Ramesh Chandra and H. R. Jindal D. A. V. Colleae Jullundur, Panjab, India
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Specific Tests for Cations of Group II
I
Most published schemes of analysis for the group of rations which form sulfides insoluble in acidic solution ("Group 11") call for a separation into subgroups. The authors have found that it is possible to take advantage of certain specific chemical behavior of the cations to develop a rapid scheme to find each metal in the single solution containing the whole group. This type of problem can be used as a student assignment. The suggestions here presented are illustrative; details of procedure will be supplied to interested readers by the authors, upon request. Mercury: In acidic solution, mercury(I1) will deposit on a copper foil after a brief contact. Arsenic, antimony, and bismuth ions are reduced to a metallic deposit very slowly. Thus the test is specific.' Lead: Lead dioxide oxidizes benzidine to "benzidine blue." Only bismuth interferes. The interference due to bismuth is eliminated if the test is carried out with the alkaline e x t r a ~ t . ~ Bismuth: I n the reduction of bismuth hydroxide by alkaline sodium stannite, only mercury and copper interfere. HgClz (bp 277'C) can be eliminated from the solution by ignition on a porcelain chip. The interference due to copper, if not present in a disproportionate excess, is slow and insignificant so that the addition of KCN is not usually necessary.
Copper: All metals except copper, cadmium, and arsenic will form precipitates with aqueous ammonia. Cadmium, arsenic, and the low concentration of mercury present do not interfere with the detection of copper as the ferrocyanide in acidic solution. Cadmium: If the above ammoniacal solution is acidified well with 5 M HC1 and heated with an adequate amount of zinc mercaptide of toluene-3,4dithiol, mercury and copper will precipitate completely, whereas cadmium remains in so1ution.a Arsenic is removed incompletely. Cadmium is therefore precipitated as CdS in ammoniacal solution in which arsenic does not precipitate as the sulfide. Arsenic: Concentrated nitric acid converts Asz& into HAs08, which gives yellow precipitate with ammonium molybdate. Antimay: The well-known Rhodamine B test for antimony (V) is specific, if benzene is used to extract the violet ~omplex.~ Tin: I n acidic solution, tin(1V) will be reduced to tin(I1) by metallic iron which precipitates copper, etc. The resulting tin(I1) gives a silky white precipitate with HgCI,. No cation of the group interferes with the test. Other specific tests5 have proved to be satisfactory. CLARK,R. E. D.,
AND
NEVILLE,R. G., J. CHEM.EDUC.,
36, 393 (1959).
CHARLOT, G ~ NET, AL., "Rapid Detection of Cations," 2nd ed., Chemical Publishing Company, New York, 1954, p. 30. FEIGL,FRITZ,"Spot Tests, Inorganic Applications," 4th ed., Elsevier Publishing Co., Amsterdam, 1954, p. 68.
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' CHARMT,GASTON, op. it., p. 65.
FEIGL,F., op. it., p. 104; CHARLOT, GASTON, op. it., p. 67; or BURNS,R. L.,AND GRUEN, F. M., J . CHEM.EDUC.,38.41lL11 (1961).
