Oxalic Acid

OXALIC ACID, ethane-diacid, (COOH)₂·2H₂O. Oxalic acid is very widespread in nature. It is found both in the free state (in certain fungi) and, chiefly, in the form of salts. The potassium salt of oxalic acid, both acid and neutral, is found in sorrel, rhubarb, belladonna, and a number of other plants. The calcium salt of oxalic acid is extremely common in roots, leaves, and bark in the form of crystals of various shapes: a) druses—aggregates of crystals having a stellate shape, b) raphides—bundles of needle-like crystals, and c) single crystals. In the animal organism, oxalic acid is found as a normal product of metabolism and originates, firstly, directly from the oxalic and citric acid salts of food and, secondly, as a result of the incomplete combustion of carbohydrates and other dietary constituents. Oxalic acid is found in the urine in oxaluria (see), in urinary sediments (see Urine), in renal and bladder stones, in the mucus of the gallbladder, and so forth. In pathological conditions (diabetes, jaundice), the amount of oxalic acid excreted in the urine can reach 1.5 g per day. Industrially, oxalic acid is obtained by fusing coniferous wood sawdust with alkalis, whereupon the resulting oxalic acid salts (oxalates) are decomposed by acids. Oxalic acid can also be obtained through the action of Aspergillus niger or acetic acid bacteria on carbohydrates. Oxalic acid crystallizes in the form of colorless monoclinic prisms with a specific gravity of 1.64, soluble in 10 parts of water and 2.5 parts of alcohol, with a strongly acidic reaction. When heated to about 100°, oxalic acid loses water, turning into a white powder; at a higher temperature, it decomposes, yielding, depending on conditions, carbon dioxide, water, carbon monoxide, and formic acid: (COOH)₂ → CO₂ + HCOOH, CO + H₂O. Dehydrating agents (ZnCl₂, P₂O₅, H₂SO₄, etc.) act on oxalic acid in a similar manner. Oxidizing agents convert oxalic acid into carbonic acid. The identification of oxalic acid is carried out using calcium salts, which yield a precipitate of calcium oxalate that is insoluble in water and acetic acid, and readily soluble in hydrochloric and nitric acids. Oxalic acid finds no medical application, but is of considerable toxicological interest. When ingested internally, oxalic acid and its soluble salts cause poisoning symptoms that very frequently (up to 50%) have a fatal outcome (see Poisoning). Pathological-anatomical findings: the gastric mucosa is swollen and hyperemic, and the esophagus is also largely affected. In the duodenum, there are scabs and ecchymoses. The blood is fluid and bright scarlet (in fresh corpses). In the kidneys, there are calcium oxalate infarcts, and the tubules are clogged with white crystals. In forensic-chemical investigations, the characteristic shape of calcium oxalate crystals is established, and the crushed and dried material is extracted with acidified HCl in boiling alcohol; the filtrate is evaporated to dryness on a water bath, the residue is dissolved in water, and the described reactions are performed. S. Shubin.