(c) Conversion of the Trisulphide of Arsenic into the Arsenomolybdate of Ammonia.—The purified sulphide is oxidised by nitric acid, the acid solution is rendered alkaline by ammonia, and then precipitated by a molybdenum solution, made as follows:—100 grms. of molybdic acid are dissolved in 150 c.c. of ordinary ammonia and 80 of water; this solution is poured drop by drop into 500 c.c. of pure nitric acid and 300 c.c. of water; it is allowed to settle, and, if necessary, filtered. The molybdic solution must be mixed in excess with the liquid under treatment, the temperature raised to 70° or 80°, and nitric acid added in excess until a yellow coloration appears; the liquid is then passed through a tared filter, and dried at 100°. It contains 5·1 per cent. of arsenic acid [3·3 As].[792]
[792] Champion and Pellett, Bull. Soc. Chim., Jan. 7, 1877.
(d) Conversion of the Sulphide into Metallic Arsenic.—If there should be any doubt as to the nature of the precipitated substances, the very best way of resolving this doubt is to reduce the sulphide to metal; the easiest method of proving this is to dissolve in potash and obtain arsine by the action of aluminium; or if it is desired to evolve arsine from an acid solution with zinc in the usual way, then by dissolving a slight excess of zinc oxide in potash or soda, and dissolving in this the arsenic sulphide; the zinc combines with all the sulphur, and converts the sulpharsenite into arsenite; the zinc sulphide is filtered off, and the filtrate acidified and introduced into Marsh’s apparatus. The original process of Fresenius was to mix the sulphide with carbonate of soda and cyanide of potassium, and place the mixture in the wide part of a tube of hard German glass, drawn out at one end to a capillary fineness. Carbonic anhydride, properly dried, was passed through the tube, and the portion containing the mixture heated to redness; in this way the arsenical sulphide was reduced, and the metal condensed in the capillary portion, where the smallest quantity could be recognised. A more elaborate and accurate process, based on the same principles, has been advocated by Mohr.[793]
[793] Mohr’s Toxicologie, p. 57.
A convenient quantity of carbonate of soda is added to the sulphide, and the whole mixed with a very little water, and gently warmed. The yellow precipitate is very soon dissolved, and then the whole is evaporated carefully, until it is in a granular, somewhat moist, adhesive state. It is now transferred to a glass tube, open at top and bottom, but the top widened into a funnel; this tube is firmly held perpendicularly on a glass plate, and the prepared sulphide hammered into a compact cylinder by the aid of a glass rod, which just fits the tube. The cylinder is now dried over a flame, until no more moisture is to be detected, and then transferred into a glass tube 4 or 5 inches long, and with one end drawn to a point (the weight of this tube should be first accurately taken). The tube is connected with the following series:—(1) A chloride of calcium tube; (2) a small bottle containing nitrate of silver solution; (3) a hydrogen-generating bottle containing zinc and sulphuric acid. The hydrogen goes through the argentic nitrate solution, leaving behind any sulphur and arsenic it may contain; it is then dried by chloride of calcium, and streams in a pure dry state over the cylinder of prepared sulphide (no error with regard to impurities in the gas is likely to occur; but in rigid inquiries it is advisable to heat a portion of the tube, previous to the insertion of the cylinder, for some time, in order to prove the absence of any external arsenical source); when it is certain that pure hydrogen, unmixed with air, is being evolved, the portion of the tube in which the cylinder rests is heated slowly to redness, and the metallic arsenic sublimes at a little distance from the source of heat. Loss is inevitable if the tube is too short, or the stream of hydrogen too powerful.
The tube after the operation is divided, the portion soiled by the soda thoroughly cleansed, and then both parts weighed; the difference between the weight of the empty tube and the tube + arsenic gives the metallic arsenic. This is the process as recommended by Mohr; it may, however, be pointed out that the glass tube itself loses weight when any portion of it is kept red-hot for some little time; and, therefore, unless the crust is required in the original tube, it is better to divide it, carefully weigh the arsenical portion, remove the crust, and then re-weigh. The method is not perfectly accurate. The mirror is not pure metallic arsenic (see [p. 571]), and if the white alkaline residue be examined, arsenic will be detected in it, the reason being that the arsenical sulphide generally contains pentasulphide of arsenic as well as free sulphur. Now the pentasulphide does not give up metallic arsenic when treated as before detailed; nor, indeed, does the trisulphide, if mixed with much sulphur, yield an arsenical crust. It is, therefore, of great moment to free the precipitate as much as possible from sulphur, before attempting the reduction.