[787] Hydric sulphide gas has been liquefied, and is now an article of commerce, being sold in iron bottles.
The impure sulphide is washed with water, then with alcohol, then with carbon disulphide, then, after having got rid of the lead, again with alcohol, and finally with water; it is then dissolved in ammonia, the ammonia solution filtered, and the filtrate evaporated to dryness on a sand-bath, at a somewhat high temperature; in this way it is freed from sulphur and, to a great extent, from organic matter; after weighing, it may be purified or identified by some of the following methods:—
(a) Solution in Ammonia and Estimation by Iodine.[788]—The filter is pierced, the sulphide washed into a flask by ammonia water (which need not be concentrated), and dissolved by warming, filtered from any insoluble matter, and estimated by iodine and starch.
[788] P. Champion and H. Pellett, Bull. Soc. Chim. (2), xxvj. pp. 541-544.
(b) Oxidation of the Sulphide and Precipitation as Ammonia Magnesian Arseniate, or Magnesia Pyro-Arseniate.—The tersulphide, as before, is dissolved in ammonia (not omitting the filter-paper, which should be soaked in this reagent), the solution filtered, and evaporated to dryness. The dry residue is now oxidised by fuming nitric acid, taking care to protect the dish with a large watch-glass (or other cover) during the first violent action; the dish is then heated in the water-bath until all the sulphur has disappeared, and only a small bulk of the liquid remains; it is then diluted and precipitated by “magnesia mixture.”[789] The fluid must stand for several hours, and, if the arsenic is to be determined as the usual ammoniacal salt, it must be passed through a weighed filter, and washed with a little ammoniacal water (1 : 3). The solubility of the precipitate is considerable, and for every 16 c.c. of the filtrate (not the washings) 1 mgrm. must be allowed. The precipitate, dried at 100°, 2(NH4MgAsO4)H2O, represents 39·47 per cent. metallic arsenic.
[789] Magnesia Mixture:—