Section I.—Of the Tests for Oxalic Acid.
Oxalic acid is commonly in small crystals of the form of flattened six-sided striated prisms, transparent, colourless, free of odour, very acid to the taste, and permanent in the air. Two other common vegetable acids, the citric and tartaric acids, present a totally different crystalline form. In general appearance it greatly resembles the sulphate of magnesia, for which it has been so often and so fatally mistaken. So close, indeed, is the resemblance, that repeatedly, on desiring several persons to point out which was the poison and which the laxative, I have found as many fix on the wrong as on the right parcel. The sulphate of magnesia has of course a very different taste, being strongly bitter. Various plans have been devised for preventing the accident to which this unlucky resemblance has given rise. The best of them imply the use of a safeguard by the patient before he takes his laxative draught. It seems to have escaped the notice of those who have proposed the plans in question, that, if accidents are to be prevented in this manner, by far the simplest and most effectual security will be to let the public know, that a laxative salt ought always to be tasted before being swallowed. Its solubility has been much overrated by some chemists. It does not appear to me soluble in less than eleven parts of water.
In determining the medico-legal tests for oxalic acid, it will be sufficient to consider it in two states,—dissolved in water,—and mixed with the contents of the stomach and intestines or vomited matter. If the substance submitted to examination is in the solid state, the first step is to convert it into a solution.
1. In the form of a pure solution, its nature may be satisfactorily 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.