Development of an Improved Rapid Method for Determining Microconcentrations of Copper in High-Purity Indium by Amalgam Accumulation Polarography
Abstract Generated abstract
An improved rapid procedure is described for determining trace copper in high-purity indium by amalgam accumulation polarography without separating the indium matrix. The authors reduce analysis time by shortening nitrogen purging to three minutes, modifying sample heating and evaporation conditions to limit copper adsorption, and using a better amalgamated mercury electrode that permits higher polarographic sensitivity and a shorter accumulation time of six minutes. Repeated analyses of a high-purity indium sample gave a mean copper content of 2.5 × 10^-5 percent with an error of ±0.34 × 10^-5 percent, and the proposed procedure reached a limiting sensitivity of 4 × 10^-6 percent while requiring fewer personnel than the colorimetric plant method.
Full Text
N. A. MESYATS, A. A. KAPLIN, M. S. ZAKHAROV, G. K. TYCHKINA
DEVELOPMENT OF AN IMPROVED RAPID METHOD FOR DETERMINING TRACE CONCENTRATIONS OF COPPER IN HIGH-PURITY INDIUM BY AMALGAM ACCUMULATION POLAROGRAPHY
(Presented by the scientific seminar of the Department of Physical Chemistry)
Previously [1] we developed a method for determining trace concentrations of copper \((10^{-5}\%—10^{-6}\%)\) in high-purity indium by amalgam polarography without separating the bulk of the indium. The method consists of the following. A 0.2 g weighed portion of pulverized indium is dissolved in 11 N \( \mathrm{HNO}_3 \); the solution is evaporated to dryness, and the resulting indium nitrate is dissolved in 2 ml of background solution, 1 M \( \mathrm{H}_3\mathrm{PO}_4 \). Nitrogen is passed through the solution for 10 minutes, copper is accumulated for ten minutes in a mercury drop, and the anodic polarogram of copper is recorded. The dissolution current is recorded with a galvanometer at a sensitivity of \(1.4 \cdot 10^{-8}\) A/mm. The duration of the analysis of two indium samples and one blank by this method is 3–3.5 hours when performed by one laboratory assistant. However, for practical purposes it is desirable to have a more rapid method. Therefore, we investigated the possibility of shortening the analysis time by
a) shortening the time of passing nitrogen through the solution,
b) shortening the accumulation time (transition to higher sensitivity).
Passing nitrogen through the solution is necessary to remove oxygen, whose presence distorts the shape of the copper wave, and to wash from the walls of the beaker ions of the metal being determined that were adsorbed on them during evaporation of the acid.
From the investigations we carried out, it was found that passing nitrogen through the solution for three minutes is sufficient for satisfactory removal of oxygen under the conditions of the analysis. In this case the copper wave is analogous to the wave obtained when nitrogen is passed through the solution for 10 minutes.
In order to eliminate possible adsorption of copper on the walls of the beaker, the conditions of heat treatment of the sample were changed. The dissolution temperature was lowered from 100–150° to 60–50°C, and the solution is evaporated not to dryness, but to a volume of 0.1–0.2 ml.
In the previously developed method, the use of a galvanometer of higher sensitivity (limiting galvanometer sensitivity \(1 \cdot 10^{-9}\) A/mm) was hindered by the presence of large residual currents (Fig. 1) on the mercury electrodes with a platinum contact used earlier, which was associated with insufficient amalgamation of the platinum surface.
In work [2], a procedure was developed for obtaining a mercury electrode with good amalgamation of the mercury contact. The investigations carried out showed that, using an electrode prepared by this procedure, it is possible to work successfully on a polarograph with a sensitivity of \(8.4 \cdot 10^{-9}\) A/mm. At the same time, it appears possible to reduce the accumulation time from 10 to 6 min.
Fig. 1. Residual-current polarograms:
a) electrode with a well-amalgamated surface of the plate;
b) electrode with insufficient amalgamation of the surface of the platinum contact.
Fig. 2. Polarograms of the analysis of a high-purity indium sample for the content of copper microimpurity in it:
a) polarogram of the blank experiment (1.5 ml of nitric acid was evaporated in a clean beaker);
b) polarogram of the indium sample;
c) polarogram of the indium sample + addition of 0.1 μg of standard copper solution.
On the basis of the investigations carried out, we propose an improved rapid procedure for determining copper in high-purity indium.
Procedure
0.2 g of finely ground In is dissolved in a quartz beaker checked for cleanliness in 1.5 ml of 11 N HNO\(_3\) at \(\sim 50 \div 60^\circ\)C. After the indium has dissolved, the solution is evaporated to 0.1–0.2 ml and a background of 2 ml of 1 M H\(_3\)PO\(_4\) is added. The copper concentration is determined by the method of additions of a standard copper solution.
