ACETIC ACID, the organic acid which gives to vinegar (from which it derives its name) its characteristic sharp taste, is the second of the series of aliphatic acids, formic acid (q.v.) being the first. Its formula is
and it occurs in nature in plant juices occasionally as free acid, but more generally as calcium or potassium salts. Three processes are available for large-scale production of acetic acid, CH•COOH.
Several species of bacteria con vert dilute alcoholic liquors into impure acetic acid, in the form ordinarily known as vinegar. This bacterial fermentation, which takes place in the presence of air and a certain amount of nutri ent phosphates and nitrogenous material necessary for the feed ing of the bacteria, involves the oxidation of ethyl alcohol (see ALCOHOL) into dilute acetic acid. A more concentrated acid is obtainable by neutralizing the dilute acid with lime to form cal cium acetate and then distilling a mixture of this salt with strong sulphuric acid. In the oxidation of alcohol to acetic acid the living acetic bacteria may be replaced by specific enzymes (unor ganized ferments) or even by spongy platinum or palladium since either of these, in the presence of air, oxidizes the alcohol first to acetaldehyde and then to acetic acid.
2.
Distillation of Wood.—A considerable pro portion of the acetic acid of commerce is obtained by the destruc tive distillation of wood, the hard woods yielding 4.7o to 6.5% of the concentrated acid.
The starting point in the indus trial synthesis of acetic acid is calcium carbide made in the electric furnace from limestone and coal. The acetylene evolved from the carbide by addition of water is converted into acetaldehyde by passing into hot dilute sulphuric acid containing mercuric sulphate. The acetaldehyde is further oxidized by air or oxygen in the presence of a catalyst consisting of the oxides of metals such as iron, vanadium and uranium, acetic acid being thereby formed.
Anhydrous acetic acid, usually termed glacial acetic acid owing to its ice-like crystallization at 16.7°C., has a very pungent odour and boils at I 18°C. to a heavy inflammable vapour. Its alkali salts are soluble in water. Ferrous and ferric acetate are used as mordants in dyeing (q.v.). Normal lead acetate is known in commerce as sugar of lead and basic copper acetates are known as verdigris (q.v.).
Acetic acid and the acetates are detected by heating with ethyl alcohol and sulphuric acid ; ethyl acetate is evolved and recog nized by its fragrant odour. When heated with arsenious oxide, acetates give rise to the malodorous and highly poisonous cacodyl oxide.
Glacial acetic acid is sometimes used as a caustic for corns. Vinegar (with 5% acetic acid) is taken as a cure for obesity, but this application is unde sirable since continued use may so injure the mucous membrane of the stomach as to cause indigestion and a morbid reduction in weight. The soluble acetates, and especially potassium acetate, are useful medicinal agents for after absorption in the blood, they are oxidized to carbonates, thus acting as remote alkalis increasing the alkalinity of the blood or reducing the acidity of the urine without the disturbing influence of alkalis on the digestive tract.
