Oxalic Acid, , occurs in colourless, transparent, oblique, rhombic prisms, which have an intensely sour taste, and are soluble in nine parts of cold water, and much more freely in boiling water. When heated to ( C.) the crystals lose their two equivalents (or 28.5 per cent.) of water, and the residue, consisting of the anhydrous acid, , becomes opaque. When the crystallised acid is rapidly heated to about ( C.), or when it is warmed with strong sulphuric acid, it is decomposed into carbonic acid and carbonic oxide gases, and into water. Oxidising agents, such as binoxide of manganese, peroxide of lead, nitric acid, &c., convert oxalic into carbonic acid, and on this property is based a good method of determining the commercial value of the black oxide of manganese. One of the most powerful of the organic acids, it expels carbonic acid and many other acids from their salts. The acid itself, which, like its soluble salts, is poisonous, is very widely diffused throughout the vegetable kingdom, sometimes in the free state, but more frequently as a salt of lime, as in rhubarb and many lichens. In the animal kingdom it never occurs except in minute quantity, and in combination with lime. It is produced by the action of either caustic potash or nitric acid upon most organic compounds of natural occurrence. Its most common mode of preparation is by the oxidation of starch or sugar by nitric acid. The organic compound and the nitric acid are heated in a flask till all effervescence has ceased, after which the solution is evaporated, and the oxalic acid separates in crystals on cooling. The most important salts are the oxalate of ammonia, , used as a test for lime, and the binoxalate of potash or salt of sorrel, , also known as essential salt of lemons, and which is popularly used for removing ink-stains or for cleaning brass.
The best test for this acid is the production of a white precipitate (of oxalate of lime), on the addition of any soluble salt of calcium. The precipitate is insoluble in water, in solution of potash, and in acetic acid, but dissolves in the mineral acids. A solution of nitrate of silver also gives a white precipitate of oxalate of silver, which explodes when heated.
In consequence of its employment in cotton-printing, bleaching straw, &c., oxalic acid is more accessible to the general public than many other poisons; and on this account instances of suicide from the swallowing of this acid are by no means uncommon. Cases of accidental poisoning, moreover, sometimes occur from its being sold by mistake for Epsom salts. Large doses destroy life very rapidly. With the view of converting the free acid in the stomach into an insoluble and inert salt, chalk, whiting, or lime-water, with full draughts of milk, should be administered with the least possible delay. Salt of sorrel is almost as poisonous as the pure acid.