Hydrogen sulfide selective precipitation

Group II consists of six different cations, all of which form very insoluble sulfides (Figure A). These compounds are precipitated selectively by adding hydrogen sulfide, a toxic, foul-smelling gas. at a pH of 0.5. At this rather high H+ ion concentration. 0.3 M, the equilibrium... 

Sulfides with widely different solubilities and solubility products can be selectively precipitated by adding S2 ions to the solution removed from the chlorides in the first step (see Fig. 11.20). Some metal sulfides (such as CuS, HgS, and Sb2S3) have extremely small solubility products and precipitate if there is the merest trace of S2" ions in the solution. Such a very low concentration of S2 ions is achieved by adding hydrogen sulfide, H2S, to an acidified solution. A higher hydronium ion concentration shifts the equilibrium... 

A popular source of sulfide ions is hydrogen sulfide gas. This is a versatile reagent capable of precipitating a number of metal sulfides, some of which can be precipitated selectively from solution by suitably adjusting the solution pH. In the aqueous medium the following reactions take place by which hydrogen sulfide releases, in two stages, sulfide ions to the medium ... 

L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen, J.W. Geus, Production of nickel-on-alumina catalysts from preshaped bodies K.P. de Jong, Deposition-precipitation onto preshaped carrier bodies — possibilities and limitations P.J. van den Brink, A. Scholten, A. van Wageningen, M.D.A. Lamers, A.J. van Dillen, J.W. Geus, The use of chelating agents for the preparation of iron oxide catalysts for the selective oxidation of hydrogen sulfide, all in Preparation of Catalysts V, Elsevier, Amsterdam, 1991. 

Separation of clement traces is also possible with precipitation reactions. This technique permits rapid and extensive concentration in a relatively uniform matrix, but it is not very selective. Aluminum hydroxide, magnesium hydroxide, iron hydroxide, and hydrogen sulfide have all been used for trapping trace amounts of various elements[101], [118]-[I20], as have such organic precipitating agents as thionalide, cupferron. and dithiocarbamate [101]. [121]. [122]. A subsequent determination is carried out either directly on the separated precipitate or after restoring it to solution [118]. 

There have also been many efforts to grow particles in multiphase polymer systems, such as block copolymers. For instance, a two-phase polymer can be soaked in one reagent, which selectively absorbs into one phase. A cadmium salt could be taken up by a polyether phase. Subsequent treatment with hydrogen sulfide results in precipitation of cadmium sulfide within the polyether. While the volume fraction of sulfide formed is quite small, repeated cycles can give rise to higher fractions of the mineral. Work on precipitation in lyotropic liquid crystalline amphiphile solutions has led to composites structures. 

Hydrogen sulfide used to have importance in analytical chemistry for well over a century, in the qualitative inorganic analysis of metal ions. In these analyses, heavy metal (and nonmetal) ions (e g., Pb(II), Cu(II), Hg(II), As(III)) are precipitated from solution upon exposure to H2S. The components of the resulting precipitate redissolve with some selectivity. 

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