DETECTION OF CADMIUM IONS IN THE PRESENCE OF COPPER IONS

V. M. Aksenenko

(Presented by Professor, Doctor of Chemical Sciences A. G. Stromberg)

In the course of systematic qualitative analysis by the hydrogen sulfide scheme, cadmium and copper ions are separated in ammoniacal solution after the separation of lead and bismuth. A number of methods have been proposed for detecting cadmium ions in the presence of copper ions. These methods provide either for the separation of copper or for binding it into a complex. A method has found application in which the solubility of cadmium sulfide in hot 1 N HCl is used. Among separation reactions, the precipitation of copper by the action of potassium iodide is known [1]:

\[ 2\mathrm{CuCl}_2 + 4\mathrm{KJ} = \mathrm{Cu}_2\mathrm{J}_2 + \mathrm{J}_2 + 2\mathrm{KCl}. \]

After separation of the precipitate \(\mathrm{Cu}_2\mathrm{J}_2\), the solution is neutralized with ammonia and cadmium is detected with hydrogen sulfide or sodium sulfide. Copper can be separated from cadmium by reducing it to the elemental state through the action of a suitable metal in an acidic medium.

As complexing agents for copper in analytical practice, potassium cyanide is used; in its presence copper ions are not precipitated by hydrogen sulfide. A reaction has been proposed for detecting cadmium in the presence of copper in an acidic medium from a thiourea complex [2].

We propose a very simple and rapid method for detecting \(\mathrm{Cd}^{2+}\) in the form of \(\mathrm{CdS}\), based on the quantitative precipitation of copper ions by hydrazine sulfate from a weakly acidic solution and the subsequent separation of yellow cadmium sulfide

\[ \mathrm{CuSO}_4 + (\mathrm{N}_2\mathrm{H}_4)_2 \cdot \mathrm{H}_2\mathrm{SO}_4 = \mathrm{CuSO}_4(\mathrm{N}_2\mathrm{H}_4)_2 \cdot \mathrm{H}_2\mathrm{SO}_4. \]

Reichinstein proposed this reaction for the determination of copper [3]. However, his work contains no information on the interaction of hydrazine sulfate with ions of cadmium, lead, and bismuth, nor are there indications of the possibility of separating cadmium and copper. We studied the action of hydrazine sulfate on \(\mathrm{Cd}^{2+}\), \(\mathrm{Pb}^{2+}\), and \(\mathrm{Bi}^{3+}\) over a wide range of concentrations from 0.001 to 2 M solutions. It proved that \(\mathrm{Cd}^{2+}\) is not precipitated by hydrazine sulfate from either very dilute or concentrated solutions. Lead ions are precipitated by the action of hydrazine sulfate, but not sufficiently completely. In all experiments, lead was detected in the filtrate by the action of sodium sulfide. Bismuth ions, to a still greater extent than lead, remain—

remains in the filtrate under the action of hydrazine sulfate. Thus, of the three cations of the copper subgroup—$Cd^{2+}$, $Pb^{2+}$, $Bi^{3+}$—only $Cd^{2+}$ remains completely in the filtrate under the action of hydrazine sulfate. This property of cadmium was used by us to separate copper from it, followed by detection of cadmium in the form of CdS. $Pb^{2+}$ and $Bi^{3+}$ must be absent. We also investigated the conditions for the most complete and rapid separation of $Cu^{2+}$ from $Cd^{2+}$. From a sulfuric-acid solution, copper is completely and rapidly precipitated by hydrazine sulfate even from boiling solutions. In the presence of hydrochloric and nitric acids, complete precipitation of copper occurs only after the solution has cooled.

Procedure. The ammoniacal solution of $Cu^{2+}$ and $Cd^{2+}$ obtained in the course of the analysis after separation of lead and bismuth ions is neutralized with 2 N $H_2SO_4$; solid hydrazine sulfate is added, and the mixture is heated on a water bath in a centrifuge tube with stirring. Copper ions very rapidly separate as a blue precipitate. If the copper has been completely precipitated, then on cooling the analyzed mixture a white precipitate of hydrazine sulfate forms above the blue precipitate. After centrifuging the solution, without filtering off the precipitate, a sodium sulfide solution is added to the centrifugate. In the presence of cadmium, yellow CdS is formed.

LITERATURE

1. N. S. Krugovoi. Proceedings of the Commission on Analytical Chemistry of the Academy of Sciences of the USSR, III (IV), 325, 1951.

2. S. M. Efros. Proceedings of the Leningrad Technological Institute named after Lensovet, issue XXXV, 73, 1956.

3. Ts. G. Raikhinshtein. Zav. lab., 12, 1407, 1949.