The concentration of silver-ion, which may exist in an ammoniacal solution, evidently must decrease rapidly with increasing concentrations of the free ammonia. Now, let us imagine only sufficient free ammonia, in solution, added to a mixture of silver chloride, bromide and iodide, to keep the concentration of silver-ion, which can exist in the solution, say at [Ag+] = 6E−9, which is just 1 / 100th of the concentration of silver-ion in a saturated aqueous solution of silver bromide. Such a solution of ammonia, in contact with the three silver salts mentioned, will dissolve silver chloride, if sufficient is present, until [Cl−] = KAgCl / 6E−9 = 0.017 molar. At the same time, silver bromide would be dissolved until [Br−] = KAgBr / 6E−9 = 0.000,06 molar. In other words, silver chloride could be dissolved in some quantity, while silver bromide is dissolved only in traces (the ratio of [Cl−] : [Br−] is again about 250 : 1). When such an ammoniacal extract is acidified with nitric acid, almost pure (white) silver chloride would be precipitated and only traces of bromide would be lost. After the extraction of the chloride, an increased concentration of ammonia would lead, similarly, to a solution in which silver bromide would dissolve readily and only traces of the iodide be lost, and thus a separation of bromide and iodide may be effected.
In Hagar's method, the concentration of ammonia, required to dissolve silver chloride with but traces of bromide, is attained by the use of a solution of ammonium sesqui-carbonate,[598] in which free ammonia is present only in small concentration, as a result of the hydrolysis of the salt. After the [p305] extraction of the chloride by this solution, the bromide is extracted with a 5% solution of ammonia.
Complex Ions of Acid Ions with Other Acids.
Ammonium phosphomolybdate, (NH4)3PO4, 12 MoO3, is the salt of a complex phosphomolybdic acid, formed from phosphoric acid, O:P(OH)3, and molybdic acid, O2Mo(OH)2, by a loss of water, much as potassium dichromate is formed from potassium acid chromate [KO(CrO2)OH + HO(CrO2)OK ⇄ KO(CrO2)O(CrO2)OK]. The only difference between the two actions lies in the fact that, in the case of the dichromate, anhydride formation occurs between two molecules of a single acid; in the case of the phosphomolybdate, anhydride formation takes place between molecules of different acids, and a much larger number of molecules is involved. If we suppose the combination between the two acids to proceed symmetrically,[601] we may consider the following to be the action:
O:P(OH)3 +
3 [HO(MoO2)OH + HO(MoO2)OH + HO(MoO2)OH + HO(MoO2)OH] ⇄
O:P[O(MoO2)O(MoO2)O(MoO2)O(MoO2)OH]3 + 12 H2O.
Intermediate complex acids, containing less molybdic acid, are no doubt formed first (the action is a relatively slow one), and the action proceeds until the formation of an insoluble salt leads to the final precipitation of all of the phosphate in this form. The precipitate shows the characteristic behavior of an acid anhydride—alkalies dissolve it readily and form phosphate and molybdate—e.g. ammonium hydroxide forms [NH4]2HPO4 and (NH4)2MoO4 (exp.). Dichromates, in a similar way, are converted by alkalies into chromates, an action which may readily be followed by the change in color (exp.).
Oxidation and Reduction.
We shall discuss here only one other oxidation-reduction reaction, taken in connection with the laboratory work—the oxidation of hydroiodic acid by exposure to the air and the resistance to oxidation shown by an iodide, such as potassium iodide, under the same conditions. The following method of proximate analysis of the chief relations involved may also be used to interpret the contrast in the behavior of hydroiodic acid and that of hydrobromic or hydrochloric acid (Laboratory Manual, q. v.). In all of these cases the actual relations are rendered more complex in consequence of secondary reactions, than is indicated in the text that follows: it is intended only to outline the most effective of the factors involved and to illuminate the qualitative results observed.
Oxidation of Hydroiodic Acid by Air.
4 I− + O2 + 2 HOH ⇄ 2 I2 + 4 HO−.