STUDIES ON THE DEVELOPMENT OF A METHOD FOR DETERMINING THALLIUM IMPURITY IN HIGH-PURITY CADMIUM BY THE APA METHOD

T. I. Semenyak, V. E. Katyukhin, A. A. Kaplin

(Presented by the scientific and technical seminar of the Department of Physical and Colloid Chemistry)

Recently, a number of articles have appeared in the literature on the determination of thallium in cadmium and in products of its manufacture by polarographic methods with accumulation.

In works [1—3], thallium was first separated from the matrix by extraction and then polarographed in acidic or neutral supporting electrolytes. In work [4], thallium was determined without separation of the matrix, after first binding cadmium into a complex with EDTA.

In developing a method for determining thallium in high-purity cadmium, we tested the most commonly used supporting electrolytes. The greatest difference in potentials between the peaks of lead, thallium, and cadmium ($\Delta \varphi_{\mathrm{Cd—Tl}}$ is 0.19 V; $\Delta \varphi_{\mathrm{Pb—Tl}}$ is 0.27 V) is observed in a 1 M KOH solution.

In addition, at this alkali concentration cadmium does not yet precipitate as the hydroxide.

In a 1 M KOH solution, dependences were recorded for the oxidation current of thallium ions on the electrolysis potential, on the concentration of thallium ions, and on the electrolysis time. From these dependences it follows that the optimum accumulation potential for thallium is $-0.8$ V; under these conditions cadmium is reduced at the electrode only insignificantly if its amount in solution is less than $1 \cdot 10^{-3}$ g. If, however, the amount of cadmium in solution is greater than $1 \cdot 10^{-3}$, then a comparatively large amount accumulates on the electrode, which interferes with the determination of thallium.

In this supporting electrolyte, the influence of excess amounts of lead and cadmium on the determination of thallium was also studied. It was shown that a 30-fold excess of lead and a ten-thousand-fold excess of cadmium do not interfere with the determination of thallium.

In the course of the studies it became clear that, at thallium contents lower than $5 \cdot 10^{-8}$ g, the thallium peaks decrease gradually with subsequent electrolyses. The causes of this phenomenon, in our view, may be:

1) interaction of thallium with the electrode substrate;
2) formation of intermetallic compounds of thallium with other metals in mercury;
3) shielding of the electrode surface by a precipitate of thallium(III) oxide hydrate, which forms as a result of the anodic process.

To confirm the presence of the first of the indicated causes, electrodes of various designs were tested: a mercury drop on a platinum contact, a mercury drop and a mercury film on a silver substrate, and graphite and mercury-graphite electrodes. On all electrodes, thallium

decreases from electrolysis to electrolysis. Consequently, it may be concluded that no interaction of thallium with the electrode substrate occurs.

The possibility of the formation of intermetallic compounds of thallium with cadmium and lead (as the most probable elements) was studied by the method described in [5]. The results of the experiments showed the absence of intermetallic-compound formation between these elements.

To confirm the possibility of the appearance of Tl(III) ions in solution, we carried out experiments to determine the number of electrons participating in the electrode process during the oxidation of thallium. It was found that thallium is oxidized according to both a one-electron and a three-electron scheme. Consequently, the formation of a sparingly soluble film of Tl(OH)\(_3\) on the electrode is not excluded.

In order to prevent the formation of such a film, electrolysis was carried out in alkaline and neutral media with the addition of hydrazine hydrate, which is a reducing agent; however, positive results could not be obtained either in alkaline (1 M KOH) or in neutral (1 M NH\(_4\)F) solutions. In addition, the possibility of removing the hydroxide film from the electrode in acids was investigated. For this purpose, after each electrolysis the electrode was placed in acid solutions of various concentrations (0.1–3 N) and held at a potential of −2.5 V for 30–120 sec. After rinsing the electrode in bidistilled water, electrolysis was again carried out and the thallium peak was recorded on the polarogram. But in this case as well, it was not possible to prevent the decrease in the peaks.

The possibility of determining thallium in nonaqueous media using solvents not containing (OH\(^{-}\)) ions was investigated. Tetraethylammonium iodide in DMFA was used as the supporting electrolyte, but the sensitivity of thallium determination in this supporting electrolyte proved insufficient.

The possibility of determining thallium without separating the matrix was investigated. In this case, the cadmium peak was expected to be suppressed by organic reagents proposed by Loshkarev and co-workers [6]. The experiments showed that suppression of cadmium is achieved (with tributyl phosphate and n-butanol), but in this case thallium either is not polarographed at all or is polarographed with low sensitivity.

Thus, the results of the above-mentioned studies do not make it possible to indicate unambiguously the cause of the nonreproducibility of the anodic peaks of thallium in alkaline and neutral supporting electrolytes.

Subsequently, when an extraction method was used to separate the matrix, the thallium peaks were satisfactorily reproducible. The cause of the phenomena under discussion will probably be established in subsequent work.

In developing a procedure for determining thallium in cadmium, we initially used the vacuum-distillation method as a method for separating cadmium and thallium. However, we were unable to obtain reproducible results, which is explained by the shortcomings of the method for separating the matrix. Therefore, extraction separation of cadmium and thallium was used. Thallium was extracted from a hydrobromic acid medium with diethyl ether [7].

We estimated the amount of cadmium remaining after extraction. It was found that it does not exceed \(10^{-6}\) g/ml. Such amounts of cadmium do not interfere with the determination of thallium in polarographic analysis.

The completeness of separation of thallium from cadmium was determined on synthetic mixtures. Thallium passes practically completely into the organic phase, while cadmium remains in the aqueous phase. The completeness of thallium extraction is 94–98%.

After the investigations carried out, an amalgam-polarographic procedure was developed for determining thallium in cadmium of special purity.

The essence of the procedure is as follows: a weighed portion of cadmium (0.5 g) is dissolved in HCl with the addition of HNO₃ (to accelerate dissolution). The solution is evaporated to dryness. Then 6 ml of HCl (1:2) is added to dissolve the salt obtained, 3 ml of HBr and 2 drops of Br₂ are added to create a stable hydrobromic-acid medium, and then thallium is extracted from this medium with diethyl ether (6 ml) and determined against the background of a 1 M KOH solution.

The results of the analysis of especially pure cadmium for its thallium content are given in Table 1.

Table 1

REFERENCES

1. Ropper R. Chem. Zvesti. 18, No. 5–6, 414, 1964.
2. Kominami Bunsiro. Japan Analyst, 18, No. 2, 253, 1969.
3. I. Levin, G. F. Rodina. Zh. analitich. khimii, 23, No. 9, 1315, 1968.
4. Temmerman E., Verbeek F. J. Electroanal. Chem., 19, No. 4, 423, 1968.
5. L. F. Zaichko. Candidate dissertation, Tomsk, 1968.
6. M. A. Poshkarev, Yu. L. Guzelenko, M. G. Efremov. Republican interuniversity scientific and technical collection “Chemical Technology,” issue 17, Publishing House of Kharkov State University, 1971.
7. Collection “Cadmium, Methods of Analysis.” GOST 12072.0-71 — GOST 12072.8-71. State Committee of Standards of the Council of Ministers of the USSR, Moscow, 1972.