William H. Waggoner University of Georgia
Athens
A Nan-Cyanide Separation of Copper from Cadmium
A procedure has been developed for the separation of copper from cadmium which is based upon the observation that thiocyanate ion will precipitate copper but not cadmium from an acidic solution. To 1-2 ml of the ammoniacal solution remaining after the separatiou of bismuth, add 17 F HC2H802 until the solution is distinctly acidic. Treat this solution with solid KalSOa (approximately 0.1 g), add 4 6 drops of 3 F KH4CKS, mix thoroughly, and centrifuge. Formation of a white precipitate indicates the presence of copper in the sample. Test the supernatant liquid for completeness of precipitation by adding another drop of 3 F XHCKS. Decant the clear solution and pass in H2S for 1 min. A yellow precipitate indicates the presence of cadmium. In the above procedure, the acetic acid is added to decompose the Cu(I1) and Cd amrnines and to provide the hydrogen iou concentration necessary to liberate
SO2, which reduces Cn++ to Cu+. The copper is then precipitated as white CuCNS. Mellorl states "cupric salts with potassium thiocyanate give black cupric thiocyanate, Cu(CNS)%,which is slowly changed into white cuprous thiocyanate, CuCNS, or quickly on adding sulfurous acid." Under the conditions given here, the black Cu(I1) salt has never been observed. For the concentrations of cupric ion ordinarily present in qualitative analysis unknowns, the blue color of the cnpric ammine complex is generally sufficient as a confirmatory test. Absence of a pronounced coloration, however, does not exclude the possibility of Cu++ being present. The precipitation of CuCNS therefore can be used as a confirmation of copper as well as a separation where it is present in small amounts. MELWR, 3. W., "A Comprehensive Treatise on Inorganic and Theoretical Chemistry," Longmans, Green, and Ca., London, 1923, vol. 111, p. 103.
Volume 37, Number 8, Augusf 1960
/
41 1
