On the other hand, the addition of alkali to hydrogen sulphide, by neutralizing and suppressing the hydrogen-ion, and by forming the ‹salts› MeSH and Me_{2}S, will very greatly increase the concentrations of the hydrosulphide-ion and of the sulphide-ion. Since the constant for the secondary ionization of hydrogen sulphide shows that HS^{−} is an ‹exceedingly weak› acid, its salts, Me_{2}S, are very largely hydrolyzed, the constant for water being somewhat greater[400] than its own. According to Knox, in a 0.1 molar solution of Na_{2}S about 99% of the sulphide is hydrolyzed: Na_{2}S + H_{2}O ⥂ NaSH + NaOH. In spite of this almost complete hydrolysis, sufficient sodium sulphide remains in a solution of this [p202] substance, to yield a concentration of the sulphide-ion that is far greater than that obtained from a solution of hydrogen sulphide. In a 0.1 molar solution of sodium sulphide the concentration of the sulphide-ion is, approximately, [S^{2−}]_{alk.} = 0.9E−3, as compared with [S^{2−}] = 1.2E−15 in a saturated solution of hydrogen sulphide (25°, 760 mm.), and with 1.3E−21 in the same solution in the presence of 0.1 molar hydrochloric acid. Ammonium sulphide (NH_{4})_{2}S 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 (NH_{4})_{2}S, we find the approximate concentration[401] of the sulphide-ion [S^{2−}]_{am.} = 1.8E−6, as compared with 0.9E−3 in a similar solution of Na_{2}S. 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. [S^{2−}] 0.1 molar Na_{2}S: 0.9E−3 0.1 molar NaSH: 0.8E−5 0.1 molar (NH_{4})_{2}S: 1.8E−6 0.1 molar (NH_{4})SH: 1.4E−8 0.1 molar H_{2}S, sat. aq. sol., 25°: 1.2E−15 0.1 molar H_{2}S, 0.1 molar HCl:[403] 1.3E−21 0.1 molar H_{2}S, 0.2 molar HCl: 3.5E−22 [p203]