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

The potential of the anodic peak for the electrodissolution of the deposit from the surface of the graphite electrode is \((-0.14)\) V (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 anodic current maximum on the pre-electrolysis potential passes through a maximum at a potential of \((-1.3 \div -1.4)\) V (S.C.E.).

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

It is still difficult to give an unequivocal answer concerning the nature of the deposit formed on the electrode. However, both the data obtained by us and the literature data on the electrodissolution of molybdenum make it possible to draw the preliminary conclusion that molybdenum is concentrated on the electrode in the form of an oxide and, possibly, a mixture of oxides.

References

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 the present time, removal of oxygen from polarographed

solutions is carried out by bubbling nitrogen, argon, or hydrogen through the solution.

Technical-grade nitrogen reduces the oxygen concentration in the solution by a factor of 10–20, while argon and hydrogen reduce it by a factor of 50–100.

A further increase in the sensitivity of the stripping and film polarography analysis method is impossible without more thorough purification of the solution from oxygen. Calculations show that the oxygen content can be reduced by a factor of \(10^3\) when a relatively shallow vacuum of \((1 \div 0.5)\) mm Hg is used. Creating such a rarefaction is a fairly simple task, simpler than supplying a laboratory with argon or nitrogen and additionally purifying them. Therefore, we fabricated and tested several types of vacuum cells in which the solution was stirred with a magnetic stirrer and the air was evacuated with a Kamovskii pump. The residual pressure was monitored with either a mercury or a gauge manometer. Air was evacuated from the cell with the magnetic stirrer switched on to a pressure of \((1 \div 2)\) mm Hg over \((1 \div 2)\) min; after this, the accumulation stage and the recording of anodic peaks were carried out according to the usual procedure.

For a qualitative assessment of the vacuum removal of oxygen, experiments were simultaneously carried out on purifying the solution from oxygen with gaseous nitrogen and argon according to the usual procedure. Since the presence of oxygen lowers the magnitude of the anodic peak and increases the magnitude of the residual current, the purification effect was evaluated from these quantities. In the experiments we used standard solutions of zinc, cadmium, and lead at concentrations of \(1 \cdot 10^{-8} \div 3 \cdot 10^{-8}\) mol/l. As was to be expected, the anodic peaks with vacuum removal of oxygen were 20–30% larger than in the parallel experiment with nitrogen and purification of the solution with argon, and the residual current was 20–30% lower. Therefore, we recommend using vacuum removal of oxygen in polarographic analysis. In this case the vacuum cell can be made from a Drechsel bottle. The bottle is cut into two parts; the upper part is ground onto a piece of ordinary window glass, which then serves as the bottom of the cell. The electrodes are introduced through the central tube and the side branch. The solution under study is placed in an insert cup with a magnetic stirrer. Between the bottom of the cell and

vacuum grease is applied to the upper part. Thus, a comparatively simple methodological improvement that reduces the magnitude of interference can serve as a sufficiently effective means of increasing the sensitivity of the method of polarography with accumulation.

STUDIES ON THE DETERMINATION OF TUNGSTEN BY THE METHOD OF FILM POLAROGRAPHY WITH ACCUMULATION

L. F. Zaichko, A. A. Kalin, G. S. Shamanskaya

Film polarography with accumulation (FPA) expands the possibilities of electrochemical determination of impurities by including elements that do not form amalgams, are not reduced to the metallic state, and are more noble than mercury. Among the elements to which the method of electrochemical concentration in the form of a metal of zero valence has a number of fundamental limitations is tungsten [1]. This is due to the fact that, in practically all solutions, according to the literature data, tungsten is reduced on mercury electrodes to intermediate valence states. Certain possibilities for concentrating tungsten in the form of \(WO\) are presented by the conditions of co-deposition with other elements. Data on the polarographic behavior of \(W^{+6}\) on solid electrodes are practically absent.

We have studied the possibility of applying the FPA method to the determination of microconcentrations of \(W^{+6}\) after its preliminary concentration on a solid electrode. The experiment was carried out on an OH-102 polarograph; the rate of potential change was 500 mV/min. An electrolyzer with inserted small beakers was used; the indicator electrode was a graphite electrode, and the reference electrode was a saturated calomel electrode.

Standard tungsten solutions were prepared from the chemically pure reagent \(Na_2WO_4 \cdot 2H_2O\). Stirring of the solution and removal of oxygen were carried out with nitrogen.

In this work, anodic tungsten peaks were obtained for the first time by the FPA method against the background of 0.1 N \(NH_4Cl\) (\(pH = 7\)). The peaks are reproduced satisfactorily (relative determination error \(\pm(10 \div 15)\%\), and have a convenient for