[679] Chem. News, vol. xxxvi. p. 93.
[680] Ann. Chem. Pharm., vol. lxi. p. 297.
In the animal kingdom oxalic acid is always present in the intestinal contents of the caterpillar. In combination with lime, it is constantly found in the allantois liquor of the cow, the urine of man, swine, horses, and cats. With regard to human urine, the presence or absence of oxalate of lime greatly depends upon the diet, and also upon the individual, some persons almost invariably secreting oxalates, whatever their food may be.
§ 689. Oxalic Acid, H2C2O42H2O (90 + 36), specific gravity 1·64, occurs in commerce in prismatic crystals, very similar to, and liable to be mistaken for, either magnesic or zincic sulphates. The crystals are intensely acid, easily soluble in water (1 part requiring at 14·5° 10·46 parts of water); they are also soluble in parts of cold, and readily in boiling, alcohol. Oxalic acid is slightly soluble in cold absolute ether; but ether, although extracting most organic acids from an aqueous solution, will not extract oxalic acid.
Oxalic acid sublimes slowly at 100°, but rapidly and completely at 150°; the best means of obtaining the pure anhydride is to put a sufficient quantity of the acid into a strong flask, clamp it by suitable connections to a mercury pump, and sublime in a vacuum; in this way a sufficient quantity may be sublimed a little above 100°. It is well to remember, not only its low subliming temperature, but also that an aqueous solution, if kept at 100°, loses acid; hence all evaporating or heating operations must not exceed 98°, or there will be some loss. The effect of heat is first to drive off water, then, if continued up to about 190°, there is decomposition into carbon monoxide, carbon dioxide, water, and formic acid; the two reactions occurring simultaneously—
C2H2O4 = CO2 + CO + H2O.
C2H2O4 = CO2 + CH2O2.
Heated with sulphuric acid to 110°, the following decomposition takes place:—