in the filtrate may be:—
- Arsenic sulphide,
- Antimony sulphide,
- Tin sulphide,
and there may also be a small portion of copper sulphide, because the latter is somewhat soluble in a considerable quantity of ammonium sulphide.
The filtrate from the original hydric sulphide precipitate will contain, if present, the sulphides of zinc and chromium in solution.
INVESTIGATION OF THE SULPHIDES SOLUBLE IN AMMONIUM SULPHIDE, VIZ., ARSENIC, ANTIMONY, TIN.
The ammonium sulphide solution is evaporated to dryness in a porcelain dish, strong nitric acid added and again dried. To this residue a little strong caustic soda solution is added, and then it is intimately mixed with three times its weight of a mixture composed of 2 of potassic nitrate to 1 of dry sodium hydrate. This is now cast, bit by bit, into a red-hot porcelain crucible. The whole is heated until it has melted into a colourless fluid.
Presuming the original mass contained arsenic, antimony, and tin, the melt contains sodic arseniate, sodic pyro-antimonate, sodic stannate, and tin oxide; it may also contain a trace of copper oxide.
The melt is cooled, dissolved in a little water, and sodium bicarbonate added so as to change any caustic soda remaining into carbonate, and to decompose the small amount of sodic stannate; the liquid is then filtered.
The filtrate will contain the arsenic as sodic arseniate; while on the filter there will be pyro-antimonate of soda, tin oxide, and, possibly, a little copper oxide.