Sulfide salts solubility

Sulfide (S ) is a bivalent monoanion produced from the decomposition of metal sulfide salts. It occurs in groundwaters, hot springs, and wastewaters. It is also formed from the bacterial reduction of sulfate. Sulfide salts in solid wastes in contact with an acid can produce hydrogen sulfide. H2S, which is highly toxic. In an aqueous sample, sulfide may be present as dissolved H2S and HS , dissolved metallic sulfide, and acid-soluble metallic sulfide contained in suspended particles. All these soluble and insoluble sulfides and dissolved H2S and HS together are termed as total sulfide. The sulfide remaining after the removal of suspended solids is termed the dissolved sulfide. Copper and silver sulfides are insoluble even under acidic conditions. Therefore, these two sulfides are not determined in the following tests. 

Since metal sulfide salts differ dramatically in their solubilities, the sulfide ion is often used to separate metal ions by selective precipitation. For example, consider a solution containing a mixture of 10 3 M Fe2+ and 10-3 M Mn2+. Since FeS (Ksp = 3.7 X 10-19) is much less soluble than MnS (Ksp = 2.3 X 10-13), careful addition of S2 to the mixture will precipitate Fe2+ as FeS, leaving Mn2+ in solution. 

Thus the most insoluble sulfide salts, such as CuS (Ksp = 8.5 X 10-45) and HgS (Ksp = 1.6 X 10 54), can be precipitated from an acidic solution, leaving the more soluble ones, such as MnS (Ksp = 2.3 X 10-13) and NiS ( Ksp = 3x10 21), still dissolved. The more soluble sulfides can then be precipitated by making the solution slightly basic. This procedure is diagrammed in Fig. 8.12. 

Stannous sulfide, SnS, is obtained as a dark brown precipitate by addition of hydrogen sulfide or sulfide ion to a solution of a stannous salt. Stannic sulfide, SnSo, is formed in the same way from stannic solution it is yellow in color. Stannic sulfide is soluble in solutions of ammonium sulfide or sodium sulfide, producing the sulfostannate... 

Thus, only sulfide salts with very low solubility products, such as CuS or HgS, will precipitate. 

At higher sulfide concentration, more soluble sulfide salts such as FeS and ZnS precipitate. 

Many sulfide salts react with acids to form gaseous hydrogen sulfide, H2S. The low solubility of H2S in water helps the reaction to proceed. 

FIGURE 12.10 Percentage uptake of the rice plant of from sulfide salts in relation to the solubility product constant. (Adapted from Engler, R. M. and Patrick, W. H., Jr., Soil ScL, 119, 217, 1975.)... 

Aluminmn s sulfide salt shows a high degree of solubility, so no precipitation occurs. But at the low pH of the test solution, where the concentration of the sulfate ion is very limited, cations with very small solubility products, e.g., lead sulfide, will precipitate. 

The selectivity of the method is given first by the ability to stay dissolved in a solution containing tartrate, second by the color of the sulfide precipitate, and finally by the fact that the sulfide salt dissolves in sodium hydroxide. The first property distinguishes it from bismuth(III) and the other cations forming insoluble oxides in neutral or alkaline solutions. But since the test does not show that a precipitate is formed in pure water, which dissolves when tartrate is added, all water-soluble cations are not excluded. So it should be viewed as a trick to facilitate dissolution only and not a part of the identification. The color of the sulfide precipitate is unique, and it is the most important criterion for a positive identification if there is any doubt when judging the result, preparing a positive control would be constructive. The solubility of the sulfide salt in sodium hydroxide is a characteristic shared with, for example, the sulfide salt of arsenate, and in classic inorganic separation the sulfide precipitate solubility in hydrochloric acid or polysulfide is used instead. " ... 

Group 2 cations form sulfide salts which are insoluble in dilute hydrochloric acid, HCl, solution. Be careful not to confuse these cations with the alkaline earth metal cations from group II in the periodic table which all form sulfide salts that are soluble even in water. We shall consider only Bi t, Cd2+, Cu2+, Pb2+, and Hg2+, though additional cations also form insoluble sulfide salts in dilute HCl solution. Assuming that Ag+ and <32 t and much of the Pb2+, have already been isolated (since they also exhibit this behavior), the group 2 cations are isolated from a mixture of cations because they precipitate black (except for yellow CdS), insoluble... 

Whereas some conducting polymers can be processed by synthesis via a soluble precursor, polyfphenylene sulfide) is soluble or can be melt-processed. Doping with AsFj (which can be accelerated by the presence of AsFj) forms a conductive polymer salt with o 1 S cm , but also tends to make the film brittle. 

Sulfide ion is often used to separate metal ions because the solubilities of sulfide salts span a wide range and depend greatly on solution pH. For example, Cu and Zn can be separated by bubbling H2S gas through an acidified solution containing these two... 

Any soluble sulfide salt plus any strong acid produces toxic and flammable H2S gas ... 

All the solubility products are low, certainly low enough to meet very strict emission legislation. Some salts, especially sulfides are especially low indeed, sulfide salt precipitation can lead to very effective effluent treatment. In practice however these limits are seldom achieved. The numbers in Table 14.5 are obtained in ideal laboratory conditions, for practical use they can only be used as a guide. Many other factors affect the efficiency of precipitation of these the most important is the presence of an excess of other ions or complexing agents. For example low levels of citrate can strongly affect iron precipitation. 

Sulfide ion is often used to separate metal ions because tiie solubilities of sulfide salts span a wide range and depend greatly on the pH of flie solution. Cu and Zn, for example, can be separated by bubbling H gas flirougji a properly acidified solution. Because CuS (Kgp = 6 X 10 ) is less soluble than ZnS (Kgp = 2 X 10 ), CuS precipitates from an acidified solution (pH = 1) while ZnS does not (Figure 17.21 ) ... 

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