Silver Chromate, Ag₂CrO₄

Silver Chromate, Ag₂CrO₄, is obtained as a reddish-brown precipitate on mixing solutions of soluble silver salts with those of chromate or dichromate of potassium - preferably the former. When prepared by decomposition of the compound Ag₂CrO₄.4NH₃ by heat, or by boiling the dichromate with water, it is obtained in a crystalline condition. The latter method of preparation, or concentration of a solution of silver chromate in dilute acetic acid, yields a greenish-black modification. The differences in colour have been connected with differences in density; but, according to Kohler, the colour of pure silver chromate is always greenish black, the red substance, supposed to be a separate modification, being a mixture of silver chromate and solid solutions of the latter with nitrates or other salts present. Pure unfused silver chromate has a density of 5.625 at 25° C.

Silver chromate is almost insoluble in water, glacial acetic acid, and in solutions of potassium chromate, but soluble in those of ammonia, caustic alkalies, nitrates, and in dilute acetic acid. A concentrated solution of ammonium nitrate is a good crystallising medium for silver chromate. With chlorine, above 200° C., silver chloride, chromium trioxide, and oxygen are produced. The solution in ammonia contains the compound Ag₂CrO₄.4NH₃, which forms crystals isomorphous with the corresponding ammoniacal sulphate.

The precipitation of silver chromate under certain conditions has attracted considerable attention since Liesegang observed that when a drop of silver nitrate solution is placed on a gelatin slab impregnated with potassium chromate or dichromate, a precipitate of silver chromate is produced which is not continuous, but which forms in rings separated by clear zones, as diffusion of the silver nitrate through the gel proceeds. These rings are now known as Liesegang rings, and the phenomenon as periodic precipitation. If the reaction takes place in a test- tube, layers of precipitate are produced.

Various theories have been advanced in explanation. Ostwald suggested that supersaturation took place, followed by precipitation, which cleared the immediate neighbourhood of the reactants, and it was therefore necessary for the silver nitrate to diffuse further before super- saturation was again reached. Hatschek, however, shows that the periodic precipitation takes place in conditions which render super- saturation impossible. Williams and Mackenzie maintain that the silver chromate is precipitated according to the usual rules of the solubility product, and does not behave in any way as a protected colloid but as a crystalloid. More recent work suggests that whenever precipitation takes place, the precipitate first passes through the colloidal condition, coagulation of the molecules into the large precipitated particles being a gradual growth and not an instantaneous transformation. If, therefore, a protective colloid such as gelatin is present, the precipitation will be retarded, since the colloidal particles of silver chromate will tend to adsorb the gelatin, and will therefore remain for a longer time in the colloidal condition. The space between the successive rings is thus determined by the difference between the rates of diffusion and of precipitation in the gel.