Amalgam Polarographic Determination of Zinc, Cadmium, and Indium in Lead with Electrolytic Separation of Lead
Abstract Generated abstract
This paper examines the determination of trace zinc, cadmium, and indium in high-purity lead by amalgam polarography with accumulation, a task hindered by the large lead matrix. The authors investigate preliminary electrolytic separation of lead from nitric acid solution as anodically deposited lead dioxide, varying current density and electrolysis time and then measuring impurities by the method of additions on a film amalgamated electrode. The study identifies practical separation conditions, chiefly electrolysis at about 100 mA/cm² for 1.5 hours, and estimates impurity losses after matrix removal at about 10% for cadmium, 9 to 19% for zinc, and 25% for indium. Results on artificial mixtures indicate the feasibility of determining 10^-4 to 10^-6% of these impurities in a 10^4 to 10^5 fold excess of lead.
Full Text
AMALGAM-POLAROGRAPHIC DETERMINATION OF ZINC, CADMIUM, AND INDIUM IN LEAD WITH ELECTROLYTIC SEPARATION OF LEAD
V. E. Katokhin, A. A. Kaplin, A. G. Stromberg
(Presented by the scientific seminar of the Department of Chemistry)
The determination of a number of impurities, for example Zn and In, in high-purity lead by the method of amalgam polarography with accumulation is difficult, while some elements (Cd and others) cannot at all be determined by this method without separating the main mass of lead. Separation of most impurities can be carried out by precipitating lead with sulfuric acid in the form of PbSO₄.
- However, a disadvantage of this method is the need for prolonged standing of the solution over the precipitate to achieve sufficiently complete precipitation of lead.
We have investigated the possibility of determining \(10^{-4}\)—\(10^{-6}\%\) of impurities Zn, Cd, In in high-purity lead by the method of amalgam polarography with accumulation, with preliminary electrolytic separation of the matrix. According to literature data [2–3], electrolytic separation of Pb is most expediently carried out at the anode in the form of PbO₂ by electrolysis of nitric-acid solutions in the presence of a considerable excess of free nitric acid. In this case, at the cathode, according to known reactions, mainly hydrogen ions and \(\mathrm{NO_3^-}\) will be reduced, and at the anode—\(\mathrm{OH^-}\) ions and lead ions in the form of PbO₂.
Experimental Part
Electrolytic separation of lead was carried out in a glass beaker with a capacity of 10—11 ml. As the anode, a cylinder of platinum sheet with a surface area of 5 cm² was used, which was placed in a cylindrically rolled platinum mesh with a surface area of 30 cm². Power was supplied from an SV-6 rectifier according to the circuit given in [3].
The initial solution when working with synthetic solutions: to 4 ml of a solution of spectrally pure Pb with a concentration of the latter of \(1 \cdot 10^{-2}\) mol/l, 1 ml of HNO₃ (9—12 N) and standard solutions of Zn, Cd, In were added in such an amount that the concentration of these elements in the solution was \(1 \cdot 10^{-6}\)—\(1 \cdot 10^{-5}\) mol/l.
The solution was stirred with a magnetic stirrer and by purging with nitrogen. A current of specified magnitude was passed through the electrolytic cell; in this process PbO₂ accumulated on the anode in the form of a dense microcrystalline brown precipitate. Periodically this precipitate was removed from the anode in a solution of nitric acid with the addition of several drops of H₂O₂. After separation of the main mass of lead, the solution was evaporated to dryness; an indifferent ...
the electrolyte on which the polarogram of the elements being determined was recorded. An OH-101 polarograph was used; a film amalgamated electrode served as the cathode. The concentration of the elements was determined by the method of additions.
In the course of the work, we investigated the influence of current density and electrolysis time on the degree of lead deposition and estimated the losses of Zn, Cd, and In after electrolytic separation of the bulk of the lead. The results of the investigations are given in Tables 1 and 2.
