The fact that ammonium salts prevent the precipitation of magnesium hydroxide was formerly explained as being due to the formation of "double salts," such as MgCl_{ 2},NH_{ 4}Cl. It is true that such double salts exist, but, if their formation should prevent the precipitation of magnesium hydroxide, possibly by including the magnesium as part of a negative ion or radical,[347] MgCl_{ 3}, then the corresponding double salts of magnesium chloride with potassium and sodium chloride (‹e.g.› MgCl_{ 2},KCl), which are just as stable as the ammonium salts, should show the same behavior; but, as a matter of fact, the addition of potassium chloride does not interfere with the precipitation of the hydroxide by either [p169] potassium hydroxide or ammonium hydroxide (‹exp.›). So the explanation is untenable. Obviously, a ‹specific interference of ammonium salts with the precipitating power of ammonium hydroxide is involved›. But that is exactly what the law of chemical equilibrium, applied to the ionization of ammonium hydroxide, would demand: As a weak base, which is little ionized in pure aqueous solutions, it is ‹very much weaker as a base, produces a far smaller concentration of the hydroxide-ion, when readily ionizable salts of ammonium are added to the solution›,[348] and the precipitation of magnesium hydroxide, and of metal hydroxides in general, depends on the concentration of the hydroxide-ion, which is a factor in the solubility-products of bases.
The precipitation of magnesium hydroxide, in particular, depends on the relation of the product [Mg^{ 2+}] × [HO^{ −}]^2 to the solubility-product constant of magnesium hydroxide. For the saturated aqueous solution, the product [Mg^{ 2+}] × [HO^{ −}]^2 is equal to the solubility-product constant, and from the solubility of magnesium hydroxide (see the table) the value of the constant is found to be 15E−12 at 18°. The concentration of magnesium-ion is 0.000,154 in this solution. In a 0.1 molar solution of magnesium sulphate, which is ionized to the extent of 37.3%, the concentration of magnesium-ion is 0.0373, and it would require a concentration ‹greater› than 2E−5 of hydroxide-ion to precipitate any magnesium hydroxide.[349] Now, the concentration of the hydroxide-ion, in an ammoniacal solution, can easily be reduced far below this value by the addition of ammonium chloride, nitrate, sulphate or other readily ionizable ammonium salt to the solution, and then magnesium hydroxide will ‹not› be precipitated.