STUDIES ON THE DETERMINATION OF \(5 \cdot 10^{-5}\)—\(10^{-7}\%\) IMPURITIES OF COPPER, LEAD, CADMIUM, BISMUTH, ANTIMONY, AND NICKEL IN CRYSTALS OF COMPLEX COMPOSITION BY THE APN AND AKPN METHODS

N. V. PODKORYTOVA, A. A. KAPLIN, N. T. RUD

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

The intensive development of quantum electronics places high demands on the quality of the materials used. The polarographic method of analysis is finding increasingly broad application for the determination of impurities in pure substances, especially its variants—the APN and AKPN methods [2—4].

Salts and crystals of composition \( \mathrm{KH_2PO_4} \) and \( \mathrm{CsH_2AsO_4} \) are new objects for analytical chemistry; there is one work on the chemical-spectral determination of a whole series of impurities in rubidium and cesium arsenates [1], with such determination being preceded by complicated and lengthy sample treatment and extraction concentration of the impurities. Solutions of water-soluble crystals and salts of the above composition can in most cases be used as backgrounds for the determination of impurities, which eliminates the stage of decomposition and additional treatment of samples.

The aim of the work is to carry out studies for developing procedures for the determination of \(5 \cdot 10^{-5}\)—\(1 \cdot 10^{-7}\%\) Cd, Pb, Cu, Bi, Sb, Ni in crystals of \( \mathrm{KH_2PO_4} \) and \( \mathrm{CsH_2AsO_4} \).

All studies were carried out on an OH-101 polarograph (Hungary). A circulating electrolyzer for the AKPN method [2] was used in the work. The cathode was a mercury film electrode on a silver substrate, and the reference electrode was a saturated calomel electrode.

As the background we investigated 0.1 M and 0.05 M solutions of the salts \( \mathrm{KH_2PO_4} \) and \( \mathrm{CsH_2AsO_4} \). For the determination of Cu, Pb, Cd, a 0.1 M solution of the salts was selected, and for the determination of Bi, Sb—the same solutions acidified with hydrochloric acid to 0.25 M.

It was established that the potential for the simultaneous determination of Cu, Pb, and Cd in solutions of the salts \( \mathrm{KH_2PO_4} \) and \( \mathrm{CsH_2AsO_4} \) is the potential \(\varphi = -1.7\ \mathrm{V}\), and for Bi, Sb—\(\varphi = -0.5\ \mathrm{V}\).

A study of the dependence of the magnitudes of anodic currents on the electrolysis time showed that after a certain time (more than 7 min) depletion of the solution begins to have an effect. Studies on the determination of Ni by the AKPN method were described earlier [3]. The dependences of the magnitudes of the anodic currents of the elements on their concentration in solutions of the salts \( \mathrm{KH_2PO_4} \) and \( \mathrm{CsH_2AsO_4} \) are linear.

The size of the weighed portion was selected on the basis of the solubility of the salts and the required sensitivity of determination of the elements. Studies showed—

showed that a 0.2 g weighed portion of the sample being analyzed ensures determination of the above-mentioned elements with the required sensitivity and practically does not distort the shape of their cathodic and anodic peaks, with the exception of the cathodic peaks of nickel against the background of CsH₂AsO₄; upon addition of this salt, the cathodic peaks of nickel decrease owing to the rapid formation of a sparingly soluble precipitate.

For analysis of CsH₂AsO₄ salt for nickel content, a procedure was used involving decomposition of the salt with HBr and subsequent removal of As(III) in the form of AsBr₃ at \(t = 160\text{--}180^\circ\mathrm{C}\).

Course of analysis of KH₂PO₄

Into a clean quartz beaker, 5 ml of supporting electrolyte is added (0.1 M KH₂PO₄ solution). A 0.2 g weighed portion of the salt is dissolved in the supporting electrolyte with gentle heating. Argon is passed through the solution for 10 min; then electrochemical preconcentration of the metals is carried out at \(\varphi = -1.7\ \mathrm{V}\), \(\tau = 3\) min. The peak potential of Cd is \(-0.56\ \mathrm{V}\); Cu—\((+0.05)\); Pb—\((-0.43)\ \mathrm{V}\).

For determining Bi and Sb, 2 drops of concentrated HCl are added to the analyzed solution, and preconcentration of these elements is carried out at \(\varphi = -0.5\ \mathrm{V}\) for 5 min. The potential of the anodic peak of Bi is \((-0.07)\ \mathrm{V}\), and that of Sb is \((-0.20)\ \mathrm{V}\). The solution is then evaporated to dryness at \(t = 120\text{--}130^\circ\mathrm{C}\); 4 ml of 0.1 M KCl solution and 1 ml of 0.1 M KSCN solution are added to the residue, and the solution is polarographed for nickel at \(\varphi = -1.7\ \mathrm{V}\), \(\tau = 2\) min; the dissolution stage is carried out in the capillary after the pump is switched off; the dissolution potential is \(\varphi = 0.01\ \mathrm{V}\), \(\tau_{\mathrm{p}} = 1\) min. Recording of the cathodic polarogram is begun at \(\varphi = -0.25\ \mathrm{V}\) after holding the electrode at this potential for 1 min; the potential of the cathodic peak of nickel is \((-0.58)\ \mathrm{V}\).

