EXP. Ferric hydroxide is readily precipitated when 2.7 grams of ferric chloride FeCl_{ 3}, 6 H_{ 2}O, 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.
[326] See Smith, ‹General Inorganic Chemistry›, p. 414, ‹General Chemistry for Colleges›, p. 277; Remsen, ‹Inorganic Chemistry (advanced course)›, p. 158.
[327] Mg^{ 2+} 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.
[330] T. S. Moore, ‹J. Chem. Soc.› (London), «91», 1379 (1907).
[331] From equation (I) we obtain directly, by the application of a simple mathematical transformation, that [NH_{ 3}] + [NH_{ 4}OH] / [NH_{ 4}OH] = ‹k›_{ NH_{ 3}} + 1 = ‹k′›, and, therefore, [NH_{ 4}OH] = [NH_{ 3}] + [NH_{ 4}OH] / ‹k′›. Inserting this value for [NH_{ 4}OH] in equation (II) and transferring ‹k′›, we have [NH_{ 4}^{ +}] × [HO^{ −}] / ([NH_{ 3}] + [NH_{ 4}OH]) = ‹k›_{ base} / ‹k′› = K.
[332] Water of crystallization, found in many of the precipitates used in qualitative analysis, will, as a rule, be indicated only in the formulas used in Part III, in the study of the reactions of ions.
[333] See Part III as to the method of vaporizing magnesium.
[334] ‹Cf.› Findlay, ‹Z. phys. Chem.›, «34», 409 (1900).
[335] The solubility of barium sulphate at 18°, according to the table, is 0.0023 / 233, or 1E−5 mole per liter, and the salt may be considered completely ionized at this dilution.
[336] The solubility (see the table) is 0.114 / 183.6, or 6.2E−4 mole per liter, 84% of which is ionized (according to conductivity determinations, Kohlrausch and Holborn, ‹loc. cit.›, p. 200). Hence [Sr^{ 2+}] = [SO_{ 4}^{ 2−}] = 0.00062 × 0.84 = 0.0005.
[337] To establish equilibrium, prolonged "digesting" is sometimes required. Double salts, solid solutions and mechanical enclosures are liable to interfere with the completeness of such separations by fractional precipitation. Resolution and reprecipitation will then usually effect a sufficiently accurate separation for most purposes. On account of the possibility of such complications, the conditions for a successful separation, within limits such as those described in the text, must in all cases be investigated.