DEVELOPMENT OF A METHOD FOR DETERMINING \(10^{-8}\)—\(10^{-7}\%\) BISMUTH IN INDIUM UNDER OPTIMAL CONDITIONS WITHOUT SEPARATION OF INDIUM

A. A. KAPLIN, A. G. STROMBERG, A. N. POKROVSKAYA

Some variants of analytical problems and possible ways of solving them in the ATN method by selecting optimal conditions have been considered earlier. On this basis, rapid procedures were developed for determining \(10^{-6}\)—\(10^{-5}\%\) copper and bismuth together with copper in high-purity indium. We then set ourselves the task of selecting certain optimal parameters in order to achieve maximum sensitivity in the determination of bismuth in indium; some initial conditions (use of a 7-77-4B polarograph, mercury dropping electrode, \(S = 10^{-2}\ \text{cm}^2\), \(W = 10^{-2}\ \text{V/sec}\); sample weight \(0.2\ \text{g}\)) remained the same as in the rapid procedure. The parameters of the rapid procedure for determining bismuth were: \(g = 0.2\ \text{g}\); \(V = 2\ \text{ml}\); \(S = 10^{-2}\ \text{cm}^2\); \(\varphi = 0.05\ \text{V}\); \(i = 3 \cdot 10^{-9}\ \text{A/mm}\); \(\bar{\beta} = 25\); \(K_{\mathrm{e}} = 1000\ \text{g}\ \text{A}\cdot\text{cm}/\text{g-ion}^{-1}\); \(h_i = 6\ \text{mm}\); \(h_c = 9\ \text{mm}\) \((\alpha = 0.997)\). Under the conditions of these experiments \(\gamma_* = 0.016\) and \(\tau = 6\ \text{min}\); the minimum determinable concentration was:

\[ a,\%=\frac{100 A \delta v h_c}{zFWg\gamma_*}\cdot i = 3\cdot 10^{-6}\%. \]

Analysis of this relationship and of the listed parameters shows that the sensitivity of the procedure can be increased 180-fold: by increasing \(\gamma\) 60-fold and using \(i_* = 1\cdot 10^{-9}\ \text{A/mm}\) (7-77-4B) instead of \(3\cdot 10^{-9}\ \text{A/mm}\). With a constant electrode size under the experimental conditions, after additional amalgamation the constant for the developed procedure, \(i_*\), was reduced to \(1.5\cdot 10^{-9}\ \text{A/mm}\) (\(\bar{\beta} = 25^\circ\), maximum compensation, \(K = 10\)). We increased the degree of depletion of the solution by increasing the accumulation time to \(60\ \text{min}\), reducing the volume of the working solution to \(0.5\ \text{ml}\), and correspondingly increasing \(K_{\mathrm{e}}\) to \(2000\ \text{g}\ \text{A}\cdot\text{cm}/\text{g-ion}^{-1}\) while stirring the solution by nitrogen bubbling. Under the new conditions \(\gamma_* = 0.76\) \((b = 1.44)\), and the calculated sensitivity of the procedure is \(a = 4\cdot 10^{-8}\%\). The method of sample treatment remained the same as in the rapid procedure. For working with volumes of \(0.5\ \text{ml}\), special quartz beakers were made; the dimensions of the tube for nitrogen bubbling, the connecting bridge, and the supporting glass tube of the cathode were correspondingly reduced.

Course of the analysis

A \(0.2\ \text{g}\) sample of finely ground indium is dissolved in \(1\ \text{ml}\) of \(7\ \text{N}\) HCl at \(t = 60\)—\(70^\circ\). After dissolution of the sample, the solution is evaporated—

is evaporated to dryness and kept for 3–5 min at \(t = 250^\circ\) to remove \(\mathrm{SbCl_3}\). Add \(0.5\) ml of \(0.25\) N HCl, cool the solution, and place the beaker in the electrolyzer. Accumulation is carried out at \(\varphi = -0.4\) V; \(\tau = 60\) min; stirring is by nitrogen bubbling. After completion of the accumulation, the polarogram of bismuth is recorded in the region \(-0.15\)—0 V \((\varphi_\mathrm{p} = -0.05\) V). Preliminary anodic dissolution of copper is carried out at \(\varphi = -0.15\) V by stopping the potentiometric drum. The bismuth concentration in indium is determined by the method of additions of a standard solution after repeated accumulation.

For analysis, samples of indium of grade In—00 were taken, the bismuth content of which, according to the chemical-spectral analysis of the plant laboratory, was below \(1 \cdot 10^{-7}\%\). The results of the amalgam-polarographic determination of bismuth by the developed procedure in one of the samples are given in Table 1.

Table 1

Analysis of an indium sample for bismuth content
\(g = 0.2\) g; \(V = 0.5\) ml;
\(\tau = 3600\) sec; \(S = 10^{-2}\ \mathrm{cm^2}\); \(i = 1.5 \cdot 10^{-9}\ \mathrm{a/mm}\)

Thus, the bismuth content in the sample is \(a(0.95) = 3.4 \pm 0.5 \cdot 10^{-8}\%\). The duration of the procedure when analyzing two samples and one blank is 6–7 hours when one laboratory assistant is working.

The accumulation time \((\tau = 60\) min) according to this procedure, although 2–3 times greater than that usually used in the APN method, is optimal for the given task \((a = 10^{-8}\%)\) when the solution is stirred by nitrogen bubbling, on a mercury dropping electrode, and when working on a 7-77-4B polarograph at maximum compensation. This time can be significantly reduced by increasing \(K_\mathrm{e}\) or by using finer sensitivities of the polarograph and correspondingly increasing the compensation current. For example, the possibility has been shown of using \(i = 1.08 \cdot 10^{-10}\ \mathrm{a/mm}\) LP—60; 1(3) in the determination of bismuth on the same surface. Under these conditions, \(\tau\) is reduced to 8–10 minutes and, consequently, the analysis time to 60–80 minutes: 2 parallel samples and one blank.