DEVELOPMENT OF A PROCEDURE FOR ELECTROLYTE REPLACEMENT IN THE APN METHOD USING A FILM ELECTRODE

V. F. Yankauskas, A. A. Kaplin, A. G. Stromberg

(Presented by the Scientific-Methodological Seminar of the KhTF)

Previously, several variants were considered for applying the transfer procedure using a mercury dropping electrode in selecting optimal conditions in the APN method. There [1] an electrolytic cell for work using the electrode-transfer procedure (electrolyte replacement) was also presented and described.

The aim of the present work was to develop methodological issues and to refine the design of the electrolyzer in order to eliminate losses of impurities concentrated in the electrode during electrolyte replacement, using a mercury film electrode as the cathode. The advantage of the film electrode over the dropping electrode lies in its greater sensitivity and resolving power [2].

The use of a mercury film electrode for the purpose of electrolyte replacement is complicated by the fact that the concentration of metal in the surface layer of the mercury film on the electrode is considerably higher than in a drop. Therefore, it could be assumed that the magnitude of impurity losses associated with oxidation of the surface layer during electrolyte replacement with the circuit opened on the film would be greater than on the drop. We studied individual operations that are part of the electrode-transfer procedure.

Losses of impurities concentrated in the electrode during the accumulation and dissolution of lead, indium, cadmium, and zinc were evaluated against a background of 0.25 M $\mathrm{CH_3COONH_4} + 0.1M\mathrm{H_2C_4H_4O_5}$ with the following intermediate operations: removal of electrodes with the concentrated impurities into air, transfer of electrodes without rinsing into an intermediate solution, transfer of electrodes to remove oxygen from the bulk of the background for oxidation of the amalgam, and transfer of electrodes with the circuit open. As in work with the dropping electrode, it was shown that opening the circuit, the presence of oxygen in the solution used to record the anodic peak, and the absence of intermediate rinsing of the electrodes lead to impurity losses (50–60%). Therefore, an improvement was introduced into the electrode design: during electrolyte replacement, the mercury film electrode is immersed in a microvolume of solution located in the drawn-out end of an agar-agar bridge.

With the modified electrode design, the completeness of determination of elements during electrolyte replacement was evaluated. Table 1 gives the results of determinations for lead and zinc. Since the degree of reversibility of the oxidation process on the two backgrounds is different, the completeness of determination was calculated not from the peak height but from its area ($q$, coulombs). The sign +#+#+#+#+#+

Table 1

Assessment of the completeness of determination of tin(2+) and lead
when using the electrode-transfer technique.
Experimental conditions: \(V=4\) ml; \(\varphi=-1.2\) V (relative to sat. cal. el.); \(\tau=180\) s;
\(C=8\cdot10^{-8}\) ml/l; \(S=0.11\ \text{cm}^2\); \(i=2\cdot10^{-8}\) A/mm

| \multicolumn{2}{c}{6 M HCl} | \multicolumn{2}{c}{2 M CH\(_3\)COOH + 2 M CH\(_3\)COONa} | \multicolumn{2}{c}{Completeness of determination, %} |
|---:|---:|---:|---:|---:|---:|
| Sn\(^{2+}\), \(q\cdot10^{?}\), C | Pb\(^{2+}\), \(q\cdot10^{?}\), C | Sn\(^{2+}\), \(q\cdot10^{?}\), C | Pb\(^{2+}\), \(q\cdot10^{?}\), C | Sn\(^{2+}\) | Pb\(^{2+}\) |
| 7.39 | 6.34 | 6.60 | 5.81 | 89 | 90 |
| 13.73 | 6.86 | 12.67 | 6.86 | 92 | 100 |
| 46.99 | 6.08 | 46.20 | 6.88 | 98 | 112.5 |
| 10.56 | 8.37 | 9.90 | 7.66 | 94 | 91 |
| 21.65 | 15.05 | 18.48 | 13.20 | 84 | 87.5 |
| 22.44 | 14.26 | 20.59 | 13.73 | 91 | 96.0 |

“A” denotes the area under the peak recorded in the initial solution; “B” denotes that recorded after changing the electrolyte in the solution used for recording the peak.

Table 2 gives data on the completeness of determination of indium and cadmium during accumulation in a solution of 0.25 M CH\(_3\)COONH\(_4\) + 0.1 M H\(_2\)C\(_4\)H\(_4\)O\(_6\) and anodic dissolution against the background of 0.5 M KOH + 0.25 M ethylenediamine, and of zinc, respectively, in solutions of 0.5 M KOH and 0.25 M CH\(_3\)COONH\(_4\) + 0.1 M H\(_2\)C\(_4\)H\(_4\)O\(_6\).

From Tables 1 and 2 it follows that, when the electrolyte is changed with a closed circuit and in an inert atmosphere using the developed electrolyzer design, the losses do not exceed 20%, which is within the limits of experimental error.

Table 2

Assessment of the completeness of determination of indium, cadmium, and zinc when using
the electrode-transfer technique.
(For experimental conditions, see the caption to Table 1)

The investigations carried out on the possibility of applying the electrode-transfer technique (electrolyte change) were used by us in developing a number of procedures for determining impurities in high-purity lead without chemical separation of the matrix. The data presented in Tables 1–2 also represent one of the variants for selecting optimal conditions using the electrode-transfer technique: the choice, for anodic dissolution, of a supporting electrolyte with the most reversible process of oxidation of the metal from the amalgam. Other conditions being equal, the sensitivity of the determination (peak height) is then maximal.

Conclusions

For the first time, a procedure has been developed for the quantitative determination of impurities using the transfer technique with the application of a film amalgamated electrode to increase the resolving power of the APN method.

References

1. A. A. Kanygin. Dissertation. TPI, Tomsk, 1966.
2. V. A. Igolinskii. Dissertation, TPI, Tomsk, 1963.