§ 707. Metallic Arsenic, at. wt. 75, specific gravity of solid 5·62 to 5·96, sublimes without fusion in small quantities at 110° (230° F.) Guy. It occurs in commerce in whitish-grey, somewhat brittle, crystalline masses, and is obtained by subjecting arsenical pyrites to sublimation in earthen retorts, the arsenic being deposited in suitable receivers on sheet iron. Metallic arsenic is probably not poisonous, but may be changed by the animal fluids into soluble compounds, and then exert toxic effects—volatilised metallic arsenic is easily transformed in the presence of air into arsenious acid, and is therefore intensely poisonous.
§ 708. Arsenious Anhydride—Arsenious Acid—White Arsenic—Arsenic, As2O3 = 198; specific gravity of vapour, 13·85; specific gravity of opaque variety, 3·699; specific gravity of transparent variety, 3·7385. Composition in 100 parts, As 75·75, O 24·25; therefore one part of metallic arsenic equals 1·32 of As2O3. It is entirely volatilised at a temperature of 204·4°.
In analysis it is obtained in brilliant octahedral crystals as a sublimate on discs of glass, or within tubes, the result of heating a film of metallic arsenic with access of air. It is obtained in commerce on a very large scale from the roasting of arsenical pyrites. As thus derived, it is usually in the form of a white cake, the arsenious acid existing in two forms—an amorphous and a crystalline—the cake being generally opaque externally, whilst in the centre it is transparent. According to Kruger, this change from the crystalline to the amorphous condition is dependent upon the absorption of moisture, no alteration taking place in dry air. Both varieties of arsenious anhydride are acid to test-paper.
The solubility of arsenious acid is often a question involving chemical legal matters of great moment. Unfortunately, however, no precisely definite statement can be made on this point, the reason being that the two varieties of arsenic occur in very different proportions in different samples. Both the amorphous and crystalline varieties having very unequal solubilities, every experimenter in succession has given a different series of figures, the only agreement amid the general discrepancy being that arsenic is very sparingly soluble in water.
The statement of Taylor may, however, be accepted as very near the truth, viz., that an ounce of cold water dissolves from half a grain to a grain. According to M. L. A. Buchner,[701] one part of crystalline arsenious acid dissolves after twenty-four hours’ digestion in 355 parts of water at 15°; and the amorphous, under the same condition, in 108 of water. A boiling solution of the crystalline acid, left to stand for twenty-four hours, retains one part of acid in 46 of water; a similar solution of the amorphous retains one of arsenic in 30 parts of water, i.e., 100 parts of water dissolve from 2·01 to 3·3 parts of As2O3.
[701] Bull. de la Société Chem. de Paris, t. xx. 10, 1873.
Boiling water poured on the powdered substance retains in cooling a grain and a quarter to the ounce; in other words, 100 parts of water retain ·10. Lastly, arsenious acid boiled in water for an hour is dissolved in the proportion of 12 grains to the ounce, i.e., 100 parts of water retain 2·5.
K. Chodomisky[702] has investigated the solubility of recrystallised arsenious acid in dilute acids, and his results are as follows:—100 c.c. of 1·32 per cent. hydrochloric acid dissolves 1·15 grm. As2O3 at 18·5°. 100 c.c. of 6 per cent. hydrochloric acid dissolves 1·27 grm. at 18·5°. 100 c.c. of pure hydrochloric acid of the ordinary commercial strength dissolves 1·45 grm. As2O3. 100 c.c. of dilute sulphuric acid at 18° dissolves about 0·54 grm.; at 18·5° from 0·65 to 0·72 grm.; and at 80° from 1·09 to 1·19 grm.