[424] See Blyth, ‹Poisons, etc.›, p. 608 (1895), in regard to the detection of traces of lead. [p216] CHAPTER XII «THE COPPER AND SILVER GROUPS» (‹Continued›).—«THE THEORY OF COMPLEX IONS» We will now turn to the consideration of a series of reactions involving the behavior of so-called "complex ions," which are very frequently met with in the various analytical groups and which offer valuable methods of separation and identification of ions. The behavior of silver nitrate solution towards ammonia forms a convenient point of attack in taking up the general subject. «Action of Ammonia on Silver Nitrate.»—Addition of ammonium hydroxide solution to silver nitrate (‹exp.›) results in the formation of a brown precipitate of silver oxide (and silver hydroxide). We may consider the supernatant liquid to be saturated with silver hydroxide (this is in equilibrium with silver oxide), and for the saturated solution we may put [Ag^{+}] × [HO^{−}] = K_{AgOH}. If more ammonium hydroxide is added to the mixture, the precipitate dissolves readily. The excess of ammonium hydroxide must increase the concentration of hydroxide-ion and, if no other action occurred, we should, according to the principle of the solubility-product, expect that the precipitate would thereby be slightly increased (p. 145), rather than that it should be dissolved so readily. Since solution results even when the value of the one factor, [HO^{−}], of the product is increased, we must suspect that the value of the other factor, the concentration [Ag^{+}] of silver-ion, is in some way made much smaller by the addition of the excess of ammonium hydroxide. Recalling the fact that aluminium hydroxide is soluble in excess of sodium hydroxide, as the result of its amphoteric character, a solution of sodium aluminate NaAlO_{2} being obtained, we might suspect that silver hydroxide also has amphoteric properties, ‹i.e.› that it might be capable of ionizing into "argentate ions," AgO^{−}, and hydrogen ions, AgOH ⇄ AgO^{−} + H^{+}. If such be the case, the nonionized silver hydroxide is in equilibrium, not only with the solid phase, [p217] but also with two sets of ions, Ag^{+} + HO^{−} ⇄ AgOH AgOH ⇄ AgO^{−} + H^{+} AgOH ⇄ AgOH ↓, and we must have ‹two› solubility-product constants, one corresponding to the basic ionization (see above) and the other corresponding to the acid ionization, and [AgO^{−}] × [H^{+}] = K′_{AgOH}.