Sulfide salts

Potassium removal is required because the presence of potassium during electrolysis reportedly promotes the formation of the a-Mn02 phase which is nonbattery active. Neutralization is continued to a pH of approximately 4.5, which results in the precipitation of additional trace elements and, along with the ore gangue, can be removed by filtration. Pinal purification of the electrolyte Hquor by the addition of sulfide salts results in the precipitation of all nonmanganese transition metals. 

Kolis et al. reported the synthesis of some metal sulfide salts of homolep-tic lanthanide ammine complexes using supercritical ammonia as a reaction medium (Scheme 12) [49]. They proposed that these reactions proceed via a... 

Some of these salts dissolve easily in water —- all the nitrates (salts of nitric acid) and most of the chlorides (salts of hydrochloric acid). Many salts, on the other hand, are insoluble — most of the carbonates (salts of carbonic acid) and most sulfides (salts of hydrosulfuric acid). 

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. 

Mercury is a silver-colored metallic element in fluid form (symbol Hg atomic no. 80). Its symbol derives from the Greek name for the element hydrargyros. It is found as the sulfide salt in cinnabar, the source of the pigment vermilion. 

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. " ... 

In the next step, 0.05 ml of sodium sulfide solution R is added to the hot test solution, upon which no precipitation must occur. This reveals the presence of cations cable of forming insoluble sulfide salt even at neutral pH. 

The chemical basis for the determination is the formation of a colloidal precipitate of insoluble heavy metal sulfide salt, as shown below using lead as an example. 

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