Sulfide acid-insoluble

Group II Pb2+, Cu2+, Hg2+, Cd2+, Bi3+, and Sn4+ After the insoluble chlorides have been removed, the solution is treated with H2S to precipitate the cations of group II as insoluble sulfides—PbS, CuS, HgS, CdS, Bi2S3, and SnS2. Because the solution is strongly acidic at this point ([H30 + ] 0.3 M), only the most insoluble sulfides precipitate. The acid-insoluble sulfides are then removed from the solution. 

The optimum conditions for forming the acid and ammoniacal sulfide precipitates were also studied. Whether a moderate or a large excess of thioacetamide is added appears to have little or no effect on the completeness of precipitation. Precipitation of the acid-insoluble sulfides occurs slowly at room temperature however, precipitation may be hastened by heating the solution to just below boiling. To ensure complete precipitation and an easily filtered precipitate, the solution should be filtered only after the residue has settled foir several hours or overnight. An examination of the filtrate showed complete removal of antimony, arsenic, bismuth, cadmium, copper, and lead only molybdenum was not completely removed. 

The precipitates from the two aliquots cannot be combined since traces of acid-insoluble sulfides are inevitably occluded with the am-moniacal precipitate and vice versa. 

Acid-insoluble sulfides CuS, CdS, Bi2S3, PbS, HgS, AS2S3, Sb2S3, SnS2... 

Group 2. Acid-insoluble sulfides After any insoluble chlorides have been removed, the remaining solution, now acidic, is treated with H2S. Only the most insoluble metal sulfides—CuS, 61283, CdS, PbS, HgS, AS2S3, Sb2S3, and SnS2—precipitate. (Note the very small values of for some of these sulfides in Appendix D.) Those metal ions whose sulfides are somewhat more soluble— for example, ZnS or NiS— remain in solution. 

Group 3. Base-insoluble sulfides and hydroxides After the solution is filtered to remove any acid-insoluble sulfides, it is made slightly basic, and (NH4)2S is added. In basic solutions the concentration of is higher than in acidic solutions. Thus, the ion products for many of the more soluble sulfides are made to exceed their K p values and precipitation occurs. The metal ions precipitated at this stage are Al +, Cr Fe +, Zn +, Ni +, Co +, and Mn + (The Al +, and Cr ions do not form insoluble sulfides instead they precipitate as insoluble hydroxides, as Figure 17.23 shows.)... 

When testing for molybdenum in mixtures containing metal salts, it is best to boil the test material with sodium carbonate or to fuse it with potassium sodium carbonate, to form water-soluble alkali molybdate. If a mixture of acid-insoluble sulfides is being tested, the sulfur can be roasted off and the alkali molybdate then dissolved out by digesting with alkali hydroxide. 

A separation of from the other acid-insoluble sulfides is based on the in-solubihty of HgS in 2-M HNO3 (separation from Cu, Cd, Pb and Bi) and in (NH4)2S (separation from Sn, As, Sb etc.). After the sulfide is dissolved in aqua regia or HCl plus CIOs and the excess oxidant is decomposed, mercuiy is confirmed by adding SnCU, which gives Hg2Cl2] (white) or Hg (black) or a mixture... 

Reduced chalcogens. Phosphoric acid, being a very weak oxidant, is not reduced by any of the reducing acids. The phosphates of metals forming acid-insoluble sulfides are transposed by H2S alkali sulfides transpose these and many other phosphates. The metal sulfide remains as a precipitate except with Cr or Al, which form hydroxides. Phosphoric acid or a phosphate will be found in the solution. 

The solubility of the As sulfides in yellow (NH4)2S2 separates As with Sn and Sb from the other (acid-insoluble) sulfides, oxidizing the AS2S3 ... 

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