N. Isenberg, H. J. Kreger
and RT H. ~ i d d l e t o n University of Wisconsin Kenosha Campus
I
Erron in the Detection of Cadmium Light-colored copper-thioacetamide complexes
One of the common mistakes in the analysis of the copper-arsenic group occurs when students report the presence of cadmium because they observe the formation of a light-colored cloudiness in solutions containing copper(I1) ions when they add thioacetamide. The inexperienced analyst may erroneously identify this light-colored material as cadmium sulfide. This mistake is likely to be made with analytical schemes which do not use cyanide ions to complex the copper(I1) ions and which call for the removal of the copper(I1) ons before cadmium sulfide is precipitated. In such a procedure the precipitation of copper(I1) sulfide may be achieved by forming the chlorocadmate ion with hydrochloric acid and by adding thioacetamide to this solution. As soon as the thioacetamide is added to an acidic solution containing copper(I1) ions, a light-colored cloudiness can be observed. The same cloudiness is also observed when thioacetamide is added to basic and neutral solutions which contain copper(I1) ions. However, when acidic, basic, or neutral solutions containing copper(I1) ions are treated with hydrogen sulfide, black copper(I1) sulfide is precipitated immediately and no lighbcolored material is seen. When the light-colored reaction mixture containing thioacetamide and copper(I1) ions is heated in a water bath, the color changes gradually to dark yellow, then to brown, and finally to black. The light-colored cloudiness may also be observed with analytical schemes which call for the addition of cyanide ions to prevent the precipitation of copper sulfide. In this case, however, the error occurs only when an insufficient amount of cyanide is added. Addition of cyanide to reaction mixtures which have turned cloudy will result in a clear solution. Thus it is important to take adequate precautions in the identification of cadmium in the presence of copper.
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I n the schemes which use cyanide ions one must make certain to add enough cyanide to reduce all the copper(11) ions to copper(I), and to form the dicyano cuprate (I) complex before one preceeds with the thioacetamide treatment. When cyanide is not used in the analysis, the formation of a lighecolored cloudiness is almost a!ways observed before black copper sulfide precipitates. Here it is important to allow sufficient time for the formation of the copper sulfide and to remove it from the reaction mixture before proceeding with the test for cadmium. I n analytical procedures using low concentrations of copper(I1) ions (0.02 M) the light-colored cloudiness is presumably due t o the formation of sulfur,' which is formed from sulfide when copper(I1) is reduced to copper(1). With greater conceutrations of copper(I1) ions (0.75 M), it is also possible to isolate a copper(1) thioacetamide complex.', 2, a The reduction of copper(I1) to copper(1) is of interest because several complexes of copper(1) have been reported in which copper in its lower oxidation state is more stable than in its +2 oxidation state.4 This reduction of copper(I1) to copper(1) may thus yield free sulfur and a copper(1) thioacetamide complex. The following observations indicate that complex formation is involved in our work: A white solid could be obtained from our reaction mixture when thioacetamide was added to a solution
Swrm, E. H., A N D ANSON,F.C., Talanla, 3,296 (1960). CLIFPORD,A. F., Ynorganic Chemistry of Q~talitative Analysis," Prentice-Hall, Ine., Englewood Cliffs, N.J., 1961, pp. 1
1
14% 412.
containing copper(I1) ions. This solid was washed and was clissolvcd in hot dilrlte nitric acid. Addition of an excess of an~n~onium hydroxide to this nitric acid solution resultcd in a blue color. Some of the solid mat,erial was transferred to a slightly acidic solution after washing. The mixture was
heated and a black precipitate fonned. This precipitate was soluble in hot dilute nitric acid. Addition of ammonium hydroxide to this nitric acid solution resulted in a blue color. The precipitate turned black in ainmonium hydroxide."
Volume 43, Number 8, August 1966
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