SEPARATION OF ARSENIC AND COPPER IN QUALITATIVE ANALYSIS JOHN T. STOCK University of Connecticut, Stom
THATammonium polysulfide dissolves cupric sulfide slightly is well known (4); some copper may thus pass into Group IIB of the usual sulfide scheme of qualitative analysis. Less usual and hence disconcerting is the extensive, or even complete, dissolution of a copper-containing sulfide precipitate. This may occur when copper and arsenic are present together. The enhanced solubility of cupric sulfide in ammonium ploysulfide, caused by presence of the arsenic sub-group, has been known for over a century (6, 6). Davies and Monro studied the effect quantitatively by precipitating 200-mg. portions of copper and variable amounts of arsenic with hydrogen sulfide (1). They treated the precipitate with ammonium polysulfide of approximate composition (NH&SZ and determined as thiocyanate the copper dissolved out with the arsenic. Their data show that the weight of copper extracted is markedly dependent upon the amount of arsenic present. Working with 0.5-ml. portions of an "unknown" solution which was about 0.05 M with respect to each copper(I1) and arsenic(III), it was noticed that when a dark sulfide precipitate was obtained, behavior with ammonium polysulfide was satisfactory. In many cases, however, students obtained a brown or buff precipitate; this form always dissolved extensively in ammonium polysulfide and possibly contains copper as a thioarsenite rather than as sulfide. I n an attemot to exolain the erratic results., hvdro" chloric acid addedto 0.5-ml, portions of the Uunknown" t o give acidities ranging from 0.2 to 2 M. Each portion was heated to-incipient boiling, then carried t o the fume hood and treated with hydrogen
sulfide. Persistence of erratic behavior showed that the acidity is not the controlling factor. Davies and Monro do not indicate the temperature at which their hydrogen sulfide precipitations were performed. Further 0.5-ml. portions of the solution were therefore adjusted to a fixed acidity of 1 M; each portion was then maintained a t a selected temperature during treatment with hydrogen sulfide. At 95" or above, the precipitate was invariably dark and behaved quite satisfactorily in the subsequent extraction with ammonium polysulfide. I t is thus clear that, when copper and arsenic are likely t o be present together, to merely heat the solution before treatment with hydrogen sulfide is inadequate. Inevitable cooling during transport to the fume hood invites erratic results, especially when operating on a small scale. The 75- X 10-mm. test tube containing the solution should be placed in boiling water, so that treatment with hydrogen sulfide may be performed a t or near 100". Use of a simple pumptype microgenerator (2,s)eliminates any risk of ignition of hydrogen sulfide by the flame used for heating. Charles R. Green and Jule A. Miller carried out most of the experimental work. LITERATURE CITED (1) D ~ W E Sc., , A. D. MONRO, J. them. Sac.,1927,2385, (2) HEATH,P., Analyst, 79,787(1954). (3) STOCK, J. T., m n P. HEATH.Analyst, 76,496(1951). (4)STOCK; J. T:, AND P. HE.&, " s i d i - 8 ~ i l eInorganic Qualitative .4nalysis," Chemical Publishing Co., New York,
- *a. p. sTORCH, L., B ~ 17.,?,
"0
~16, . ,2016 (1883). (6)W0~1.~~,F.,Annalen,34,236(1840). (5)
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