Arsenious sulphide sol (double decomposition) Preparation
For the preparation of arsenious sulphide sol, a saturated solution of pure arsenious oxide is made in clean water by boiling in a clean flask. The solution is cooled to about 50°C and the undissolved oxide is removed by filtration and it is diluted to about three times its volume with clean water. Purified H2S is slowly passed through the clear solution. The H2S reacts to form arsenious sulphide. The solution becomes yellow and almost all the arsenious sulphide remains in the colloidal state. If any precipitate is formed it should be removed by filtration through sintered crucible or glass wool and not through filter paper. Purified H2 or CO2 or N2 is now slowly bubbled through the sol till the excess H2S is removed. A clear, quite stable sol of arsenious sulphide can thus be prepared.
A still more convenient method is to add a small quantity of thioacetamide or thiourea to the original arsenious oxide solution in water. The thio compounds undergo slow hydrolysis forming H2S which reacts to form an arsenious sulphide. The sol thus produced contains arsenious sulphide particles even smaller than the sol prepared by direct bubbling of H2S. While the sol made by the first process is yellow in color, the sol obtained by the action of theo compound appears yellow with a bluish tinge in transmitted light. This shows that the particles of arsenious sulphide are at least 0.01μ (10 nm).