Oxalic Acid, an important and powerful acid discovered by Scheele in 1770, or as claimed by some by Bergman; symbol, H2C2O4, 2H20; chemical equivalent, 126. It occurs in vegetables, animals, and rarely in minerals, as in the form of sesquioxalate of iron in humboldtite. Of the juices of plants it is a frequent constituent. Its name is derived from its giving to the leaves of the wood sorrel (oxalis acetosella) their very acid taste. In this and in the common sorrel (rumex acetosa) it occurs combined with potash as binoxalate of potash. Combined with lime, it gives solidity to many lichens, and is found in the roots of rhubarb, valerian, and other plants. It is found in a free state in the bristles of the chick pea (cicer arktinum). It is artificially produced by the oxidation of sugar or of starch by nitric acid. Scblesinger'a method, recommended by Berzelius, is to dissolve one part of dry loaf sugar in 8¼ parts of nitric acid of specific gravity 1.38, and heat in a flask till effervescence, caused by the escape of carbonic acid and nitric oxide, ceases. The solution is then evaporated by a water bath to one sixth of its bulk, and the acid crystallizes on cooling. The product varies greatly in quantity accordjng to the manner in which the nitric acid is applied.
The crystals are colorless transparent prisms of four or six sides. They have a very sour taste, and dissolve in nine parts of cold or about one part of boiling water. In a very dry atmosphere they effloresce slightly, and gently heated they become opaque, and lose two atoms (28.5 per cent.) of water, their composition then being H2C3O4. The crystals may crumble to powder, and even be almost wholly sublimed, without decomposition; but the other atom of water is expelled only at a decomposing heat,_ when the compound is converted into carbonic and formic acids and carbonic oxide. If the whole of the water be abstracted by treatment with strong sulphuric acid, the elements of dry oxalic acid are instantly resolved into equal volumes of carbonic acid and carbonic oxide. Two salts of oxalic acid are of especial importance, the binoxalate of potashand oxalate of lime. The former, known as salt of sorrel, sometimes improperly called salt of lemons, is used to remove ink stains from linen, which it does by forming a soluble double salt of potassium and the metal whose oxide or compound produces the stain. For lime oxalic acid has a very strong affinity, separating it from its solution in much stronger acids, and converting it into an insoluble oxalate.
The acid is consequently an excellent test of the presence of lime in solutions. - Oxalic acid is a corrosive poison, extremely virulent and rapid in its effects when taken into the stomach in large doses; and from the resemblance of its crystals to those of Epsom salts, it has often been sold and administered instead of this purgative with fatal effects. Emetics and the stomach pump may be immediately applied, but the true antidote is copious draughts of water containing pulverized chalk or magnesia. These neutralize the acid, forming with it an insoluble oxalate of lime or magnesia, either of which is harmless. The salts formed by oxalic acid with potassa or ammonia are also poisonous, and consequently these alkalies are not to be used as antidotes. Some of the plants mentioned above, which contain oxalate of potassa, are efficient antiscorbutics, and the acid itself has been used for this purpose. - Oxalic acid is largely employed in calico printing for discharging colors; it is also used for cleaning the straw of bonnet makers and the leather of boot tops, and for removing stains of ink and iron rust from fabrics. Many tons of oxalic acid are now made weekly in England for the calico printers, by heating saw dust with a mixture of hydrate of potash.
A concentrated solution of mixed caustic soda and potash, of specific gravity 1.35, is prepared, containing two atoms of hydrate of soda to one of hydrate of potash. Saw dust is introduced in order to form a thick paste, and this is placed in layers on heated iron plates, and stirred constantly while the temperature is gradually raised. The heat is continued for three or four hours, taking care to avoid charring. The mass becomes thoroughly dry, and finally contains 28 to 30 per cent, of oxalic acid in combination with soda. Hydrate of lime converts the oxalate of soda into oxalate of lime, from which oxalic acid is obtained by treatment with sulphuric acid.