= CO, + CO + Oxalic Carbonic Carbonic Water acid acid gas oxide This reaction affords one of the best means of ob taining carbonic oxide for use in the laboratory. Oxidizing agents, such as manganese dioxide, peroxide of lead, nitric acid, etc.. 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.
Oxalic acid is one of the most powerful of the organic acids, and expels carbonic acid and many other acids from their salts. The acid itself and its soluble salts are poisonous. This acid is very widely diffused throughout the vegetable king dom. Sometimes it occurs in a free state (as in Boletus selpharcus), but much more as a salt, either of potash. as in the different species of Oxalis from which genus the acid was originally obtained and derives its name) and of Rumex: or of soda. as ill various species of Salieornia and Salsola; or of lime, as in rhubarb and many lichen,. In the animal kingdom it never occurs except in minute quantity and in combination with lime. Oxalate of lime is found in a crystalline shape. both in healthy and morbid urine, and is the chief constituent of the urinary calculus known from its rough exterior as the Mulberry calculus.
Oxalic acid is produced b' the action either of hydrate of potash or of nitric acid upon most organic compounds of natural occurrence. Its most common mode of preparation is by the oxi dation of starch or sugar by nitric acid. The or ganic 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 molecule of oxalic acid is composed of two carboxyl groups (C001-1) and is represented graphically by the following formula: Of the numerous oxalates the most important arc the oxalate of lime (in consequence of its physiological and pathological relations) ; the neutral oxalate of ammonia, which is the best test for the detection of lime in solution (in con sequence of the extreme insolubility of the re sulting oxalate of lime) and the acid oxalate of potash, which is employed in various manu facturing processes, as well as for removing ink and rust stains. When the neutral oxalate
of potash is mixed with sulphate of iron, a double oxalate of iron and potassium is pro duced. This compound is a strong reducing agent, and has the power of separating metals (for instance, silver) from their salts; this prop erty is made use of in photography.
The best test for oxalic 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 printing cotton, bleaching straw, etc.. oxalic acid is more accessible to the general public than many other poisons, and on this account, instances of sui cide from the swallowing of this acid are by no means uncommon. Cases of accidental poison ing, moreover, sometimes occur by its being sold by mistake for Epsom salts. Large doses destroy life very rapidly. The symptoms of oxalic acid poisoning are a burning acid taste, with a sense of constriction or suffocation; vomiting, great pain in the region of the stomach, convulsions, cold perspirations, and general collapse speedily follow; and respiration shortly before death be comes slow and spasmodic. 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 he ad ministered with the least possible delay. Salt of sorrel is almost as poisonous as the pure acid. See ANTIDOTE.