STUDIES ON THE DETERMINATION OF TRACE IMPURITIES IN HIGH-PURITY LEAD

V. F. Yankauskas, V. M. Pichugina, A. A. Kaplin, V. I. Kuleshov

(Presented by the scientific-methodological seminar of the Chemical-Technological Faculty)

For the simultaneous determination of copper and bismuth (along with other impurities) in high-purity lead, the chemical-spectral method is most often used [1]. However, the latter is time-consuming and labor-intensive.

The aim of our studies was to develop a method for the simultaneous determination of copper and bismuth in high-purity lead without separation of the matrix, by the method of amalgam polarography with accumulation (APA) [2]. A comparison of the half-wave potentials of copper, bismuth, and lead on various backgrounds shows [3] that there is a considerable difference between the half-wave potentials of these elements (up to −0.3–0.4 V). This makes it possible to accumulate copper and bismuth at potentials more positive than the potential at which lead reduction begins. The optimal variant is the determination of copper and bismuth from a solution of lead nitrate after dissolving a weighed portion of lead in nitric acid. However, against the background of lead nitrate, the anodic waves of copper and bismuth are practically not separated. The experimental data we obtained for the anodic waves of copper and bismuth on different backgrounds (Table 1) showed that, on the backgrounds studied, it is not possible to find conditions

Table 1

Potential of the anodic peaks of Pb, Cu, Bi on several backgrounds

that would be simultaneously optimal both for the stages of sample treatment and accumulation, and for anodic oxidation and peak registration. Thus, on a background of 0.2 M HCl, bismuth is oxidized reversibly (width

the notch is minimal); however, copper is oxidized irreversibly; lead precipitates. Against a background of 0.25 M CH₃COONH₄ + 0.1 M H₂C₄H₄O₆, copper is oxidized reversibly (the notch width is minimal), but the anodic peak of bismuth is masked by the current of anodic oxidation of mercury; the presence of tartrate ion causes precipitation of lead tartrate.

The difficulties in solving the problem of the simultaneous determination of copper and bismuth without separating the lead matrix were eliminated by us by using the technique of electrode transfer (change of electrolyte) [4]. The essence of the transfer technique is that the accumulation stage is carried out in one supporting solution, and the anodic dissolution of the metals from the amalgam in another. The design and dimensions of the electrolyzer were described earlier [4]. A mercury-film electrode with a surface area of 0.16 cm² was used as the cathode [5]. Oxygen was removed from the solution by bubbling purified nitrogen through it [6].

As the background for the accumulation of copper and bismuth we used a solution of the lead sample in nitric acid. As the background for the anodic dissolution of bismuth, a 0.2 M hydrochloric acid solution was used, and for the anodic dissolution of copper, 0.25 M CH₃COONH₄ + 0.1 M H₂C₄H₄O₆. The technique of transferring the electrodes from one electrolyte to another makes it possible in this case to use the advantages of three backgrounds. The solubility of lead nitrate in water (dilute nitric acid) is sufficiently high and amounts to 0.5 g/ml. The potential of the anodic notch of lead in this background is −0.40 V (vs. SCE). Therefore, at a preliminary-electrolysis potential equal to −0.35 V (vs. SCE), the comparatively small amount of lead reduced on the electrode does not hinder the recording of the anodic notches of copper and bismuth during anodic dissolution of the amalgam.

When the transfer technique is used, special attention should be paid to the washing solution for the electrodes. Lead cations form, with tartaric and hydrochloric (>0.1 M) acids, films insoluble in these backgrounds on the surface of the mercury, which passivate the mercury electrode. As a result, upon repeated transfers of the mercury electrode, the height of the anodic notch of copper and bismuth decreases almost to zero. Therefore, as the washing solution for the electrodes in the determination of copper, 0.1 M ammonium acetate was chosen, and in the determination of bismuth, a 0.05 M hydrochloric acid solution.

With transfer, the completeness of the determination of bismuth (%) is: 98; 102; 94; 85, and of copper: 96; 103; 95; 105; 100. Thus, the losses of copper and bismuth during transfer do not exceed 10–15%, which is quite satisfactory, considering the required accuracy of the analysis. On the basis of the investigations carried out, the following procedure is proposed for the determination of copper and bismuth in lead without separation of the matrix.

