative to the SCE the peak disappears. At this same potential, a large catalytic current of hydrogen is observed. The disappearance of the radium peak at electrolysis potentials more negative than −0.4 V relative to the SCE is apparently connected with the catalytic evolution of hydrogen.

Conclusions

Anodic peaks of the electro-dissolution of radium from the surface of a graphite electrode have been obtained. The magnitude of the anodic peak depends linearly on the concentration of radium in solution in the concentration range \(5 \cdot 10^{-9}\)—\(1 \cdot 10^{-5}\) mol/L, which makes it possible to apply the method of film polarography with accumulation for the determination of trace amounts of radium.

REFERENCES

1. Kh. Z. Brainina, V. B. Belyavskaya. Coll. “Methods for the Analysis of Chemical Reagents and Preparations,” issue 5–6, Moscow, IREA, 1963, p. 129.
2. A. R. Brown, W. Watt. Industrial Carbon and Graphite, London, Society of Chemical Industry, 1958, p. 86.
3. O. A. Songina, V. G. Barikov. Coll. “Chemistry and Chemical Technology,” 2, Alma-Ata, 1964, p. 121.
4. V. I. Kuleshov, A. G. Stromberg. Coll. “Methods for the Analysis of Chemical Reagents and Preparations,” issue 5–6, Moscow, IREA, 1963, p. 37.
5. R. J. Magee, W. H. Douglas. J. Inorg. and Nucl. Chem., 26, 10, 1707—1712, 1964.
6. R. D. Gullard, T. A. Osborn, G. Wilkinson. J. Chem. Soc., July, 4107—4110, 1965.
7. V. I. Kravtsov, I. I. Shereshevskaya. “Elektrokhimiya,” 7, 3, 1971, 407—411.
8. V. I. Shapiro. Abstract of dissertation, Kishinev, 1970.
9. G. Grube, E. Kesting. Z. Electrochem., 39, 948, 1933.
10. F. E. Beamish. Analyt. Chim. acta, 44, 253, 1969.
11. N. A. Ezerskaya. Coll. “Modern Methods for the Analysis of Materials,” Moscow, “Metallurgiya,” 1969.
12. P. Began. Chem. Zvesty, 17, 7, 517, 1963.
13. Ya. Geyrovskii, Ya. Kuta. Fundamentals of Polarography. Moscow, “Mir,” 1965.
14. O. A. Efremenko, V. E. Shlenskaya, I. P. Alimarin. Izv. AN SSSR, ser. khim., No. 8, 1673, 1968.