[405] The precipitation of sulphides, from a solution containing much more of the hydrosulphide-ion than of the sulphide-ion, is comparable with the precipitation of mercuric oxide, HgO, and of silver oxide, Ag2O, by sodium or potassium hydroxide.
[406] On account of the presence of a small, unknown amount of sulphuric acid in the original solution, resulting from the hydrolysis of ferrous sulphate, the exact value of [S2−] in the first solution cannot be calculated without further examination; but, according to the values given in the table on page [202], the value of x, indicating the growth in the concentration of S2−, is at least 1012, if 2 equivalents of NaOH, and 109, if 2 equivalents of NH4OH are used to convert the 0.1 molar hydrogen sulphide into the corresponding sulphide Me2S, of 0.1 molar concentration.
[407] [S2−] is exactly the same in the two products, when equal volumes of the zinc and ferrous sulphate solutions are mixed and the mixture is saturated with hydrogen sulphide; zinc sulphide is precipitated.
[408] The difference in the values obtained, when hydrolysis is considered or neglected, is very considerable. Vide Bodländer, on the solubility of calcium carbonate, Z. phys. Chem., 35, 23 (1900), and Stieglitz, Carnegie Institution Publications, No. 107, 249 (1909).
[409] In carrying out this fractional precipitation a very dilute solution of ammonium sulphide is used, so as to prevent the mechanical enclosure of black ferrous sulphide, which would discolor the white sulphide. The ammonium sulphide solution should be saturated with hydrogen sulphide, to prevent the precipitation of green ferrous-ferric oxide by an excess of free ammonia. It is best to prepare a set of the precipitates and to preserve them in well-stoppered vessels, and not to try to take the time and care necessary to effect a perfect fractionation as a lecture experiment. The presence of the ferrous sulphate, in the supernatant liquid above the first precipitate of zinc sulphide, may be readily demonstrated by pouring off some of the solution and adding an excess of ammonium sulphide to it. Of course, it is also perfectly legitimate, and easier, to precipitate first zinc sulphide from a pure zinc sulphate solution and then to add ferrous sulphate solution to the mixture and to preserve the mixture. If the zinc sulphide were not the less soluble, it would be rapidly converted into the black ferrous sulphide. (See p. [165], and see below, pp. [210], [213], where similar transformations are carried out as lecture experiments.)
[410] When 10% of the zinc in a 0.1 molar solution has been precipitated, 0.01 molar sulphuric acid has been formed. For the sake of a rough approximation, the acid may be considered completely ionized and then [H+] = 0.02, which is 200 times the value of [H+] in a saturated H2S solution (p. [200]); if the presence of a little sulphuric acid in the original zinc sulphate solution, resulting from a slight hydrolysis of the salt, is ignored, the concentration of the sulphide-ion is decreased roughly (200)2 or forty thousandfold, while the concentration of zinc-ion falls 10%. The corrections, that have been indicated, would change the quantities involved, but they would not modify the character of the result.
[411] This proportion of acid, making the concentration of the hydrogen-ion, approximately, [H+] = 0.2, is used, not because it represents the minimum concentration of the hydrogen-ion, which will prevent the precipitation of zinc sulphide in 0.1 molar zinc sulphate solution, but because it represents the practical conditions under which the precipitation of zinc sulphide is avoided, when the copper and arsenic groups are precipitated in qualitative analysis (see p. [213]).
[412] Nickel and cobalt sulphides are also precipitated by hydrogen sulphide in the presence of free acetic acid, if sodium or potassium acetate is added, to suppress the hydrogen-ion of the acetic acid (p. [112]).
[413] They would be found in the copper group.
[414] The sulphate, of this composition, is obtained by drying the crystallized sulphate in an air bath at 100–105°.