Description of the determination

Into a quartz beaker of 6–7 ml capacity, 0.3 g of finely ground lead is placed and dissolved in 3 ml of 6 M HNO₃ at 150–160° in an air bath. The solution is evaporated to dryness at the same temperature, and the lead nitrate is dissolved, with gentle heating, in 3 ml of 0.1 M HNO₃ in the same beaker. The beaker is cooled and placed in the electrolyzer. After a ten-minute removal of oxygen from the solution and air by passing nitrogen, the impurities are accumulated on the electrode at a potential of −0.35 V (vs. SCE). After completion of accumulation, the cover with the electrodes is raised upward so that the electrodes are located above the upper level of the beakers, but inside the electrolyzer. The nitrogen is not turned off. Between the row of positioned

between the cathode and the agar–agar bridge, so that the circuit is not broken. After turning the lid, the electrodes are lowered for 15–20 sec into 0.1 M CH₃COONH₄ to wash off Pb(NO₃)₂; the lid with the electrodes is again raised, turned, and the electrodes are lowered into the background solution for copper. In the same way, after accumulation, the anodic peaks of bismuth are recorded. In this case the background for recording the anodic bismuth peak is a 0.2 M hydrochloric acid solution. The copper and bismuth contents are evaluated by the method of additions of standard solutions.

The results of the statistical treatment for one batch of high-purity lead for copper and bismuth are given in Table 2. It is evident from the table that, at a copper and bismuth content in the investigated lead samples of about \(10^{-5}\%\), the error in determining copper, on the average from 16 determinations, is 10%, and the error in determining bismuth, on the average from 16 determinations, is 20%.

Table 2

Results of the analysis of lead for the content of copper and bismuth impurities. Background for anodic dissolution of copper: 0.25 M CH₃COONH₄ + 0.1 M H₂C₄H₄O₆, and for anodic dissolution of bismuth: 0.2 M HCl; \(A = 0.3\) g; \(L_{\mathrm{el}} = 0.35\) V (relative to the saturated calomel electrode); \(V = 3\) ml; \(\tau = 6\) min; \(i^* = 8 \cdot 10^{-9}\) A/mm; \(W = 200\) mV/min

Note. \(\bar{x}\) is the arithmetic mean (% impurity in the sample); \(S_{\bar{x}}\) is the root-mean-square error of an individual measurement; \(t_{0.95}\) is Student’s coefficient; \(E\) is the absolute error; \(E_1\) is the relative error; \(n\) is the number of experiments.

The sensitivity of the proposed determination procedure (the minimum determinable content of copper and bismuth in lead), with a peak height of 5–10 mm, electrolysis duration of 30 min, instrument sensitivity of \(3—4 \cdot 10^{-9}\) A/mm, solution volume of 3 ml, sample weight of 0.3 g, and stirring of the solution by nitrogen bubbling, is \(1 \cdot 10^{-6}—5 \cdot 10^{-7}\%\). In practice, because of the lack of sufficiently pure lead, reduced lead samples (down to 0.005 g) were taken in these experiments, but the impurity content is calculated for the full lead sample (0.3 g). The duration of the analysis (two parallel samples and one blank) is about 6 hours.

Conclusions

Using the technique of transferring the electrodes (changing electrolytes), an accelerated procedure has been developed for the amalgam-polarographic determination of Cu and Bi from a single lead sample without separation of the matrix. The sensitivity of the determination is \(1 \cdot 10^{-6}—5 \cdot 10^{-7}\%\). The duration of the analysis is 6 hours.

References

1. Edited by I. P. Alimarin. Methods of Analysis of High-Purity Substances. “Nauka,” 1965.
2. A. T. Stromberg, E. A. Stromberg, Zavodskaya laboratoriya, 27, 3, 1961; 30, 261, 1964.
3. T. A. Kryukova, T. V. Aref’eva, S. I. Sinyakova. Polarographic Analysis. Khimizdat, Moscow, 1959.
4. A. A. Kaplin. Dissertation, TPI, Tomsk, 1966.
5. V. A. Igolinskii. Dissertation, TPI, 1963.
6. A. G. Stromberg, Yu. N. Zhikharev. Zavod. labor., 31, 1185, 1965.