In solutions of ammonium hydroxide the concentration of hydroxide ion for 18° is found from the relation [NH4+] × [HO−] : ([NH4OH] + [NH3]) = 18E−6 (p. [161]). (1) For ammonium hydroxide, for instance in 0.2 molar solution, and in the absence of any ammonium salt, [HO−] = [NH4+] = 0.0019.[350] This concentration of the hydroxide-ion should be more than sufficient to precipitate magnesium hydroxide in a 0.1 molar solution of the sulphate.[351]
Exp. 100 c.c. of a mixture which should contain magnesium sulphate in 0.1 molar solution would require 246 / (10 × 10) or 2.46 grams of MgSO4, 7 H2O. [p170] This weight of the salt is dissolved in 50 c.c. of water and 50 c.c. of 0.4 molar solution of ammonium hydroxide is added to it. Magnesium hydroxide is precipitated. (The precipitation is incomplete because the ammonium sulphate, formed in the reaction, reduces the ionization of ammonium hydroxide.)
(2) In the presence of 0.25 molar ammonium chloride, the chloride being dissociated to the extent of 80%,[352] the concentration of the hydroxide-ion in a 0.2 molar ammonium hydroxide solution is reduced to 18E−6.[353] According to the above calculation this is too small a concentration to precipitate magnesium hydroxide in a 0.1 molar solution of the sulphate.
Exp. 1.35 grams ammonium chloride (the weight corresponding to 100 c.c. of a mixture containing ammonium chloride in 0.25 molar solution) is dissolved in 50 c.c. of 0.4 molar ammonium hydroxide and 2.46 grams of magnesium sulphate, dissolved as before in 50 c.c. of water, is added to the mixture. No precipitate of magnesium hydroxide is formed.
In a similar fashion and for the same reason, ammonium salts interfere more or less with the precipitation of other hydroxides, for instance with the precipitation of manganous, nickelous, cobaltous, ferrous, zinc, cupric and cadmium hydroxides. But ammonium salts do not prevent the precipitation of aluminium, chromium and ferric hydroxides, which are much less soluble than the hydroxides just mentioned, and as much weaker bases, (Chap. X) are also much less readily ionized. They are precipitated by smaller concentrations of the hydroxide-ion than are the hydroxides of the first group, and their precipitation may be made quantitative.
Exp. Ferric hydroxide is readily precipitated when 2.7 grams of ferric chloride FeCl3, 6 H2O, the weight of the chloride required to give 100 c.c. of a 0.1 molar solution, is dissolved in as little water as possible. The solution is added to the mixture of magnesium sulphate, ammonium chloride and ammonium hydroxide obtained in the previous experiment, in which no magnesium hydroxide was precipitated.
Chapter IX Footnotes
[326] See Smith, General Inorganic Chemistry, p. 414, General Chemistry for Colleges, p. 277; Remsen, Inorganic Chemistry (advanced course), p. 158.
[327] Mg2+ may also be precipitated as a carbonate with the other ions of the group (see Part III).
[328] In the case of ammonium nitrate a different form of decomposition, namely into nitrous oxide and water, predominates, and, in the case of ammonium nitrite, decomposition into nitrogen and water takes place so readily, that the decomposition of the salt takes that direction chiefly.