[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 H_{2}S 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°. [415] ‹Cf.› the corresponding experiment with zinc sulphate, p. 207. [416] [S^{2−}] = ‹k› / [H^{+}]^2. See equation (IV), p. 201. [417] Any immediate precipitation of cadmium sulphide will be prevented by the addition of 10 c.c. of concentrated acid (sp. gr. 1.19) to 50 c.c. of the 0.1 molar solution, and 15 c.c. will completely prevent any precipitation of the sulphide. Of course, a smaller excess would prevent the precipitation of small quantities of the sulphide (‹e.g.› a half milligram of cadmium), which should easily be found in 50 c.c. (see p. 214).