[712] Chevers, Med. Jurisprudence for India, p. 116.
Arsenious acid is used by dentists to destroy the nervous pulp of decayed and painful teeth, about the twenty-fifth of a grain (2·5 mgrms.) being placed in the cavity. A common formula is arsenious acid, 2; sulphate of morphine, 1; creasote, q. s. to make a stiff paste. There is no record of any accident having resulted from this practice hitherto; but since the dentist seldom weighs the arsenic, it is not altogether free from danger.
(5) Pigments, &c.—King’s yellow should be As2S3, the trisulphide of arsenic or orpiment. It is frequently adulterated with 80 to 90 per cent. of arsenious acid, and in such a case is, of course, more poisonous. King’s yellow, if pure, yields to water nothing which gives any arsenical reaction.
A blue pigment, termed mineral blue, consists of about equal parts of arsenite of copper and potash, and should contain 38·7 per cent. of metallic arsenic (= to 51·084 As2O3H) and 15·6 of copper.
Schweinfurt green (Syn. Emerald-green), (CuAs2O4)3Cu(C2H3O2)2 is a cupric arsenite and acetate, and should contain 25 per cent. of copper and 58·4 per cent. of arsenious acid. In analysis, the copper in this compound is readily separated from the arsenic by first oxidising with nitric acid, and then adding to the nitric acid solution ammonia, until the blue colour remains undissolved. At this point ammonium oxalate is added in excess, the solution is first acidified by hydrochloric or nitric acid, and, on standing, the copper separates completely (or almost so) as Oxalate, the arsenic remaining in solution.
Another method is to pass SH2 to saturation, collect the sulphides on a filter, and, after washing and drying the mixed sulphides, oxidise with fuming nitric acid, evaporate to dryness, and again treat with nitric acid. The residue is fused with soda and potassic nitrate, the fused mass is dissolved in water, acidulated with nitric acid, and the copper is precipitated by potash; the solution is filtered, and in the filtrate the arsenic is precipitated as ammonio-magnesian arseniate or as trisulphide.[713]
[713] P. Gucci, Chem. Centrbl., 1887, 1528.