A. A. KAPLIN, B. F. NAZAROV, M. S. ZAKHAROV, V. S. ZHIKINA

STUDIES ON IMPROVING THE RELIABILITY OF A STATIONARY ELECTRODE WITH A PLATINUM CONTACT IN THE METHOD OF AMALGAM ACCUMULATION POLAROGRAPHY

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

One of the most promising methods for determining \(10^{-5}\%\) and smaller amounts of impurities in semiconductor materials is the method of amalgam polarography with accumulation on a stationary mercury drop [1].

In practice, electrodes of two types are used: with an extruded Hg drop and with a drop deposited on a Pt, Ag, or Au contact. Electrodes with a Pt contact are the most widely used.

In developing a procedure for determining \(10^{-5}\)—\(10^{-6}\%\) copper in high-purity indium [2], with polarography on a pure supporting electrolyte, the electrodes with a Pt contact used by us proved to be unsuitable to a considerable degree.

When polarograms were recorded in the region of the anodic peak of copper (\(\varphi_a = +0.05\ \text{V}\) (sat. cal. el.)) against a \(1\ \mathrm{M}\ \mathrm{H_3PO_4}\) background, anomalous serrations were observed, hindering the determination of copper (Fig. 2, curve 1б).

In addition, on such electrodes a considerable residual current is observed, caused by incomplete amalgamation of the platinum surface (Fig. 2, curves 1а, 2а), which lowers the sensitivity of the determinations.

The present work is devoted to a study aimed at developing a method for treating the platinum surface and conditions for its satisfactory amalgamation in order to eliminate the indicated shortcomings.

The studies showed that there is a relationship between the appearance of considerable residual currents and anomalous serrations on the polarogram, on the one hand, and the quality of the platinum contact and the homogeneity of its surface, on the other (Fig. 1).

It was shown that the ability of the contact surface to undergo amalgamation depends on the conditions of analysis.

During polarography in acidic supporting electrolytes and in the positive region of potentials, the ability of the contact surface to undergo uniform amalgamation is lost more rapidly than in alkaline and neutral supporting electrolytes.

The possibility of eliminating inhomogeneities of the platinum surface before amalgamation was studied by treating the latter in \(\mathrm{HNO_3}\), hot \(\mathrm{HNO_3}\), a mixture of \(\mathrm{HNO_3} + \mathrm{HCl}\), in \(\mathrm{KOH}\), over a broad range of positive and negative potentials (\(+0.5\ \text{V}—2.0\ \text{V}\), relative to the sat. cal. el.).

It has been shown that in most cases such treatment does not improve the quality of the surface of the platinum contact (the notches do not disappear, and amalgamation is nonuniform).

We have established that satisfactory results can be obtained by heating the contact for 5–10 minutes in the flame

Fig. 1. Relationship between the properties of the Pt contact and the properties of the Hg drop deposited on this contact.
a) 1 and 2—polarogram without accumulation on high-quality platinum; 3 and 4—polarogram without accumulation on an Hg drop deposited on the same contact. Sensitivity \(8 \cdot 10^{-3}\) mA/mm, comp. 60.
b) 1—polarogram without accumulation on a damaged Pt contact; 2—polarogram without accumulation on an Hg drop deposited on this contact. Sensitivity \(8 \cdot 10^{-3}\) mA/mm, maximum compensation.

of a gas burner. In Fig. 2 (curves \(1a, 2a, 1b, 2b, 1v, 2v, 1g, 2g\)) the polarogram without accumulation is presented for polarography in the potential region \((-0.3\ \text{V} - (-0.2)\ \text{V},\ \text{vs. S.C.E.})\) for the electrode before treatment, and in Fig. 2 (curves \(3a, 4a, 3b, 4b, 3v, 4v, 3g, 4g\)) after treatment of it in the flame of a gas burner. Comparison of the polarograms shows that heating the electrode considerably improves the properties of the latter. On such electrodes anomalous notches are not obtained, and they amalgamate well.

