OXALIC ACID. A vegetable acid, first discovered in the juice of the Oxalis aostosello ; it was afterwards ascertained that the same acid might be produced ar tificially by the action of the nitric acid upon sugar; this process yields it in slen der prismatic crystals, intensely sour, and soluble in about ten parts of cold wa ter. These crystals consist of 1 atom of real acid, and 3 of water : the equivalent of the acid is 86 ; and in its anhydrous state, as it exists in the dry oxalates, it is constituted of 2 atoms of carbon (6X2) =12, and 3 of oxygen (8X3)=24: so that it may be represented by an atom of car bonic acid and one of carbonic oxide. Solutions of oxalic acid, or of soluble ox alates, yield an insoluble precipitate in solutions containing lime and its salts : hence its use in the laboratory as a test of the presence of that earth. The solu tion of oxalate of ammonia is generally used for the purpose. Oxalic acid is a powerful poison, and, from its resem blance to Epsom salt, it has sometimes been sold and mistaken for that harmless aperient. In such cases, the best anti dote is a mixture of ci.slk and water, and ;where it is immediately administered it generally prevents the accession of fatal symptoms : it forms an insoluble oxalate of lime, which is inert.
It is usually prepared upon the small scale by digesting four parts of nitric acid of specific gravity 1.4, upon one part of sugar in a glass retort ; but on the large scale, in a series of salt-glazed stoneware pipkmns, two thirds filled, and set in a water bath. The addition of a little sul phuric acid has been found to increase the product. 15 pounds of sugar yield fully 17pounds of the crystalline acid. This acid exists in the juice of wood sor rel, the oxalis acetosella, in the state of a bioxalate ; from which the salt is extract ed as an object of commerce in Switzer land, and sold under the name of salt of sorrel, or sometimes, most incorrectly, under that of salt of lemons.
Some prefer to make oxalic acid by act ing upon 4 parts of sugar, with 24 parts of nitric acid of specifie gravity 1.220, heat ing the solution in a retort till the acid begins to decompose, and keeping it at this temperature as long as nitrous gas is disengaged. The sugar loses a portion of its carbon, which, combining with the oxygen of the nitric acid, becomes car bonic acid, and escapes along with the deutoxide of nitrogen. The remaining carbon and hydrogen of the sugar being oxidized at the expense of the nitric acid, generate a mixture of two acids, the ox alic and the malic. Whenever gas ceases to issue, the retort must be removed from the source of heat, and set aside to cool ; the oxalic acid crystallizes, but the final p remains dissolved. After draining these crystals upon a filter funnel, if the brown ish liquid be further evaporated, it will furnish another crop of them. The re siduary mother water is generally regard ed as malic acid, but it also contains both oxalic and nitric acids ; and if heated with 6 parts of the latter acid, it will yield a good deal more oxalic acid at the expense of the malie. The brown crystals now formed being, however, penetrated with nitric, as well as malic acid, must be allowed to dry and effloresce in warm dry air, whereby the nitric acid will be got rid of without injury to the oxalic. A second crystallization and efflorescence will entirely dissipate the remainder of the nitric acid, so as to afford pure oxalic acid at the third crystallization. Sugar affords, with nitric acid, a purer oxalic acid, but in smaller quantity, than saw dust, glue, silk, hairs, and several other animal and vegetable substances.
Oxalic acid occurs in aggregated prisms when it crystallizes rapidly, but in tables of 4 and 6 sides when crystallized more slowly.