Precipitate ammonium sulfide group precipitates

Detection. — Uranium is precipitated in the Third Group and in the analysis undergoes numerous characteristic changes. (1) When present as a uranyl salt the addition of ammonia produces a precipitate of ammonium dim-anate, (NHOjUjOj, which appears as a yellow powder which is slowly transformed by ammonium sulfide to UO2S, dark brown, soluble both in HC1 and (NH<) 2C03. (2) When NaOH is added to a solution of a uranyl... 

Of all the ions in Groups 2 and 3 of the analytical scheme, only copper, cadmium, nickel and zinc give soluble complexes with ammonia. Zinc is the only one of these ions that yields a sulfide which is soluble in dilute sulfuric acid. Accordingly, when an ammoniacal extract of these ions is treated with ammonium sulfide, all these ions are precipitated as sulfides. On acidification with 2 N sulfuric acid, only the zinc goes into solution. 

Group VIII sulfides are obtained by the low-temperature precipitation described above. However, a preferred synthesis involves the direct sulfidation with hydrogen sulfide of ammonium hexachlorometallates. A good example is found in the synthesis of RuS2 and Rh2S3 (28). However, synthetic techniques and the solid-state chemistry of the Group VIII sulfides are not as well developed as those for the Group VII sulfides, and much research needs... 

If a drop of the test solution is added to a solution of a few eg ammonium molybdate in 50 % sulfuric acid, a blue color appears immediately due to the reduction of Mo i to Mo. Alkali sulfides react similarly but sulfites, even in large amounts, do not. For this reason, it may be assumed that free (unstable) H2S2O8 acts as a reductant towards Mo i through its SH groups. Sulfides can be eliminated by previous treatment of the test solution with a cadmium salt, to precipitate CdS, and centrifuging. 

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