Studies on the determination of copper in zinc telluride by the APN method
Abstract Generated abstract
This study examines the determination of trace copper in zinc telluride by anodic polarographic methods, with particular attention to interference from tellurium in different supporting electrolytes and at different electrodes. The authors evaluate alkaline, tartaric acid, and succinic acid backgrounds, test extraction of tellurium with methyl isobutyl ketone, and find that residual tellurium can suppress the copper peak, while a succinic acid medium adjusted with ammonium hydroxide to pH 5 to 6 gives a distinct, reversible copper signal. On this basis, they propose a procedure involving acid decomposition of ZnTe, repeated hydrochloric acid treatment, extraction of tellurium as TeCl4, destruction of residual solvent, and polarographic measurement by additions. The method is reported to determine about 10^-5 to 5 x 10^-6 percent copper in ZnTe and can also be used for simultaneous determination of lead and cadmium.
Full Text
STUDIES ON THE DETERMINATION OF COPPER IN ZINC TELLURIDE BY THE APC METHOD
A. A. KAPLIN, Z. S. MIKHAILOVA
(Presented by the scientific-methodological seminar of the Department of Physical Chemistry)
In addition to the methods we developed earlier [1] for determining a number of impurities in compounds of the type \(A_{II}B_{VI}\), studies were carried out on the determination of Cu in ZnTe.
For this purpose, the extraction behavior of tellurium and copper was studied on a number of backgrounds. It was found that copper on a KOH background is irreversibly oxidized and, when tellurium is added, the copper peak decreases; the decrease is more noticeable on a background of 0.5 N KOH than on 3 N KOH. The greater the basicity of the solution, the more extensive is the hydrolysis of tellurium. When copper is determined without preliminary separation of tellurium on a KOH background, a white colloidal solution is observed in which analysis cannot be carried out. Thus, it was established that the determination of copper is impossible in the presence of a base—tellurium.
In order to separate tellurium, the possibility of extracting its carbonate complex with methyl isobutyl ketone was investigated. Tellurium is extracted quantitatively, but together with it copper partially passes into the organic phase. The influence of tellurium on the polarographic behavior of copper at a graphite electrode was then studied. Tellurium is oxidized at the graphite electrode at \(\varphi=-0.45\ \text{V}\) (relative to the graphite electrode). At low tellurium concentrations (\(10^{-5}\)—\(10^{-6}\ \text{mol/l}\) in solution), the copper peak is observed at \(\varphi_{1/2}=-0.7\ \text{V}\) (relative to the graphite electrode). With increasing tellurium concentration, the copper peak decreases and, in the presence of \(10^{-3}\ \text{mol/l}\) tellurium in the solution, practically disappears.
The reproducibility of the determination of copper at the graphite electrode in our experiments was unsatisfactory; therefore, in subsequent work a mercury film electrode was again used.
In [2] a method is described for determining trace amounts of Cd, Cu, Ni, Pb, and Zn in tellurium, based on preliminary separation of tellurium by extraction with methyl isobutyl ketone from 7 N HCl, followed by polarography of the impurities on a tartaric acid background. We tried to apply this procedure to the analysis of zinc telluride.
Tellurium in HCl forms a stable yellow complex, \(\mathrm{TeCl}_4\), and, upon extraction with methyl isobutyl ketone, passes into the organic phase. The influence of tellurium on the copper peak on a tartaric acid background was studied. It was established that at a tellurium concentration of \(6\cdot10^{-5}\ \text{g}\) in solution, the copper peak does not decrease in the presence of tellurium. With an increase in the tellurium concentration, the copper peak begins to decrease, and upon addition of \(1.4\cdot10^{-4}\ \text{g}\) Te to the solution, the copper peak is completely suppressed.
It may be assumed that the interfering effect of elemental tellurium deposited on the electrode consists in a decrease in the overvoltage—
hydrogen evolution, diffusion limitations for impurities, and the formation of sparingly soluble semiconductor compounds on the electrode.