In the work, a comparison was made of the properties of electrodes before and after heating in the flame of generator gas + air, gas + \(O_2\), and a gasoline burner.

It was established that the best results are obtained when heating in a gas–oxygen flame.

It may be assumed that during heating, particles adsorbed in the course of the analysis are removed, which makes the surface of the contact homogeneous. This circumstance leads to the fact that, on the polarogram in the positive region of potentials, anomalous notches interfering with the analysis are not obtained.

During the growth of the mercury drop, very often several drops form on the platinum surface instead of a single Hg drop. When such an electrode is used, distorted analytical results are obtained.

The appearance of several mercury drops is also associated with nonuniformity of the surface of the platinum contact. A series of solutions for amalgamating the surface of the platinum contact was investigated.

The most satisfactory composition is Hg(NO$_3$)$_2$—0.05%, 0.5 M NH$_4$OH and 0.05 M Trilon B [3].

Fig. 2. Comparison of electrode quality before and after ignition. Curves 1a, 2a, 1b, 2b, 1c, 2c, 1g, 2g are polarograms without accumulation on a series of electrodes before treatment of the platinum surface. Curves 3a, 4a, 3b, 4b, 3c, 4c, 3g, 4g are polarograms without accumulation on the same electrodes after ignition in the flame of a gas burner.

We selected the optimal conditions for amalgamating the contact and developed a procedure for amalgamating platinum and growing a mercury drop on the platinum contact.

The most satisfactory amalgamation is achieved in a solution of the above-mentioned composition, at a continuously varying current strength (from 1 to 10 mA) for 10–15 sec.

In Fig. 3, a, b, c photographs are shown of mercury drops on platinum contacts that had deteriorated during the analysis; in Fig. 3, g is shown one of the contacts amalgamated by the proposed procedure.

Procedure for preparing the electrode

After ignition of the electrode, a mercury drop is grown on the platinum contact in a saturated solution of Hg$_2$(NO$_3$)$_2$ for 30 sec at a current of 25 mA. Sometimes the mercury does not completely cover the surface of the contact, and the drop grows with an irregular shape (Fig. 3). In this case the drop is thoroughly washed and transferred to an electrolyzer containing the solution for amalgamation, and for 10–15 sec, at a continuously varying current strength (from 1 to 10 mA), additional amalgamation is carried out.

After two- or threefold amalgamation, the drop is shaken off, the electrode is washed and transferred to a solution of Hg(NO$_3$)$_2$, where an Hg drop is grown in the usual way ($i = 25$ mA, $\tau = 30$ sec).

Fig. 3, a, b. Mercury droplet formed on a defective contact.

Fig. 3, c. Mercury droplet formed on a defective contact.

Fig. 3, d. One of the preliminarily amalgamated platinum contacts.

For subsequent analyses, the drop is shaken off the electrode and a new drop is grown without additional amalgamation.

Thus, annealing the electrode and additional amalgamation significantly improve the properties of the platinum contact.

Use of the electrode obtained by the method described makes it possible to substantially increase the reproducibility of analyses and the sensitivity of amalgam-polarographic methods.

In conclusion, we express our gratitude to Prof. A. G. Stromberg for valuable advice in carrying out and discussing the work.

REFERENCES

1. A. G. Stromberg, E. A. Stromberg. Determination of ultramicroconcentrations of a substance in solution by the method of amalgam polarography on a stationary dropping electrode under continuously varying potential (review). Zavodskaya laboratoriya, 27, 1, 1961.

2. A. G. Stromberg, M. S. Zakharov, N. A. Mesyats, A. A. Kaplin, R. S. Tyutyunnikova. Determination of microimpurities of Pb, Zn, Bi and Cu in indium by the method of amalgam polarography with accumulation on a stationary Hg drop. Collection of Works of IREA (in press).

3. Rolf Neeb. Anodische Amalgamvoltammetri. Z. anal. Chem. 180, 161, 3, 1961.