Treatise on Poisons by Sir Robert Christison

determined by the following process. The acidity of the fluid is first to be established by its effect on litmus-paper.—A small portion is next to be tested with ammonia, which, if the solution of the acid be sufficiently concentrated, will produce a radiated crystallization, as the oxalate of ammonia formed is much less soluble than oxalic acid itself. This property, according to Dr. O’Shaughnessey, distinguishes it from every other acid.[393] The remainder of the fluid is next to be subjected to the following reagents. _Hydrochlorate of lime_ causes a white precipitate, the oxalate of lime; which is dissolved on the addition of a drop or two of nitric acid,—and is not dissolved when similarly treated with hydrochloric acid, unless the acid is added in very large proportion. The easy solubility of the oxalate of lime in nitric acid distinguishes the precipitate from the sulphate of lime, which the present test might throw down from solutions of the sulphates, and which is not soluble in a moderate quantity of nitric acid without the aid of heat. The insolubility of the oxalate of lime in hydrochloric acid on the other hand distinguishes the precipitate from the tartrate, citrate, carbonate and phosphate of lime, which the test might throw down from any solution containing a salt of these acids. The last four precipitates are redissolved by a drop or two of hydrochloric acid; but the oxalate is not taken up till a large quantity of that acid is added. _Sulphate of lime_ in solution causes a white precipitate with oxalic acid, and not with any other.[394] _Sulphate of copper_ causes a faint bluish-white, or greenish-white precipitate, which is not redissolved on the addition of a few drops of hydrochloric acid. The precipitate is the oxalate of copper. It is redissolved by a large proportion of hydrochloric acid. This test does not precipitate the sulphates, hydrochlorates, nitrates, tartrates, citrates. But with the carbonates and phosphates it forms precipitates resembling the oxalate of copper. The oxalate, however, is distinguished from the carbonate and phosphate of copper by not being redissolved on the addition of a few drops of hydrochloric acid. _Nitrate of silver_ causes a dense, white precipitate, the oxalate of silver; which, when collected on a filter, dried and heated, becomes brown on the edge, then fulminates faintly and is dispersed. The object of the supplementary test of fulmination is to distinguish the oxalate of silver from the numberless other white precipitates which are thrown down by the nitrate of silver from solutions of other salts. The property of fulmination, which is very characteristic, requires, for security’s sake, a word or two of explanation, in consequence of the effect of heat on the tartrate and citrate of silver. The citrate when heated becomes altogether brown, froths up, and then deflagrates, discharging white fumes and leaving an abundant, ash-gray, coarsely fibrous, crumbly residue, which on the farther application of heat becomes pure white, being then pure silver. The tartrate also becomes brown and froths up, but does not even deflagrate, white fumes are discharged, and there is left behind a botryoidal mass, which, like the residue from the citrate, becomes pure silver when heated to redness. Another distinction between the oxalate and tartrate is that the former continues permanent at the temperature of ebullition, while the latter becomes brown. The preceding process or combination of tests will be amply sufficient for proving the presence of oxalic acid, free or combined, in any fluid, which does not contain animal or vegetable principles.