V. A. IGOLINSKII, V. Z. BASHKATOV, L. F. ZAICHKO

A HIGH-RESOLUTION POLAROGRAPH FOR AMALGAM POLAROGRAPHY AND ITS APPLICATION

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

In connection with the development of new branches of technology, the role of ultrapure materials has recently increased extraordinarily [1]. Closely connected with the task of obtaining and studying high-purity materials is the task of developing sufficiently sensitive, accurate, and rapid methods for their analysis.

Instrumental methods of analysis are widely used for the analysis of high-purity materials. One of the most promising methods of instrumental analysis is amalgam polarography with accumulation [2], which has much higher sensitivity than classical polarography.

The essence of the method of amalgam polarography with accumulation is that, by electrolysis over a certain time, the metal being determined is concentrated on a stationary mercury electrode in the form of an amalgam.

Then, at a linearly varying potential, the curve of anodic dissolution is recorded; the current peak obtained in this process characterizes, by its position, the nature of the metal, and by its height, the concentration.

Along with an enormous advantage over classical polarography—significantly (by 3–4 orders of magnitude) greater sensitivity—amalgam polarography also has certain drawbacks. One of the principal drawbacks of amalgam polarography is its rather low resolving power, which hinders the simultaneous determination of a large number of elements.

The resolving power of polarography may be defined as the reciprocal of the half-peak width in volts.

At present, the highest resolving power is possessed by alternating-current polarography, which is considerably inferior in sensitivity to amalgam polarography; in it, the polarograms are rather narrow peaks.

Thus, along with the task of increasing sensitivity, an equally important task arises—the task of increasing the resolving power of the method of amalgam polarography. For this purpose, we have for the first time constructed a high-resolution polarograph for amalgam polarography.

The design is based on the idea of automatically stopping the change of potential at the cell upon reaching the maximum (or mini-

minimum) of the current on the volt-ampere curve (or on its derivative). The design of this polarograph differs from the usual ones by the presence of a special radio-engineering unit—an “attachment.”

Fig. 1 shows the circuit of the attachment developed for the oscillographic polarograph OP-1 (or OP-2).

Fig. 1. Circuit of the attachment to the oscillographic polarograph OP-1 or OP-2.

The attachment automatically, at the moment when the maximum (or minimum) current is reached on the volt-ampere curve (or on its derivative), switches on for a specified time (this time is determined by the time constant of the circuit \(R_5C_1\)) a voltage that charges the capacitor of the linearly varying-voltage generator.

As a result, the change in voltage at the cell ceases, and the electrode potential remains constant for the specified time.

Thus, after the peak has been reached, dissolution of the metal from the amalgam occurs at constant potential. After some time the current falls practically to zero owing to the almost complete dissolution of the metal from the very small volume of the mercury electrode\(^1\) (this constitutes an essential feature of amalgam polarography). By stopping the change in potential, it is possible to eliminate the descending, diffusion branch of the polarogram and to preserve the region of potentials that it occupied, where the peak of one more element can be accommodated.

This considerably increases the resolving power of the method of amalgam polarography and makes it equal to (or even greater than) the resolving power of alternating-current polarography, while using considerably simpler apparatus. The high resolving power makes it possible to determine simultaneously a large number of impurities. To test the attachment and establish its resolving power, we chose the elements Cd and Tl, which, against a background of \(0.1\,N\) \((\mathrm{NH}_4)_2\mathrm{SO}_4\), give peaks at potentials equal respectively to \(-0.54\) V and \(-0.47\) V relative to the saturated calomel electrode.

When Cd and Tl are present together, their peaks practically completely merge with one another (Figs. 2, 1 and 3) (the difference in potentials

\(^1\) In the case of cathodic peaks the current falls because of propagation of the diffusion front into the depth of the solution. Therefore the attachment can also be used when recording cathodic polarograms.

peaks of 0.07 V, and it is impossible to measure their heights separately (and, consequently, impossible to determine their concentrations). When the same peaks are recorded with the attachment, they are completely separated, forming entirely separate peaks on the polarogram (Fig. 2, 2), for which the height can easily be measured and the concentration determined. Thus, the attachment makes it possible to completely separate peaks for which the potential difference is only 70 millivolts.

In Table 1 are given the half-peak widths of thallium and cadmium obtained against a background of 0.1 N \((\mathrm{NH}_4)_2\mathrm{SO}_4\), without the attachment and with the attachment, and the corresponding resolving power.

For comparison, the third graph gives data on the half-peak width and resolving power for alternating-current polarography. The half-peak width for alternating-current polarography was calculated from the formula [3]:

\[ a=\frac{0.088}{z}\ \text{volt}, \]

where \(a\) is the half-peak width in volts, and \(z\) is the number of electrons participating in the electrode process, which is equal for \(\mathrm{Tl}^+\) and \(\mathrm{Cd}^{++}\) to 1 and 2, respectively.

Fig. 2. Peaks of cadmium and thallium in their joint presence (conditions see Table 1): 1, 3—without the attachment; 2—with the attachment.

Table 1

It is evident from Table 1 that the resolving power of the polarograph with the attachment is somewhat higher than the resolving power of alternating-current polarography, which is at present considered the highest.

The polarograph with the attachment was used for the determination of antimony in high-purity tin. Determination of antimony by means of an ordinary polarograph is made difficult by the fact that, against a background of 0.5 N HCl, antimony gives a peak that practically completely merges with the curve of anodic dissolution of mercury (Fig. 3, 1, 3), and therefore under these conditions it is impossible to measure the height of the antimony peak.

When recorded on the polarograph with the attachment, the antimony peak is practically completely separated from the mercury dissolution curve and can be easily

measurements (Figs. 3, 2, 4); in the course of the study the proportionality of the peak height to the concentration of antimony in the solution was established, and thus

Fig. 3. Antimony peaks against a background of 0.5 N HCl: 1, 3 — without the attachment; 2, 4 — with the attachment.

the possibility was shown of using the polarograph with the attachment for the determination of antimony in high-purity tin.

Conclusions

1. A high-resolution polarograph for amalgam polarography has been developed and constructed on the basis of the OP-1 and OP-2 oscillographic polarographs.

2. Using thallium and cadmium as examples, the high resolving power of the new instrument has been demonstrated.

3. The possibility has been shown of using the polarograph for the determination of antimony in high-purity tin.

LITERATURE

1. Methods for the Determination and Analysis of Rare Elements. Collection of articles, Academy of Sciences of the USSR (pp. 5–10), 1961.

2. Z. Kublik. Electrolytic enrichment and determination of traces (microanalysis). Wiad. Chem., No. 8 (170), 499–528, 1961.

3. S. B. Tsfasman. Electronic Polarographs. Metallurgizdat, 1960.