If a fragment of pure copper be boiled with pure hydrochloric acid for ten minutes, no discolouration occurs. If now a solution containing arsenic be added, the copper turns black or grey, from formation of an alloy of copper and arsenicum. On drying the copper, and heating it in a small glass tube closed at one end, the arsenicum is oxidized, with production of crystals of As2 O3. Organic matter does not interfere. Antimony, sulphides, and some metals produce a similar grey deposit, but do not yield a crystalline sublimate. Mercury also precipitates on the copper, but the sublimate consists of metallic globules.
Any compound of arsenicum, mixed with dried potassium cyanide and carbonate of soda, introduced into a piece of hard glass tubing drawn to a point, and heated in a slow stream of dry carbon dioxide, gives a deposit, in the narrow portion, of the whole of its arsenic in the metallic form (Fresenius).
As2 O3 heated in a tube with dry potassium or sodium acetate gives cacodyl-oxide (Bunsen) of an exceedingly offensive alliaceous odour.
In solution, arsenious acid gives:—
1. With ammonio-silver nitrate (prepared by adding silver nitrate to dilute ammonia till a precipitate just forms) a yellow precipitate of silver arsenite, soluble in ammonia and in nitric acid.
2. With ammonio-cupric sulphate (prepared by similarly treating cupric sulphate), a bright green precipitate of cupric arsenite.
3. With sulphuretted hydrogen a yellow colour (the intensity of this has been proposed as a method of estimating small quantities of arsenic by comparison à la Nessler), but no precipitate till hydrochloric or other acid be added, when yellow arsenious sulphide falls. This is a most delicate test, as arsenious sulphide is only soluble to the extent of one part in one million of water (Fresenius, Quant. Anal. p. 137), and not much more soluble in acids. The precipitate may be weighed, or treated as already mentioned (pp. 386-7). Tin and cadmium solutions also give yellow sulphides, but they are insoluble in ammonia, and do not yield the other tests.
4. Stannous chloride (protochloride of tin) gives a brown deposit of metallic arsenic. With acids containing traces of arsenic it gives a brown colour.
DOSES.
1. Medicinal (British Pharmacopœia).—Acidum arseniosum (As2 O3), 1/60 to 1/12 grain in solution. Liquor arsenicalis (solution of potassium arsenite), 2 to 8 minims. Liquor arsenicalis hydrochloricus (solution of chloride of arsenic), 2 to 8 minims. Sodæ arsenias (sodium arsenate), 1/16 to ⅛ grain. Liquor sodæ arseniatis (solution of the preceding), 5 to 10 minims. Ferri arsenias (ferric arsenate), 1/16 to ½ grain.