Development of a Method for Determining Trace Impurities of Indium in Lead by Amalgam Accumulation Polarography
Abstract Generated abstract
A procedure is developed for determining trace indium impurities in lead by amalgam polarography with accumulation on a film electrode. The method combines preliminary extraction of indium from weakly acidic solutions with alkylphosphoric acids, removal of lead by nitric acid reextraction, transfer of indium into hydrochloric acid, destruction of residual organic phase with hydrogen peroxide, and polarographic measurement in 0.5 M KOH with 0.1 M EDTA. Experiments on supporting electrolytes and synthetic solutions indicate effective separation from large excesses of lead and several other metals, with reported indium recoveries in lead-containing synthetic mixtures of about 84 to 94 percent and an attainable determination level of \(10^{-5}\) percent in lead samples.
Full Text
DEVELOPMENT OF A METHOD FOR DETERMINING TRACE IMPURITIES OF INDIUM IN LEAD BY AMALGAM POLAROGRAPHY WITH ACCUMULATION
T. F. Ryashentseva, A. A. Kaplin, P. S. Levin
(Presented by the scientific seminar of the CTF)
The methods described in the literature for the polarographic determination of indium in lead are very cumbersome, require the performance of a large number of operations, and a considerable number of reagents are needed for the analysis; the sensitivity of the method is comparatively low—\(10^{-3}\)–\(10^{-4}\%\) [1–3].
The most widespread method for separating the bulk of lead from impurities is its precipitation in the form of sulfate [4, 5]. This method, however, does not provide sufficiently complete separation of lead from the indium impurity. More effective separation of lead as the matrix can be achieved by extracting indium with alkylphosphoric acids (APA) from weakly acidic media (2–3 N \(\mathrm{HNO_3}\), 4 N \(\mathrm{HClO_4}\)).
The aim of the present work is to develop a method for determining indium in lead by amalgam polarography with accumulation, with preliminary separation of indium from the matrix by extraction with alkylphosphoric acids. The purification of the reagents used in the work and the method of investigation are the same as in [7]. For separating indium from lead, the extraction procedure described in [6] was used; its essence consists in extracting indium APA from weakly acidic solutions, followed by removal of lead from the organic phase by re-extraction with 2–3 N \(\mathrm{HNO_3}\).
Proceeding from the fact that, in many solutions, the peak potentials of anodic oxidation of indium and lead have close values, we carried out studies to select a supporting electrolyte for polarographing indium in the presence of an excess of lead \((1:10^3)\). The optimum of the supporting electrolytes studied is 0.5 M \(\mathrm{KOH}\) + 0.1 M EDTA, which permits determination of indium in an excess of Zn, Pb, Cu, Tl, Bi, Sb \((\Delta \varphi_{\mathrm{Pb-In}} = 0.2\ \mathrm{V})\). A series of experiments was carried out for a quantitative assessment of possible losses of indium during the performance of various operations directly associated with the extraction of APA and subsequent polarography. Studies of the first stage, i.e., the completeness of extraction of indium from various media (2 N \(\mathrm{HNO_3}\), 1 N \(\mathrm{HNO_3}\), and 4.4 N \(\mathrm{HClO_4}\)) by APA, showed that indium passes completely into the organic phase. Since, for the decomposition and combustion of the organic phase, \(\mathrm{H_2O_2}\) and \(\mathrm{H_2SO_4}\) are introduced into the re-extract during evaporation, we carried out studies to determine possible losses of indium in this operation. The investigations were performed on synthetic mixtures of the reagents used in the operations. The results are summarized in Table 1. It follows from the table that it is sufficient to burn the organic phase with a 30% solution of \(\mathrm{H_2O_2}\), and that no losses of indium are observed in this case. The completeness of indium determination after all operations was studied by us on
Table 1
Determination of the optimal conditions necessary for burning off the organic phase when evaporating the reextractant
| No. | Mixture composition | Amount of indium introduced into the solution (g) | Amount of indium found after evaporation (g) | % determined |
|---|---|---|---|---|
| 1 | 10 N HCl | \(0.5 \cdot 10^{-5}\) | \(0.6 \cdot 10^{-5}\) | 120 |
| 2 | 10 N HCl | \(0.5 \cdot 10^{-5}\) | \(0.1 \cdot 10^{-5}\) | 20 |
| 3 | 5 ml 10 N HCl 0.06 ml APF 0.03 ml conc. H\(_2\)SO\(_4\) 1 ml 30% H\(_2\)O\(_2\) |
\(0.5 \cdot 10^{-5}\) | \(0.225 \cdot 10^{-5}\) | 51 |
| 4 | 5 ml 10 N HCl 0.06 ml APF 0.1 ml 30% H\(_2\)O\(_2\) |
\(0.5 \cdot 10^{-5}\) | \(0.5 \cdot 10^{-5}\) | 100 |
synthetic solutions. The results of these studies are given in Table 2, on the basis of which it may be said that the completeness of the determination of indium is 80–87%. Proceeding from the above, we propose a procedure for the polarographic determination of indium in lead.