A polarogram of the analysis of a high-purity indium sample is shown in Fig. 2. To evaluate the reproducibility of the determination of microconcen-
concentration of copper by the proposed procedure, analyses were carried out on one sample of high-purity indium. The experimental results are given in Table 1. A statistical treatment of the results obtained was performed.
Table 1
Experimental conditions: background — 1 M H₃PO₄, sample weight — 0.2 g, \(V = 2\) ml, \(\tau_{\mathrm{el}} = 6\) min,
\(r_k = 0.04\) cm, sensitivity \(= 8.4 \cdot 10^{-9}\) A/mm
| No. | Copper content in high-purity indium, \(x_i \times 10^{-5}\%\) | Deviation from the arithmetic mean, \(\bar{x} - x_i\) | Mean-square deviation, \((x - x_i)^2\) | Calculations |
|---|---|---|---|---|
| 1 | 2.6 | 0.1 | 0.01 | 1. Arithmetic mean of the measurements \(\displaystyle \bar{x}=\frac{\sum x_i}{n}=\frac{79.1}{31}=2.5.\) |
| 2 | 2.6 | 0.1 | 0.01 | |
| 3 | 2.5 | 0 | 0 | |
| 4 | 2.6 | 0.1 | 0.01 | |
| 5 | 2.8 | 0.2 | 0.04 | |
| 6 | 2.3 | −0.2 | 0.04 | 2. Root-mean-square error of individual measurements \(\displaystyle \sigma=\sqrt{\frac{11.95}{30}}=0.62.\) |
| 7 | 2.5 | 0 | 0 | |
| 8 | 2.6 | 0.1 | 0.01 | |
| 9 | 3.2 | 0.7 | 0.49 | |
| 10 | 2.2 | −0.3 | 0.09 | |
| 11 | 2.8 | 0.3 | 0.09 | 3. Maximum possible error of the measurements \(\displaystyle \Delta=3\sigma=1.86.\) |
| 12 | 2.8 | 0.3 | 0.09 | |
| 13 | 2.5 | 0 | 0 | |
| 14 | 2.2 | −0.3 | 0.09 | 4. Root-mean-square error in the arithmetic mean \(\displaystyle \sigma=\frac{s}{\sqrt{n}}=0.34.\) |
| 15 | 2.5 | 0 | 0 | |
| 16 | 3.0 | 0.5 | 0.25 | |
| 17 | 2.6 | 0.1 | 0.01 | |
| 18 | 2.4 | −0.2 | 0.04 | |
| 19 | 2.8 | 0.3 | 0.09 | |
| 20 | 3.4 | 0.9 | 0.81 | |
| 21 | 1.5 | −1.0 | 1.00 | |
| 22 | 2.8 | 0.3 | 0.09 | |
| 23 | 2.5 | 0 | 0 | |
| 24 | 2.4 | −0.1 | 0.01 | |
| 25 | 2.6 | 0.1 | 0.01 | |
| 26 | 2.8 | 0.3 | 0.09 | |
| 27 | 1.7 | −0.8 | 0.64 | |
| 28 | 1.8 | −0.7 | 0.49 | |
| 29 | 2.8 | 0.3 | 0.09 | |
| 30 | 2.7 | 0.2 | 0.04 | |
| 31 | 2.6 | 0.1 | 0.01 | |
| \(\sum = 79.1\) | \(\sum = +1.6\) | \(\sum = 11.95\) |
Consequently, the arithmetic mean of all measurements was calculated with an error of \(\pm 0.34\), i.e. \(\bar{x} = 2.5 \pm 0.34\).
The limiting sensitivity of the proposed procedure is \(4 \cdot 10^{-6}\%\). The duration of the analysis of three samples in duplicate and two blanks, that is, eight beakers, is 6 hours when performed by one laboratory technician. The duration of the colorimetric procedure for determining copper that is currently used at the plants is 6 hours when performed by three laboratory technicians.
REFERENCES
-
A. G. Stromberg, M. S. Zakharov, N. A. Mesyats, A. A. Kaplin, R. S. Tyutyunkova. Determination of trace impurities of lead, zinc, bismuth, and copper in indium by amalgam polarography with accumulation on a stationary mercury drop. Collection of Papers of the Institute of Rare Elements and Mineral Raw Materials, Nos. 5–6, 1963.
-
A. A. Kaplin, B. F. Nazarov, M. S. Zakharov, V. S. Zhikina. Studies aimed at improving the reliability of a stationary electrode with a platinum contact in the method of amalgam polarography with accumulation. The present collection.