Crude argol is purified by dissolving it in water and destroying the colour by boiling with animal charcoal. When the clear liquid obtained from this mixture by filtration is evaporated, a white crystalline substance separates out. This is potassium hydrogen tartrate or cream of tartar.
Tartaric acid is obtained from cream of tartar. The salt is dissolved in water and nearly neutralized with milk of lime. Insoluble calcium tartrate is precipitated, and potassium tartrate remains in solution. A further quantity of calcium tartrate is obtained by adding calcium chloride to the solution just mentioned. The two precipitates of calcium tartrate are then mixed and decomposed by dilute sulphuric acid, and after the calcium sulphate is filtered off, tartaric acid is obtained as a solid by evaporating the clear liquid.
The general properties of tartaric acid are well known. It is soluble in water, giving a solution which has a pleasantly acid taste.
Citric Acid. The sharp flavour of many unripe fruits is due to the presence of citric acid; the juice of lemons contains 5-6 per cent. of the acid. The free acid is obtained in a manner precisely similar in principle to that described for tartaric acid.
Oxalic Acid. Oxalic acid and its salts, the oxalates, are very widely distributed in the vegetable kingdom. These compounds are present in wood sorrel (Oxalis acetosella), in rhubarb, in dock, and in many other plants. The acid is made on a large scale by mixing pine sawdust to a stiff paste with a solution containing caustic soda and potash. The paste is spread out on iron plates and heated, care being taken not to heat the mixture to the point at which it chars. The mass is then allowed to cool, and is mixed with a small quantity of water to dissolve out the excess of alkali. This is recovered and used again.
Sodium oxalate, which is the main product of the reaction described above, is dissolved in water and treated with milk of lime, whereby insoluble calcium oxalate is obtained, which is subsequently decomposed with sulphuric acid, yielding oxalic acid.
Potassium hydrogen oxalate is sometimes called salts of sorrel, and potassium quadroxalate, salts of lemon. The most familiar use of the latter substance is in the removal of ink stains.
Oxalic acid and its salts are poisonous. The free acid has sometimes been mistaken for sugar with fatal results.
Formic Acid (L. formica, an ant) is found both in the vegetable and in the animal kingdom. If the leaf of a stinging nettle is examined with a microscope, it is seen to be covered with long pointed hairs having a gland at the base. This gland contains formic acid. When the nettle is touched lightly, the fine point of the hair punctures the skin, and a subcutaneous injection of formic acid is made, which quickly raises a blister.
The inconvenience which arises from the stings of bees and wasps, also from the fluid ejected by ants when irritated, is due to formic acid. The remedy in each case is the same; the acid must be neutralized as quickly as possible with mild alkali, such as washing soda.