Table 2
Determination of indium in synthetic solutions
| No. | Introduced In, mg | Taken Pb, mg | Reextractant | Number of washings | Found In, mg | Determination of indium, % |
|---|---|---|---|---|---|---|
| 1 | 0.002 | — | 8 N HCl | 3 | 0.0033 | 105 |
| 2 | 0.00036 | — | 10 N HCl | 6 | 0.00035 | 98 |
| 3 | 0.00037 | — | 10 N HCl | 6 | 0.0004 | 108 |
| 4 | \(1 \cdot 10^{-4}\) | — | 10 N HCl | 6 | \(1.3 \cdot 10^{-4}\) | 130 |
| 5 | \(1 \cdot 10^{-4}\) | — | 10 N HCl | 6 | \(0.8 \cdot 10^{-4}\) | 80 |
| 6 | 0.00036 | 45 | 10 N HCl | 6 | 0.00034 | 94 |
| 7 | 0.00025 | \(5 \cdot 10^{2}\) | 10 N HCl | 6 | 0.00021 | 84 |
| 8 | 0.00022 | \(5 \cdot 10^{2}\) | 10 N HCl | 6 | 0.00019 | 87 |
Determination procedure. A weighed portion of 2–3 g of finely divided lead is dissolved in 5 ml of HNO\(_3\); the solution is evaporated to moist salts, which are then dissolved in 20–30 ml of 2 N HNO\(_3\). 5 ml of this solution is transferred to a separatory funnel, 5 ml of extractant (APF) is added, and extraction is carried out for 1–2 min with vigorous shaking. To remove the lead that has passed into the organic phase, it is reextracted six times with 5 ml of 2–3 N HNO\(_3\). Indium is reextracted twice with 2 ml of 8–10 N HCl. 4 ml of the reextractant is evaporated in a quartz beaker checked for purity at a temperature of 140–150°. Burning off the organic phase present in the reextractant is carried out by adding a 30% H\(_2\)O\(_2\) solution until the residue is completely decolorized. To decompose and remove traces of peroxide, the residue is ignited at 200–250° and dissolved in the supporting solution: 0.5 M KOH + 0.1 M EDTA. Indium is determined by amalgam polarography with accumulation. An indium content in sample specimens of \(10^{-5}\)% is determined under the following analysis conditions: background 0.5 M KOH + 0.1 M EDTA. Film electrode. \(S = 3 \cdot 10^{-2}\ \text{cm}^2\); \(\varphi_{\text{el}} = -1.5\ \text{V}\); \(\tau = 1\ \text{min}\); \(i = 1 \cdot 10^{-8}\ \text{A/mm}\); \(V = 4\ \text{ml}\); \(W = 10^{-2}\ \text{V/sec}\),
Conclusions
A method has been developed for determining indium in lead using amalgam polarography with accumulation on a film electrode, with preliminary separation of indium from lead by extraction with alkylphosphoric acids.
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