The influence of pH on the copper peak in the presence of tellurium was studied against a background of succinic acid. The pH was varied from 2 to 8 by adding NH₄OH to the solution. It was found that with the addition of NH₄OH (i.e., with a change in pH from 2 to 8), the suppression of the copper peak by tellurium decreases. For the work we chose pH 5–6; under these conditions copper is oxidized reversibly, a distinct copper peak is recorded, and the half-peak width is 17 mV.
Against a background of succinic acid neutralized with NH₄OH to pH 5–6, the dependence of the current magnitude on the copper concentration in solution was recorded. The dependence is linear.
On the basis of the studies carried out, a procedure was developed for determining copper in zinc telluride.
Procedure for determining copper in zinc telluride
The essence of the procedure is that a 0.1 g sample is decomposed in a mixture of acids HCl : HNO₃ (3 : 1) and evaporated to dryness. The residue is evaporated three times with 5 ml of HCl; tellurium is extracted with methyl isobutyl ketone, after which polarography is carried out against a background of succinic acid neutralized with NH₄OH to pH 5–6.
Course of analysis
A 0.1 g sample of ZnTe is dissolved in a mixture of HCl : HNO₃ (3 : 1) and evaporated three times with 5 ml of HCl; tellurium is extracted with methyl isobutyl ketone. This operation is repeated 3 times. After this the residue is dissolved in 5 ml of 7 N HCl and the tellurium complex TeCl₄ is extracted with methyl isobutyl ketone in two 10 ml portions. The aqueous phase is poured into a beaker and evaporated. Methyl isobutyl ketone is partially dissolved in HCl; to burn off the methyl isobutyl ketone, 1 ml of HNO₃ + 0.5 ml of HClO₄ is added to the residue and evaporated at \(t = 150^\circ\)C. The residue is again dissolved in 5 ml of a 0.56 M solution of succinic acid neutralized with NH₄OH to pH 5–6, and polarographed. Conditions: \(\varphi_{\mathrm{e}} = -0.98\) V (relative to the saturated calomel electrode), \(\tau = 5'\), \(S = 2 \cdot 10^{-8}\) A/mm, \(\varphi_{\mathrm{nCu}} = -0.15\) V (relative to the saturated calomel electrode). The copper concentration is evaluated by the method of additions.
Table 1
Copper content in zinc telluride. Conditions:
\(\varphi_{\mathrm{e}} = -0.9\) V (rel. S.C.E.) \(\tau = 5'\);
\(S = 2 \cdot 10^{-8}\) A/mm;
\(\varphi = -0.15\) V (rel. S.C.E.) \(n = 5\); \(\alpha = 0.95\)
| Analysis results \(\bar{x} \cdot 10^{-4}\) | \((x-\bar{x})^2\) | \(S_{\bar{x}}\) | \(t_\alpha\) | \(\varepsilon\) | \(\varepsilon, \%\) |
|---|---|---|---|---|---|
| 3.32 | 15.786 | 0.89 | 2.8 | 1.49 | 45 |
Simultaneously with copper, this procedure makes it possible to determine Pb and Cd.
Data on the analysis of ZnTe for Cu content are given in Table 1.
Conclusions
- The influence of Te on the copper peak in various supporting electrolytes has been studied.
- A procedure has been developed for determining \(10^{-5}\)—\(5 \cdot 10^{-6}\)% Cu in ZnTe. Simultaneous determination of Pb, Cd, and Cu is possible.
LITERATURE
- A. A. Kaplin, Z. S. Mikhailova, V. M. Pichugina, V. F. Yatskauskas. “Electronic Engineering,” ser. 12, issue 1 (7), 43, 1971.
- Syües Alewlius J., Klug Otto N. Chem. Analit, Poland, 12, No. 3, 450–458, 1967; RZhKhim, 2G116, No. 2, 1968.