The content of Cu, Pb, Cd, Bi, Sb, and Ni in the sample is established by the method of additions of standard solutions. Several samples were analyzed. The sensitivity of the procedure for a 0.2 g weighed portion of salt is, for Pb—\(7 \cdot 10^{-7}\%\), for Cu—\(8 \cdot 10^{-7}\%\), Bi—\(1 \cdot 10^{-6}\%\), Ni—\(1.5 \cdot 10^{-6}\%\); analysis time is 3–2.5 h. The relative error of determination is Cu—18.2%, Pb—11%, Bi—8.8%, Ni—11.9%.

Course of analysis of CsH₂AsO₄ salt

A 0.2 g weighed portion of the salt is dissolved with gentle heating in 5 ml of 0.05 M CsH₂AsO₄ solution and placed in a cell. After oxygen is removed with argon, electrochemical accumulation of the metals is carried out at \(\varphi = -1.8\ \mathrm{V}\), \(\tau = 3\) min. The peak potential of Pb is \(-0.43\ \mathrm{V}\), Cu—\(+0.05\ \mathrm{V}\), Cd—\(-0.57\ \mathrm{V}\). For determining Bi and Sb, 2 drops of concentrated HCl are added to the analyzed solution and preconcentration of Bi is carried out at \(\varphi = -0.2\ \mathrm{V}\) for 5 min. The potential of the anodic peak of Bi is \(-0.07\ \mathrm{V}\). After this, the solution is evaporated to dryness at \(t = 120\text{--}130^\circ\mathrm{C}\). To the dry residue, 2 ml of concentrated HBr is added and evaporated to dryness at \(t = 160\text{--}180^\circ\mathrm{C}\); the operation is repeated twice after adding 1 ml portions of concentrated HBr. To the resulting residue, 1 ml of HNO₃ is added and, after evaporating the residue to dryness, nickel is determined. For this purpose, 4 ml of 0.1 M KCl solution and 1 ml of 0.1 M KSCN solution are added to the residue. Oxygen is removed with argon for 10 min; electrolysis of nickel is carried out at \(\varphi = -1.8\ \mathrm{V}\), \(\tau = 2\) min, and dissolution at \(\varphi = 0.01\ \mathrm{V}\), \(\tau = 1\) min. Recording of the cathodic polarogram is begun at \(\varphi = -0.25\ \mathrm{V}\) after holding the electrode at this potential.

of the electrode at this potential for 1 min. The potential of the cathodic Ni peak is \(\varphi=-0.5\ \mathrm{V}\). The content of Cu, Pb, Cd, Bi, and Ni in the sample is established by the method of additions of standard solutions.

Several samples were analyzed. The sensitivity of the procedure, with a weighed portion of \(0.2\ \mathrm{g}\) of salt, is: for Pb—\(1.5\cdot10^{-7}\%\), Cu—\(1\cdot10^{-6}\%\), Bi—\(1\cdot10^{-6}\%\), Ni—\(2\cdot10^{-6}\%\). The analysis time is 3.5 hours. The relative error of determination is: Cu—8.8%, Pb—20%, Bi—9.3%, Ni—28.8%.

Conclusions

1. Studies have been carried out to select the optimum conditions for the determination of Cu, Pb, Cd, Sb, Bi, and Ni in \(\mathrm{KH_2PO_4}\) and \(\mathrm{CsH_2AsO_4}\) crystals.

2. An amalgam-polarographic procedure has been developed for the determination of \(5\cdot10^{-5}\)—\(10^{-7}\%\) Cu, Pb, Ni, Bi, Cd, and Sb in salts and crystals of \(\mathrm{KH_2PO_4}\) and \(\mathrm{CsH_2AsO_4}\).

LITERATURE

1. Yu. A. Fedyashina, I. G. Yudelevich, T. T. Strokina. ZhAKh, 21, 10, 1232—1235, 1966.

2. V. I. Kuleshov. “Zavod. lab.”, 950, 1967.

3. V. I. Kuleshov. Dissertation. Tomsk Polytechnic Institute, Tomsk, 1969.

4. L. M. Artemova, Yu. I. Ivanov, E. I. Koroleva, V. I. Kuleshov, N. V. Podkorytova. In: “Preparation and Reduction of Ores” (Sibelectrostal Plant). Metallurgiya Publishing House, issue 2, 208, 1971.