OXALIC ACID (HO, This acid was discovered by Scheele, in 1776. It occurs in several plants, as in the wood sorrel (Oxalis acctosella) hence the name oxalic, and in common sorrel (Rumex =tom). In the juice of these plants, and in the roots of rhubarb, bistort, gentian, fic., it is combined with potash ; in several kinds of lichens, it is found in union with lime. It occurs as a mineral compound in combination with oxide of iron, called Hum badtite. It is an ingredient also of several urinary calculi Oxalic acid may be obtained by several processes : first, by decom posing the natural superoxalate of potash, or salt of sorrel, by means of acetate of lead, sulphuric acid, &e.; but it is best obtained by heating organic matter, such as sugar, potato starch, or even sawdust, with nitric acid in a retort. Having been first. obtained from sugar it re ceived the name of saccharine acid. Indeed most organic compounds which contain much carbon and no nitrogen, may be converted into oxalic acid by the action of nitric acid; hydrate of potash, in some cases, produces similar results, and this process applied to sawdust now yields much of the oxalic acid used in commerce. When nitric acid acts upon sugar or starch or sawdust, it ie decomposed, and yields nitric oxide gas in great quantity; the oxygen of the decomposed acid uniting with the carbon of the vegetable matter, converts it into oxalic acid : the crystals first obtained are to be dissolved in water and re-crystallised to render them perfectly pure.
Oxalic acid crystals are prismatic, colourless, and transparent ; tho primary form is a right rhombic prism. Their taste is extremely sour, and they are very poisonous. When dissolved in 200,000 times their weight of water, the solution reddens litmus paper. Exposed to a dry air, oxalic acid effloresces, losing water of crystallisation ; it dissolves in 15 parts of water at 50°, and in 9 parts at 60; at 212° it melts in its water of crystallisation; it ie soluble also in alcohol, but less so than in water.
Oxalic acid is usually considered to be monobasie, but most of its properties are those of a member of the dibasic series.
No means have yet been discovered of forming oxalic acid by com bining carbonic oxide and carbonic acid ; but when oxalic acid is heated with sulphuric acid, it resolves into these substances entirely, and in the proportions stated, the water which it contains combining with the sulphuric acid.
Oxalic acid contains water that cannot be totally expelled by heat ; the acid, unless combined with a base, always retaining one equivalent. Oxalic acid is to a certain extent volatile, and Mr. Faraday has shown that sublimation takes; placeeven at common temperatures ; at 212.° the volatilisation is much more considerable ; and when deprived of two-thirds of its water of crystallisation, it sublimes rapidly and without decomposing ; the sublimed acid is crystalline, and contains one equivalent of water.
When oxalic acid retaining the whole of its water of crystallisation is suddenly heated up to about 300°, it yields carbonic acid, carbonic oxide, formic acid, and water.
Oxalic acid forms compounds with bases, which are termed oxalates, and of these we shall describe some of the more important.
Oxalate of ammonia. [Ammosnen, oxalate of ammonia.] Bin oxalate of ammonia may also be formed, but it is not of im portance. When binoxalate of ammonia is heated it yields oxamie acid Oxalate of potash (KO, Aq.)—Oxalic acid forms three salts with potash : the neutral oxalate is obtained by saturating the acid with the alkali. It crystallises in flat rhombic crystals, which are so very soluble in water, that even when cold the latter takes up half its weight. When a solution of oxalic acid is added to this, precipitation of a leas soluble supersalt occurs.
Binoxalate of potash (KO, HO, + 2Aq.) may be prepared either by dissolving the requisite quantities of acid and alkali in water, or, as above mentioned, by adding oxalic acid to the neutral salt. It forms small rhombic crystals, which have a bitter taste and an acid reaction; they are soluble in about I0 times their weight of cold water. This salt, mixed with the quadroxalate, exists in oxalis acctosella, and they constitute the compound know by the name of the salt of sorrel, or salt of lemons, employed for removing ink-spots.
Quadraralate of potash (KO, may be formed by digesting the binoxalate in hydrochloric acid, which taking half the potash, the remainder becomes quadroxalate; or by saturating car bonate of potash with oxalic acid, and then adding three times the weight of oxalic acid already used. The crystals are prismatic, and are as little soluble in water as those of the binoxalate.
The oxalates of potash are all converted by a red heat into carbonate of potash ; their composition was first ascertained and pointed out by Dr. Wollaston as affording one of the most remarkable series of proofs of the doctrine of multiple proportions.
Oxalate and tino.calate of soda may be formed, but they are not employed.
Oxalate of lino (CaO, 2Aq.) is a very insoluble compound, and hence oxalic acid and lime, with certain restrictions, are used as tests of each other's presence. Baryta and strontia also form insoluble com pounds with oxalic acid. The remaining metallic oxalates of importance are noticed under the respective metals.
C 11 0 Oxalic ether, or oxalate of ethyl is best obtained by allowing a slow stream of absolute alcohol to flow into a retort con taining oxalic acid in a state of fusion, sad from which the water has been mostly driven off. The distillate is snahed with water, in which it is only slightly soluble, and finally dril over :blonde of calcium. Its specific gravity is 1.0929; vapour density, ; boiling point, 3632' Fahr. It is miscible with alcohol or ether. Potash and soda decom pose it, and ammonia converts it into oxamide. Potassium transforms it into carbonic ether with evolution of carbonic oxide.
Amyloxalic acid, or oxalamylic acid is formed on treating amylic alcohol with excess of oxalic acid.
Tests for oxalic acid.—The beat is the production of a white pre cipitate (oxalate of lime), with any soluble calcium salt, insoluble in acetic acid, but soluble in hydrochloric acid.