References

1. I. P. Zherebtsov, V. P. Lopatinskii, S. P. Yudina, Z. A. Michurina. Proceedings of TPI, vol. 257, 1973.

2. V. P. Lopatinskii, I. P. Zherebtsov, S. P. Yudina, L. P. Belyanina. Proceedings of TPI, same collection.

3. R. K. Eikhman, V. O. Lukashevich, E. A. Silaeva. POKh, 6, 93, 1939.

4. G. G. Coker, S. G. Plant, P. B. Turner. J. Chem. Soc., 110, 1951.

5. H. M. Grotto, C. J. Riggle, A. B. Beorse. J. Org. Chem., 29, No. 8, 2474, 1964.

6. A. Weissberger et al. Organic Solvents. Izd. IL, Moscow, 1958.

STUDIES ON THE DETERMINATION OF MOLYBDENUM BY STRIPPING POLAROGRAPHY

A. A. Kashin, V. F. Sliptchenko, N. N. Zubkova

It is known that, by the method of inverse chronopotentiometry, it is possible to determine \(10^{-3}\)–\(5 \cdot 10^{-6}\) M molybdenum on a mercury electrode [1–3]. It has been shown that the introduction of certain amounts of \(H_2O_2\) (30%) increases the sensitivity of molybdenum determination [3]. Anodic peaks of Mo(VI) have been obtained on a graphite electrode by the method of stripping voltammetry [4]; however, in that work a very complex supporting electrolyte was chosen for the determination of molybdenum.

We have studied the possibility of determining microquantities of molybdenum by the method of stripping polarography against backgrounds of simpler composition, likewise using a graphite electrode.

The experiment was carried out on an OH-102 polarograph, with a potential scan rate of 500 mV/min. In the work an electrolyzer with replaceable cups was used. Standard molybdenum solutions were prepared from the reagent \((NH_4)_2MoO_4\).

Clearly expressed anodic peaks of Mo(VI) were obtained against the background of

1.5 ml of 0.5 M \((NH_4)_2SO_4\) and 0.5 ml of 0.01 M Trilon B. The polarographic conditions were as follows: supporting-electrolyte volume, 2 ml; electrolysis time, 5 min; electrolysis potential, \((-1.4)\) V (vs. S.C.E.).

The potential of the anodic peak for electro-dissolution of the deposit from the surface of the graphite electrode is \((-0.14)\) V (vs. S.C.E.). The anodic peaks of molybdenum increase linearly with increasing concentration of molybdenum(VI) in the range \((2 \cdot 10^{-8} \div 1 \cdot 10^{-5})\) g/ml and can be used for the quantitative determination of molybdenum in solution. The dependence of the maximum electrooxidation current on the pre-electrolysis time is also linear.

The dependence of the heights of the anodic peak of Mo(VI) on the pH of the solution was studied. The peak current reaches its maximum value at pH \((4 \div 5)\). The dependence of the maximum anodic current on the pre-electrolysis potential passes through a maximum at a potential of \((-1.3 \div -1.4)\) V (vs. S.C.E.).

Using the background \(((NH_4)_2SO_4 +\) Trilon B), we obtained, for the first time, anodic peaks of Mo on a mercury-film electrode. The character of the dependences of the anodic current of molybdenum on a number of factors is analogous for mercury and graphite electrodes.

It is still difficult to give an unambiguous answer concerning the nature of the deposit formed on the electrode. However, both our results and the literature data on the electro-dissolution of molybdenum permit the preliminary conclusion that molybdenum is concentrated on the electrode in the form of an oxide and, possibly, a mixture of oxides.

Literature

1. Lagrange P., Schwing I. P. C. r. Acad. Sci. 263, 848 (1966).
2. Lagrange P., Schwing I. P. Bull. Soc. chem. France, 536 (1968).
3. Lagrange P., Schwing I. P. Analyt. Chem., 43, 1944 (1970); ibid., 37, 1939, 1972.
4. Pnev V. V., Popov G. N. and Nagarev V. G. ZhAKh, XXVIII, 2050, 1973.

VACUUM CELL FOR POLAROGRAPHIC ANALYSIS

V. I. Kuleshov, B. S. Shifris

At present, the removal of oxygen from polarographed...