Acidum Oxalicum. Oxalic Acid.
Related entry: Formic acid
H2C2O4, 2H2O = 126.048.
Oxalic acid, (COOH)2, 2H2O, may be obtained by heating sawdust with potassium or sodium hydroxide, with subsequent liberation of the acid and purification by recrystallising. A pure acid may be prepared by evaporating a mixture of cane sugar and nitric acid of specific gravity 1.38, and crystallising the product. It should leave no residue when ignited on platinum foil. It occurs in large, colourless, transparent, monoclinic crystals, which effloresce in the air. The hydrated acid melts in its water of crystallisation at 98°, and gradually becomes anhydrous, without melting, at a lower temperature. The anhydrous acid is volatile with partial decomposition at a temperature of about 150°, The commercial product may contain as impurities sodium, potassium, calcium, lead, sulphates, and organic matter.
Soluble in water (1 in 12), less soluble in alcohol, sparingly soluble in ether, insoluble in chloroform, benzene, or petroleum ether.
Action and Uses.—Oxalic acid, besides acting as an acid, exerts the specific action of the oxalates. It is a poison to all forms of animal life, and to plants. A 0.5 per cent. solution is equal to a 5 per cent. solution of phenol as a bactericide and disinfectant. It causes death by paralysis of the central nervous system. Oxalic acid and the oxalates prevent the coagulation of blood by removing the calcium present from its normal combination. They also decalcify the tissues generally. It is rarely employed in therapeutics, but doses of 6 centigrams (1 grain) three times daily have been given with apparent success in sciatica. The whole action of the oxalates is probably to be ascribed to their power of freeing the tissues from ionisable calcium. The acid is used chiefly for technical purposes, including the preparation of test and volumetric solutions, the commercial acid being purified for this purpose by recrystallisation. The antidotes in cases of poisoning by oxalic acid are milk of lime, saccharated solution of lime, lime water, chalk, whiting, or wall plaster, mixed with water. Alkalies, alkali carbonates, magnesia, and magnesium carbonate should not be given, neither should the stomach tube be used nor emetics administered unless the case be treated immediately after ingestion of the poison; it is preferable that the poison be converted into the insoluble calcium salt, and evacuation brought about by means of castor oil or an enema.
Dose.—2 to 6 centigrams (1/4 to 1 grain).