LIESEGANG RISGS B Y O L I S F. TOWER A S D EDNA E . C H A P Y A S
In 1928 Hedges and Henley' published a paper, in which they showed that in many cases Liesegang rings can be obtained by a process of coagulation. For example, they prepared a colloidal solution of silver chromate in gelatin, covered it with silver nitrate solution, and thereby obtained rings of silver chromate very similar to those of the original Liesegang experiments. They also found that it was not necessary for the diffusing electrolyte to possess an ion in common with the salt to be coagulated. By allowing a solution of aluminium sulfate to diffuse into a colloidal solution of arsenious sulfide in gelatin, rings of arsenious sulfide were obtained. Rings of cadmium sulfide and zinc sulfide had already been prepared in this laboratory by the usual method.* Experiments were therefore undertaken to see whether rings of these substances could be obtained by the method described by Hedges and Henley. Colloidal solutions of cadmium sulfide were prepared by mixing a tenth-normal solution of cadmium chloride in agar-agar solution with a tenth-normal solution of ammonium sulfide. On standing, these give a very good colloidal solution of cadmium sulfide. More concentrated solutions than these could not be used, as a precipitate of cadmium sulfide was then obtained. Above several of these colloidal solutions of cadmium sulfide in agar-agar were placed cadmium chloride solutions of 0.jy0 concentration and regularly increasing concentrations up to 2 0 % . These mere allowed to stand for several days, but no rings of cadmium sulfide were obtained. Tieither were rings obtained when the supernatant liquid contained ammonium sulfide, ferric chloride or aluminium sulfate. The method described by Hedges and Henley is, therefore, not a general one, and only appears to yield rings in certain special cases. In continuation of the work referred to above by Daus and Tower, we here wish to report the results of experiments of preparing Liesegang rings of manganese, nickel and cobalt sulfides. In all these cases a method similar t o that used before was followed. The electrolyte containing the sulfide ion was placed in the gelatinous medium, which consisted sometimes of gelatin, and sometimes of agar-agar, and the electrolyte containing the metallic ion was placed in the supernatant liquid. With gelatin the sulfide ion was furnished by ammoniumsulfide, as the gelatin did not set when containing sodium sulfide. With agar-agar either ammonium sulfide or sodium sulfide could be used. The best concentrations of sulfide ions were found to be between 0 . 2 and 0.5 normal, and for the manganese ion between 0.1 and z normal. It J. Chem. SOC.,130, 2714 (1928). * D a w and Tower: J. Phys. Chem., 33, 605 (1929).
