Tests.—Crystals of oxalic acid are met with as four-sided prisms, colorless, without odor permanent in the air, and very acid; this last character distinguishing them from crystals of sulphate of magnesia and sulphate of zinc. The crystals, when heated, melt, and are dissipated without combustion, and leaving no residue. This character is important as a means of distinguishing oxalic acid from other similar crystals. They are soluble in from eight to twelve parts of cold water. This acid may be thus recognised in solution:
1. Nitrate of silver throws down, with oxalic acid, an abundant white precipitate (oxalate of silver), which is soluble in nitric acid. The oxalate of silver, when dried and heated on platinum foil, detonates, and is dissipated in a white vapor.
2. Sulphate of calcium causes a white precipitate with oxalic acid (oxalate of calcium) which is soluble in nitric or hydrochloric, but not in any vegetable acid.
The solution containing the acid should be concentrated before testing, if it be not present in considerable quantity.
Lime water and all soluble lime salts throw down precipitates with oxalic acid; but as these are liable to be mistaken for a precipitate with sulphuric acid, it is better to use sulphate of calcium which is slightly soluble, as the test agent. A good deal of the test solution must be used, and the precipitate takes time to settle.
3. Sulphate of copper gives a faint bluish precipitate with oxalic acid (oxalate of copper), which is not redissolved by a few drops of hydrochloric acid.
These tests will not act if the solution contain nitric acid in excess, in which case the liquid must be evaporated to crystallization, and the crystals washed and redissolved in water.
These tests for oxalic acid should never be applied without previously separating it from all organic matter. This is best done by first of all acidulating the suspected fluid with acetic acid, and then adding acetate of lead, which combines with the oxalic acid to form a white insoluble salt, which may ordinarily be removed by filtration or subsidence. This filtrate, after being well washed, is to be diffused in water, and into this a current of sulphuretted hydrogen gas is to be passed for some considerable time. This will throw down the lead as sulphide, leaving the oxalic acid in the fluid; any organic matter will also be carried down. Filtration will separate the solids from the liquid containing the acid, which may now be evaporated until crystals are formed, which may be tested in the usual way.
Acetic Acid.—Although this acid, in its concentrated state, is highly corrosive, yet it is very seldom brought under the notice of the toxicologist.
In the case of a young woman reported by Orfila, death quickly occurred after several attacks of convulsions. At the subsequent post-mortem examination, the integuments of the dependent parts of the body were found very livid; the tongue and œsophagus were of a dirty brown color, the latter being intersected by a fine net-work of capillary vessels; and the interior of the stomach was interspersed with black elevations caused by the presence of coagulated blood in the sub-mucous areolar tissue. The mucous membrane was entire.