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.
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 NaAlO2 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 | ⇄ AgO− + H+, |
| ⇅ | ||
| 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.
If silver hydroxide have acid properties, the addition of an alkali must suppress the hydrogen-ion and the hydroxide will go into solution as an argentate, MeAgO. We find, however, that sodium or potassium hydroxide, which would form an argentate very much more readily than ammonium hydroxide, has no solvent action on silver hydroxide (exp.); on the contrary, quantitative experiments show that the alkali makes the hydroxide still less soluble than in pure water—as demanded by the solubility-product for the basic ionization. It is thus evident, that the solvent action of ammonium hydroxide is not due to its basic functions. We would suspect that we have here an action, in which ammonia is the active component, the product of a form of dissociation of ammonium hydroxide, of which the fixed alkalies are incapable.
The Complex Silver-Ammonium[425]-Ion.
NH3 + Ag+ + HO− ⇄ (NH3Ag)+ + HO− ⇄ (NH3Ag)OH.