STUDIES ON THE DETERMINATION OF \(10^{-7}\)—\(10^{-5}\)% NICKEL AND ZINC IN CADMIUM OF SPECIAL PURITY

N. V. PODKORYTOVA, N. I. BELOUSOVA, V. E. KATYUKHIN, N. A. PESHKOVA

(Presented by the scientific-methodological seminar of the Department of Physical Chemistry.)

The determination of impurities more electronegative than cadmium, as well as of those elements whose peaks are less than 0.2—0.3 V away from the cadmium peak, is impossible without separation of the matrix.

The principal methods used in separating cadmium from other elements are vacuum evaporation [1, 2], coprecipitation [3], electrolysis [4], and extraction [5, 6].

The aim of the present work is to develop procedures for determining nickel and zinc in cadmium of special purity, and also to establish the possibility of determining them after removal of cadmium by extraction and by evaporation in vacuum.

In the work an OH-101 polarograph (Hungary), an electrolyzer with insertable quartz cups, and a circulation electrolyzer for the AKPN method [7] were used. The working electrode was a mercury-film electrode on a silver substrate, and the reference electrode was a saturated calomel electrode.

Nickel was determined by the AKPN method [7, 8] in a supporting electrolyte consisting of 0.1 M solutions of KSCN and KCl salts in a ratio of 1:4. Zinc was determined by the APN method in a 0.1 M solution of \(\mathrm{NH_4F}\).

Preliminary studies showed that determination of \(10^{-5}\)—\(10^{-7}\)% zinc and nickel is possible only with separation of Cd. To remove the matrix, we tested an extraction procedure for separating iodide complexes of cadmium with tributyl phosphate (TBP) [5].

Studies on the coextraction of nickel and zinc with cadmium were carried out by the “introduced—found” method. The results of these studies are given in Table 1.

Table 1

Results of studies on the coextraction of Ni and Zn with Cd by TBP from 2.6 M HI \((n=4,\ \alpha=0.95)\)

On the basis of the studies carried out, the following procedure is proposed for determining nickel and zinc in cadmium of special purity. Since the determination of Ni is not interfered with by an approximately 10,000-fold excess of zinc, it is possible from a single cadmium sample (Table 2).

A 0.5 g sample of cadmium is dissolved in 5 ml of water and concentrated HNO₃ and evaporated to dryness at \(t = 120\text{–}160^\circ\)C. The residue is dissolved in 5 ml of water and quantitatively transferred into a separatory funnel. The required amount of distilled HI is added to the solution so that the HI concentration in the solution is \(\approx 2.6\) M, 10–12 ml of TBP is added, and the mixture is shaken for 1 min. The organic phase is discarded, and the aqueous phase is quantitatively transferred into a beaker and evaporated to dryness. Then 2 ml of HClO₄ is added to decompose organic residues, and the solution is evaporated until HClO₄ vapors are completely removed. The dry residue is dissolved in 4 ml of supporting electrolyte (0.1 solution of NH₄F) and polarographed for zinc.

Table 2

Analysis of cadmium samples for zinc and nickel content
(\(n = 4,\ \alpha = 0.95\))

The sensitivity of the procedure under the conditions developed is: Zn — \(2\cdot 10^{-5}\)%, Ni — \(4\cdot 10^{-7}\)%.

The electrolysis potential is \(-1.8\) V, and the electrolysis time is 1–3 min. The potential of the anodic zinc peak is approximately 1.0 V. Zinc is determined by the method of additions of standard solutions. To the solution containing the sample, 1 ml of 0.1 M KSCN solution is added and nickel is determined. After removal of oxygen with argon, electrochemical concentration of nickel is carried out at \(\varphi = -1.7\) V, \(\tau = 1\) min from the circulating solution (the electrode is located above the capillary). Removal of impurities of Zn, Pb, In and other metals, as well as Cd residues, also occurs in the circulating solution at \(\varphi = -0.3\) V, \(\tau = 1\text{–}0.5\) min, i.e., until the pen of the polarograph recorder leaves the anodic region. After the circulating pump is switched off, the electrode is lowered into the capillary, where dissolution occurs at \(\varphi = 0.0\) V, \(\tau = 1\) min. Recording of the cathodic polarogram is begun at \(\varphi = -0.25\) V; after holding the electrode at this potential for 1 min, the cathodic polarogram is recorded with a potential stop at \(-0.6\) V. The potential of the cathodic nickel peak is \(-0.58\) V. The nickel content in the sample is established by the method of additions of standard solutions. In addition, we tested the separation of cadmium from Ni impurity by vacuum distillation, which was carried out from a 0.5 g cadmium sample at \(t = 390\text{–}400^\circ\)C, at a pressure of \(10^{-1}\) mm Hg, for 1.5–2 h. Nickel is determined in the residue after distillation by the ACNP method according to the procedure described above. The results of the analysis of nickel in cadmium after its separation by distillation and extraction differ by no more than 4–5%, which makes it possible to conclude that both methods of matrix separation are suitable and effective for the determination of nickel. The polarographic completion is the same in both cases.

REFERENCES

1. L. Ya. Khlebnikov, V. N. Vasil’eva, L. A. Dvorzhetskaya. “Zavod. labor.,” 28, No. 1, 1962.
2. N. A. Poroshina, L. K. Larina, N. S. Belenkova — In: Methods of Analysis of High-Purity Substances, Moscow, “Nauka,” 400, 1965.
3. N. V. Grushina, V. I. Tseun, G. V. Khrapchenkova, M. I. Erdenbaeva, L. F. Kozin, ZhAKh, 21, 1966.
4. Temmerman E., Verlien F. Analyt. Chim. Acta, 50, No. 3, 505, 1970.
5. Jackwerth E. Z. analyt. Chem., 216, 73, 1966.
6. N. M. Kuz’min, V. S. Solomatin, A. N. Galaktionova, I. A. Kuzovleva. ZhAKh, 24, 5, 1969.
7. V. I. Kuleshov. Dissertation. Tomsk Polytechnic Institute. Tomsk, 1968.
8. V. I. Kuleshov. “Zavod. labor.,” 33, 950, 1967.