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 NH_{3} + HOH ⇄ NH_{4}OH we would have, at a constant temperature, according to the law of chemical equilibrium, [NH_{3}] × [HOH] / [NH_{4}OH] = ‹k›. For a dilute solution at a given temperature the concentration of the water may be considered a constant, and therefore [NH_{3}] / [NH_{4}OH] = ‹k› / [HOH] = ‹k›_{NH_{3}}. (I) For the ionization of ammonium hydroxide, NH_{4}OH ⇄ NH_{4}^{+} + HO^{−}, we would have in turn, [NH_{4}^{+}] × [HO^{−}] / [NH_{4}OH] = ‹k›_{base}. (II) This constant represents ‹the real ionization constant of ammonium hydroxide as a base›, and its approximate value has only recently been determined by Moore[330] and found to be about 5E−5. The ratio [NH_{3}] / [NH_{4}OH] was found to be approximately 2 at 20°. According to this result, ammonium hydroxide is really a much weaker, less readily ionized base than potassium or sodium hydroxide. The ‹efficiency of ammonium hydroxide as a base› depends [p161] on both conditions of equilibrium; the first equation states what proportion of ammonium hydroxide can exist, as such, in solution, if a given amount of ammonia is dissolved in a given amount of water, and the second equation shows the proportion of the hydroxide, which is ionized. The equations may be combined[331] into one expression, [NH_{4}^{+}] × [HO^{−}] / ([NH_{4}OH] + [NH_{3}]) = K. (III) That is, ‹the ratio of› [NH_{4}^{+}] × [HO^{−}] ‹to the total concentration of nonionized ammonium hydroxide and ammonia, is a constant›. This constant has the value 0.000,018 at 18° (as given in the table, p. 106) and, as said, comprises in a single expression a statement measuring the ‹efficiency›, as a base, of a solution of ammonium hydroxide and ammonia in aqueous solutions. The concentration of the hydroxide-ion, on which the efficiency as a base depends, can be ascertained directly from the expression, provided we know the total concentration of the ammonia and ammonium hydroxide and the concentration and degree of ionization of any ammonium salt, which may be present with the base (see p. 161). These data are easily obtained by direct measurement.