VI. The Nitrate Group. The salts of the acids of this group are readily soluble in water and specific tests for the acid ions are made; there is no group test.
VII. The Group of Organic Acids. This group need only be considered when a test for organic matter reveals its presence.
Applications of Physico-Chemical Principles and Theories.
In the following, only a few typical and interesting applications of the principles and theories to acid ions will be given; numerous other applications will suggest themselves in connection with the laboratory work on the acids.
Fractional Precipitation of Salts with a Common Ion.
[Ag+]1 × [Cl−]1 = KAgCl = 1E−10;
[Ag+]2 × [Br−]2 = KAgBr = 4E−13;
[Ag+]3 × [I−]3 = KAgI = 3E−16.
For a solution saturated simultaneously with the three silver salts, the value of the concentration of the silver-ion is the same in the three solubility-products (see p. [164]). Consequently, for such a solution, with which the three solid salts are in equilibrium, the ratios of the concentrations of the anions must be: [Cl−] : [Br−] : [I−] = KAgCl : KAgBr : KAgI = 3 × 105 : 1300 : 1. That is, if silver nitrate is added to a mixture of iodides, bromides and chlorides, silver iodide must be precipitated first, until the concentration of bromide-ion, in solution, is 1300 times as great as the concentration of the iodide-ion left in solution. Then bromide and traces of iodide of silver will be precipitated, until the concentration of chloride-ion is 300,000 times as great as the concentration of iodide-ion and some 250 times as great as the concentration of the bromide-ion. In other words, if silver nitrate is added gradually to such a mixture, iodide-ion and bromide-ion will be almost completely removed from solution before a precipitate of silver chloride can be in equilibrium with the solution. This gives us a convenient and rapid method of detecting chlorides, if present, [p304] in more than small quantities, with iodides and bromides. Silver nitrate, a few drops at a time, is added to the solution and the mixture vigorously shaken after each addition. As long as a yellow (AgI) or yellowish (AgBr) silver salt is precipitated on the addition of silver nitrate to the supernatant liquid (the precipitate settles quickly), silver nitrate is added as before; when the color becomes quite pale, the solution is filtered and silver nitrate added a drop at a time; if a pure white precipitate results finally, chloride-ion is present in the mixture (exp.).
Complex Ions.
The condition of equilibrium between silver-ion, ammonia and silver-ammonium-ion is expressed in the relation:
[Ag+] × [NH3]2 / [Ag(NH3)2+] = K = 1 / 107.