OXALIC ACID, one of the oldest known organic acids. Scheele in 1776 prepared it by oxidizing sugar with nitric acid, and showed it to be identical with the acetosellic acid obtained from wood-sorrel, with the formula H2C204, or, in its usual crystalline form, It is found in the form of its acid potassium salt in many plants, especially in wood-sorrel (Oxalis Acetosella) and in varieties of Rumex; as ammonium salt in guano; as calcium salt in rhubarb root, in various lichens and in plant cells; as sodium salt in species of Salicornia and as free acid in varieties of Boletus. It is also present in urine and in urinary calculi. It is formed in the oxidation of many organic compounds (e.g., sugar, starch and cellulose) by nitric acid, and also by the fusion of many oxygen-containing compounds with caustic alkalis, the latter method being employed for the manu facture of oxalic acid. In this process cellulose (in the form of sawdust) is made into a stiff paste with a mixture of strong caustic potash and soda solution and heated in flat iron pans to 2oo-25o° C. The somewhat dark-coloured mass is lixiviated with a small amount of warm water in order to remove excess of alkali, the residual alkaline oxalates converted into insoluble calcium oxalate by boiling with milk of lime, the lime salt separated, and decom posed by means of sulphuric acid. It is found that the sawdust obtained from soft woods is the best material for use in this process. It may be obtained synthetically (as sodium salt) by heating sodium in a current of carbon dioxide to 360° C, or by heating sodium formate to 400° C (V. Merz and W. Weith, 1882), by the oxidation of ethylene glycol and by the spontaneous hydrolysis of an aqueous solution of cyanogen gas.
The hydrated acid crystallizes in prisms which effloresce in air, and are readily soluble in water. It loses its water of crystalliza tion at 100° C, and begins to sublime at about 150-16o° C, whilst on heating .to a still higher temperature it partially decomposes into carbon dioxide and formic acid, or into carbon dioxide, car bon monoxide and water, the latter decomposition being also brought about by heating oxalic acid with concentrated sulphuric acid. The anhydrous acid melts at 189.5° C and is frequently used as a condensing agent. Phosphorus pentachloride decom poses it into carbon monoxide and dioxide, the reaction being the one generally applied for the purpose of preparing phosphorus oxychloride. When heated with glycerin to oo° C it yields formic acid and carbon dioxide; above this temperature, allyl alcohol is formed. Nascent hydrogen reduces it to glycollic acid. Potas sium permanganate in acid solution oxidizes it to carbon dioxide and water; the manganese sulphate formed has a catalytic accel erating effect on the decomposition.
Oxalic acid is very poisonous, and by reason of its great sim ilarity in appearance to Epsom salts, it has been very frequently mistaken for this substance with, in many cases, fatal results. The antidotes for oxalic acid poisoning are milk of lime, chalk, whiting or even wall-plaster, followed by evacuation brought about by an enema or castor oil. Only the salts of the alkali metals are soluble in water. Beside the ordinary acid and neutral salts, a series of salts called quadroxalates is known, these being salts containing one molecule of acid salt, in combination with one molecule of acid, one of the most common being "salt of sorrel," The oxalates are readily decom posed on heating, leaving a residue of carbonate, or oxide of the metal. The silver salt decomposes with explosive violence, leaving a residue of the metal.
Potassium ferrous oxalate, FeK2(C204)2.H20, is a strong reduc ing agent and is used as a photographic developer. Potassium ferric oxalate, FeK3(C204)3, is used in the preparation of platinotypes, owing to the fact that its solution is rapidly decomposed by sunlight, 2FeK3 (C204)3 = 2 FeK2 (C204)2 +K2C204 + 2CO2. Ethyl oxalate, prepared by boiling anhydrous oxalic acid with absolute alcohol, is a colourless liquid which boils at 186° C. Methyl oxalate, which is prepared in a similar man ner, is a solid melting at 54° C. It is used in the preparation of pure methyl alcohol (q.v.). Oxalyl chloride, a liquid boiling at 64° C (m.p. —52° C), has been obtained by the action of phos phorus pentachloride on anhydrous oxalic acid. Oxamic acid, is obtained on heating acid ammonium oxalate; by boiling oxamide with ammonia ; and among the products pro duced when amino-acids are oxidized with potassium permanga nate. It is a crystalline powder difficultly soluble in water and melting at 21o° C (with decomposition). Its ethyl ester, known as oxamethane, crystallizes in rhombic plates which melt at '15° C. Oxamide (CONF12)2, is best prepared by the action of ammonia on the esters of oxalic acid. It is also obtained by the action of hydrogen peroxide on hydrocyanic acid, or of manganese dioxide and sulphuric acid on potassium cyanide. It is a white crystalline powder which is almost insoluble in cold water. It melts at 417-419° C (with decomposition) when heated in a sealed tube (A. Michael, 1895). When heated with phosphorus pentoxide it yields cyanogen. It is readily hydrolysed by hot solutions of the caustic alkalis. Substituted oxamides are produced by the action of primary amines on ethyl oxalate.