Table 1
Influence of current density and electrolysis time on the degree of purification of the solution from lead
| No. | \(j,\ mA/cm^2\) | \(T,\ min\) | Final concentration of lead, mol/l |
|---|---|---|---|
| 1 | 10 | 30 | \(2 \cdot 10^{-4}\) |
| 1 | 10 | 120 | \(1.6 \cdot 10^{-5}\) |
| 1 | 10 | 60 | \(6 \cdot 10^{-5}\) |
| 2 | 50 | 90 | \(6 \cdot 10^{-6}\) |
| 3 | 100 | 40 | \(2 \cdot 10^{-6}\) |
| 4 | 100 | 90 | \(5 \cdot 10^{-7}\) |
Discussion of results
It follows from Table 1 that lead deposition at the anode should be carried out at a current density of \(200\ mA/cm^2\). However, since at current densities above \(100\ mA/cm^2\) the solution heats up strongly and therefore evaporates rapidly, which requires frequent replenishing and acidification of the solution, it is most convenient to carry out the process at a current density of \(100\ mA/cm^2\). The total electrolysis time is 1.5 hours.
Table 2
Magnitude of the losses of cadmium, zinc, and indium after purification of the solution from lead
| \(C_{\mathrm{н}}\cdot 10^{-6}\), mol/l | \(C_{\mathrm{к}}\cdot 10^{-6}\), mol/l | Number of experiments | Mean value \(C_{\mathrm{к}}\cdot 10^{-6}\), mol/l | Mean square | Magnitude of losses, % |
|---|---|---|---|---|---|
| Cadmium | Cadmium | Cadmium | Cadmium | Cadmium | Cadmium |
| 7 | 7; 10; 7; 7; 6; 5 | 6 | 7 | 0.8 | 0 |
| 7.2 | 6; 10; 8; 9 | 3 | 6.5 | 0.8 | 10 |
| 7.2 | 6.5; 6.5; 6.5 | 3 | 6.5 | 10 | |
| Zinc | Zinc | Zinc | Zinc | Zinc | Zinc |
| 36 | 30; 45; 30; 30 | 4 | 33.7 | 0.27 | 9 |
| 18 | 13.5; 12.9; 17.7; 15.4; 13.8; 14.1 | 6 | 14.7 | 19 | |
| 9.25 | 9.88; 8.8 | 6 | 9.4 | ||
| Indium | Indium | Indium | Indium | Indium | Indium |
| 2.4 | 1.9; 1.9; 1.9; 1.5 | 4 | 1.8 | 0.01 | 25 |
| 18 | 1.69; 1.77; 1.87; 1.87; 1.69; 1.42 | 6 | 1.8 | — | |
| 0.6 | 0.68; 0.6; 0.52 | 3 | 0.6 | — |
Designations: \(j\) — current density, \(mA/cm^2\); \(C_{\mathrm{к}}\) — final concentration of the elements, ml/l; \(C_{\mathrm{н}}\) — initial concentration of the elements.
It follows from Table 2 that the losses of the elements being determined, within the error of the method, amount to 10% for cadmium, 9–19% for zinc, and 25% for indium.
The figures given above are not limiting values when determining them by the APN method against a background of 0.5 M KOH + 0.1 EDA at this degree of purification from lead. Against this background, Zn and In can be determined at sto-
with a tenfold excess of Pb, and Cd with a tenfold excess, i.e., in our case it is possible to determine \(10^{-8}\) mole/liter Zn, In and \(10^{-7}\) mole/liter Cd.
Further studies will be directed toward reducing the lead content in the solution after separation of the lead.
Conclusions
-
The optimal conditions for the electrolytic separation of the main mass of lead from impurities have been selected.
-
A procedure has been developed on artificial mixtures for the polarographic determination of \(10^{-4}\)—\(10^{-6}\%\) Zn, Cd, In in a \(10^{4}\)—\(10^{5}\)-fold excess of lead, with preliminary electrolytic separation of the matrix.
-
The work has demonstrated the fundamental possibility of applying the AРN method for the analysis of Zn, Cd, In impurities in high-purity lead.
LITERATURE
- Methods for the Analysis of High-Purity Substances. Publishing House “Nauka,” Moscow, 1965.
- A. Klassen. Electroanalysis. Goskhimizdat, 1934.
- Fischer. Electroanalysis. Goskhimizdat, 1932.