The stronger the acid combined with ammonia, the more stable is the salt, and the higher is the temperature, at which the salt decomposes readily and rapidly. Ammonium chloride, one of the most stable of the salts, is decomposed rapidly only at about 350°, which is, however, still below red heat; ammonium carbonate, the salt of a very much weaker acid, decomposes appreciably at ordinary temperatures, and exposed to the air, it gradually disappears as ammonia, carbon dioxide and water. If the acid of the salt is volatile at the dissociation temperature of the salt, the whole salt is volatilized, and if the ammonia and volatile acid vapor reach a colder space, recombination to form the original solid salt occurs to a considerable extent (the "smoking off" of ammonium chloride). If the acid is not volatile, the salt, nevertheless, loses its ammonia at temperatures below red heat, while the acid remains. Sodium-ammonium phosphate, for instance, when heated, loses its ammonia, and sodium-dihydrogen phosphate is left as a nonvolatile residue.
Ammonium happens to form salts which closely resemble the corresponding salts of potassium in physical properties, such as [p160] solubility and insolubility, salts which could readily be mistaken for potassium salts. Advantage is taken of the chemical instability of the ammonium salts, just described, to remove ammonium completely from mixtures, by ignition, before tests for potassium are made.
Water is a far weaker acid (see table, p. [104]) than carbonic acid and it is not surprising to find the compound formed by water and ammonia, ammonium hydroxide, one of the least stable of the ammonium compounds. Even at ordinary temperature, the hydroxide is more or less decomposed, according to a reversible reaction of the same type as that found for the ammonium salts, NH4OH ⇄ NH3 + HOH.
Chemists have always been interested in the problem of the exact degree of stability of ammonium hydroxide, and, more particularly, in the problem whether ammonia gives solutions in water showing very much weaker basic strength[329] than equivalent solutions of potassium and sodium hydroxides, primarily because only a small proportion of the ammonia is combined with water to form the real base, ammonium hydroxide (which would be present then in much more dilute solution than the alkali metal hydroxides are in the solutions with which the comparison is made), or chiefly because ammonium hydroxide is much less readily ionizable than potassium or sodium hydroxide.
For the reversible action NH3 + HOH ⇄ NH4OH we would have, at a constant temperature, according to the law of chemical equilibrium,
[NH3] × [HOH] / [NH4OH] = k.
For a dilute solution at a given temperature the concentration of the water may be considered a constant, and therefore
[NH3] / [NH4OH] = k / [HOH] = kNH3.
(I)
For the ionization of ammonium hydroxide, NH4OH ⇄ NH4+ + HO−, we would have in turn,