Marcet.[1]—Ammoniacal nitrate of silver is of no use as a test for a moderately diluted solution of the oxide of arsenic, if vegetable or animal matter be present; either the colour of the precipitate is essentially altered, or no precipitate is formed at all.[2]
If the presence of arsenic is to be inferred only when the full lemon-yellow colour of the precipitate is developed, this test is not liable to any material fallacy. The presence of a phosphate, a serious obstacle according to an old way of using the silver test, is not a source of fallacy in the instance of the ammoniacal nitrate; for the yellow phosphate of silver is so soluble in the ammonia of the test, that it is not thrown down unless the phosphatic solution is very strong.—The silver test, which is extremely delicate, was proposed by Mr. Hume, a chemist of London; and in its improved state was suggested by the late Dr. Marcet. Various foreign authors have fallen into the error of supposing that nitrate of silver without an alkali precipitates oxide of arsenic: without an alkali, pure nitrate of silver gives no precipitate, or at most a bluish-white or yellowish-white haze when both solutions are strong.
Ammoniacal sulphate of copper is prepared by the same process with the last test, sulphate of copper being substituted for nitrate of silver. It is a test of very great delicacy. It causes in solutions of the oxide of arsenic an apple-green or grass-green precipitate of the arsenite of copper. The particular tint is altered apparently by trifling circumstances; but after the precipitate has stood some hours it always assumes a tint intermediate between apple-green and grass-green. The operation of this test is prevented by hydrochloric, nitric, sulphuric, acetic, citric, and tartaric acids in excess; and also by an excess of ammonia. These difficulties are obviated by manifest precautions. It is also prevented, according to Hünefeld, by muriate, nitrate, and sulphate of ammonia;[3] and by almost all vegetable infusions and animal fluids, when the oxide of arsenic is not abundant: these difficulties cannot be obviated. Even when not prevented by such fluids, its operation is often obscured, the precipitate not possessing its characteristic colour.
Ammoniacal sulphate of copper is more open to fallacies than the silver test. Of these the most important is that in some organic fluids it strikes a green precipitate, like the arsenite of copper, though arsenic be not present.[4] The solution of bichromate of potass is turned green but not precipitated by it.
On reviewing all that has now been stated regarding the liquid tests for arsenic, it will appear that there is no single test on which absolute reliance can be placed; but that the fallacies to which they are liable are generally remote, and each of them applicable to one test only. Hence if each of the three reagents, applied with due
- ↑ London Med. Chirurgical Transactions, iii. 342.
- ↑ See a paper by myself in Edin. Med. and Surg. Journal, xxii. 60, where the fallacies to which the liquid tests are liable are investigated at great length.
- ↑ Horn's Archiv für Medizinische Erfahrung, 1827, i. 230.
- ↑ Edin. Med. and Surg. Journal, xxii. 74.
