Ammonium sulphide (NH4)2S is the salt of an extremely weak acid with a much weaker base than sodium hydroxide, and it is correspondingly more completely decomposed by water. In a 0.1 molar solution of the sulphide (NH4)2S, we find the approximate concentration[401] of the sulphide-ion [S2−]am. = 1.8E−6, as compared with 0.9E−3 in a similar solution of Na2S. But the concentration of sulphide-ion is still enormously greater than its concentrations in hydrogen sulphide in the absence and in the presence of acids (see above).
The following table[402] contains a summary of the concentrations of sulphide-ion in the various solutions discussed, as well as its concentration in the presence of 0.2 molar hydrochloric acid. In the separation of the copper and arsenic groups from the zinc and aluminium groups, a concentration of hydrogen-ion corresponding to the presence of 0.15 to 0.25 molar hydrochloric acid is satisfactory for an accurate separation for ordinary purposes.
| Solution. | [S2−] |
|---|---|
| 0.1 molar Na2S: | 0.9E−3 |
| 0.1 molar NaSH: | 0.8E−5 |
| 0.1 molar (NH4)2S: | 1.8E−6 |
| 0.1 molar (NH4)SH: | 1.4E−8 |
| 0.1 molar H2S, sat. aq. sol., 25°: | 1.2E−15 |
| 0.1 molar H2S, 0.1 molar HCl:[403] | 1.3E−21 |
| 0.1 molar H2S, 0.2 molar HCl: | 3.5E−22 |
Precipitation of Sulphides by Hydrogen Sulphide.
The absolute values of the solubilities of the various sulphides, which are involved in the discussion, are known, with any degree of accuracy, only in a few cases. The aim of the discussion will be, therefore, to develop, rather, the relations in the values involved, which may be readily determined. Wherever absolute quantities can be given, they also will be referred to.
Theory of the Separation of Sulphides by Precipitation with Hydrogen Sulphide. I. Precipitation of Ferrous Sulphide.
II. Precipitation of Zinc Sulphide.
If the solution of zinc sulphate is saturated with hydrogen sulphide, under the same conditions as were used with the ferrous salt solution (exp.), or if we add the zinc sulphate solution to the mixture of ferrous sulphate and hydrogen sulphide (exp.), we immediately obtain heavy white precipitates of zinc sulphide. We would decide, therefore, on the basis of the principle of the solubility-product, that in this case [Zn2+] × [S2−] > KZnS. Since we have used hydrogen sulphide under practically the same conditions, we may consider that [S2−], in this experiment,[407] is the same as in the [p205] test with ferrous sulphate, and, by the conditions of the experiment, we have also made [Zn2+] = [Fe2+]. The two factors of the product are, therefore, the same, for the first moment, and we may put [Fe2+] × [S2−] = [Zn2+] × [S2−] = P.
Since P is smaller than KFeS and larger than KZnS, it is clear that the solubility-product constant for zinc sulphide must be smaller than that for ferrous sulphide. The solubility-product constants, for similar salts, are a measure of their solubilities in water. We may obtain their values by determining the solubilities of salts in pure water, whenever the solubility is not affected by other chemical changes. In the present instance, the quantitative measurements, that have been made in this way, are open to question, owing to the considerable hydrolysis which sulphides, as salts of a very weak acid, undergo in solutions of such extreme dilution.[408] Until such relations have been taken into account quantitatively, it is better to limit ourselves for the present to the more accessible question of relative solubility.