| Sulphate of magnesia, | 1 |
| Chloride of ammonium, | 1 |
| Solution of ammonia, | 4 |
| Water, | 8 |
Dissolve; then allow to stand for several days; finally filter, and keep for use.
The solubility of the magnesium arseniate itself, and the general dislike which chemists have to weighing in such hygroscopic material as a filter, are, perhaps, the main reasons for the variation of this old method, which has lately come into notice. Rose proposed some time ago the conversion of the double salt into the pyro-arseniate—a method condemned by Fresenius and Parnell, but examined and pronounced a practicable and accurate process by Remol, Rammelsberg, Thorpe, Fuller, Wittstein, Emerson, Macivor, Wood, and Brauner. The modification of Rose’s process, recommended by Wood,[790] and still further improved by Brauner,[791] may be accepted.
[790] Zeitschrift für anal. Chem., vol. xiv. p. 356.
[791] Ibid., xvj. pp. 57, 58.
The precipitation is effected by magnesia mixture, with the addition of half its bulk of alcohol. The solution is allowed to stand for several hours, until it is possible to decant the clear liquid from the precipitate; the latter is now dissolved in ClH, reprecipitated as before, thrown on a small filter, and washed with a mixture of one volume of ammonia, two volumes of alcohol, and three of water.
The precipitate is now dried, and transferred as completely as possible from the filter into a small porcelain crucible, included in a larger one made of platinum, moistened with nitric acid, covered and heated at first gently, lastly to a bright redness; the filter is then treated similarly, and the crucible with its contents weighed. Pyro-arseniate of arsenic (Mg2As2O7) contains 48·29 per cent. of metallic arsenic.