Sulfide salts qualitative analysis

Hydrogen sulfide causes the precipitation of sulfides from many heavy-metal salts. The classical qualitative analysis scheme depends on precipitation of the sulfides of Hg, Ph, Bi, Cu, Cd, As, Sh, and Sn under acid conditions and the sulfides of Co, Ni, Mn, Zn, and Fe under ammoniacal conditions. 

The student should remember from qualitative analysis that white zinc sulfide dissolves readily in dilute HC1, that yellow cadmium sulfide may be dissolved in concentrated HC1, but that black mercuric sulfide (one of the least soluble salts known) requires the action of a trifunctional reagent such as aqua regia (Exercise 1). 

Structural data are available (Table 30) for a range of binary, ternary and quaternary sulfides of manganese, almost invariably Mn", and these set the scene for the structures to be expected in the compounds with the more discrete polyhedra.319 Indeed, the structural pattern is established in the simple binary compound MnS. Whereas, the stable modification of this (a-MnS) is green and has the cubic rock salt structure with [MnS6] octahedra, the well-known flesh-coloured precipitates of the qualitative analysis system are metastable / - and y-modifications, which have [MnS4] tetrahedra with respectively the zinc blende or diamond (cubic) and wurtzite (hexagonal) structures. And so, in the rest of the known solids, there are almost equal numbers of four-coordinate tetrahedra and six-coordinate octahedra with no other polyhedra having been detected. 

A problem with the first route is that even with excess of the thiolate, it is difficult to avoid some further reaction leading to formation of the sulfide (R2S). The secondary reaction can be avoided by starting with thiourea (la) and obtaining the thiol via the intermediate S-alkylisothiuronium salt (2) by subsequent hydrolysis (Scheme 2). The formation of the isothiuronium salt (2) depends on the relative instability of the thione ( C=S ) bond in thiourea (la), which therefore exists mainly as the enethiol form isothiourea (lb) which then reacts with the halide. S-Benzylisothiuronium salts are used in organic qualitative analysis for the characterisation of carboxylic acids with which they form nicely crystalline... 

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