[347] In that case the salts would be called "complex" salts, salts of a complex ion, MgCl_{3}^{−}. See Chapter XII. They are really "double salts"; ‹cf.› Smith, ‹General Inorganic Chemistry›, p. 536. [348] See p. 114, and recall the laboratory experiments (Lab. Manual, p. 9, § 6), which may be given as lecture experiments at this point. [349] Calling ‹x› the concentration of the hydroxide-ion, required to saturate a 0.1 molar magnesium sulphate solution with magnesium hydroxide, we have 0.0373 × ‹x›^2 = 15E−12 and ‹x› = 2E−5. [350] Putting [HO^{−}] = [NH_{4}^{+}] = ‹y›, we have ‹y›^2 : (0.2 − ‹y›) = 18E−6. Then ‹y› = 0.0019. [351] 0.0373 × 0.0019^2 = 0.13E−6, which is considerably larger than the solubility-product constant for magnesium hydroxide, 15E−12. [352] Kohlrausch and Holborn, p. 159. Minor changes in the degrees of ionization of MgSO_{4} and NH_{4}Cl (and consequently of NH_{4}OH) occur, when the salts are present together. In a rigorous treatment, the ionization of each salt in the mixture would be calculated with the aid of Arrhenius's principle of isohydric solutions. [353] ‹Vide› the analogous calculation on p. 113. [p171] CHAPTER X «ALUMINIUM; AMPHOTERIC HYDROXIDES; HYDROLYSIS OF SALTS. THE ALUMINIUM AND ZINC GROUPS» The chemistry of the analytical reactions of the alkalies and alkaline earths is extremely simple,—it is essentially the chemistry of well-defined bases and their salts,—and the separations and identifications, as we have seen, depend almost entirely on physical differences rather than on chemical contrasts. In the aluminium and zinc groups, which are precipitated together and which will be discussed together, the chemistry of the reactions becomes very much more complex. Therefore, we shall not, as yet, consider the groups as a whole, but shall first discuss the important analytical reactions of some compounds of aluminium.