Oxalic acid crystallizes in colorless, transparent, oblique, quadrilateral prisms with two-sided summits. The crystals are inodorous, have a strongly acid taste, faintly effloresce in a dry atmosphere, redden litmus paper, and when pure are completely volatilized by heat, and without becoming blackened. They dissolve in from 8 to 11 parts of water at 15.5 C. (60 F.), in their own weight of water at 100 C. (212 F.), and in 4 parts of alcohol; the addition of a small quantity of nitric acid to the water causes them to dissolve more readily. Nearly all the oxalates are insoluble in water, excepting the alkaline. Oxalate of calcium is insoluble, and hence oxalic acid is useful as a test for calcium, and is usually employed in the form of oxalate of ammonium; if the liquor to be examined contains any free acid, this must first be neutralized, as the oxalate can only detect calcium in neutral or alkaline fluids. Oxalic acid reduced by hydrogen is converted into glycolic and acetic acids, and if the action be kept up sufficiently long the glycolic becomes wholly formed into acetic acid.Oxalic acid may be detected in any solution, by being entirely volatilized by heat; by yielding a white precipitate with nitrate of silver, soluble in nitric acid; and by giving a white precipitate with lime water, which is insoluble in water, readily soluble in nitric acid, insoluble in acetic acid, and which, when dried and heated to low redness, is converted, without blackening, into carbonate of calcium. Solution of sulphate of calcium produces a bluish-white precipitate with oxalic acid. Oxalic acid is sometimes contaminated with nitric acid, which gives a faint odor to it, and stains the cork of the bottle in which it is kept, yellow. If a very dilute solution of sulphate of indigo, containing the impure crystals, be boiled, the nitric acid present will decolorize the solution. On account of the resemblance between crystals of this acid and of magnesium sulphate, the latter has been used as an adulterant. This resemblance has also led to cases of poisoning, the person believing the acid to be Epsom salts. The acid may likewise be used for removing iron-rust and ink-stains from linen, and is employed in calico printing as a bleaching and discharge agent.
||Dr. Willmar Schwabe