The Encyclopedia Americana (1920)/Ace′tic acid
ACE′TIC ACID, an organic acid belonging in the fatty series, and resulting from the oxidation of alcohol. It is important on account both of its extensive use in the arts, and of its properties as viewed from the standpoint of the theoretical chemist. Its formula is CH3.COOH, or C2H3O.OH. It is a monobasic acid, the hydrogen in the radical CH3 not being replaceable by a metal or another radical. In its dilute state it has been known for centuries as vinegar, and in strong vinegar the characteristic odor of the acid is quite marked. It is formed naturally in the fluids of many plants, especially trees; in some cases as free acid and in other in combination, usually as acetate of lime or of potassium; also in many organic acetates, and in the oils of many seeds. It is a constituent also of certain animal fluids, as, for example, milk. It may be obtained by the oxidation, decomposition and destructive distillation of many organic bodies. It is produced, as in the manufacture of cider vinegar, by the action of the microscopic plant Mycoderma aceti, better known as “mother-of-vinegar,” or “vinegar plant,” upon weak alcohol. In the manufacture of vinegar the alcohol required for the transformation is present in the cider as the result of a previous alcoholic fermentation. It has been shown that “mother-of-vinegar” has no effect upon pure alcohol; a certain amount of albuminous and mineral matter must be present to serve as food for the plant. A very pure form of acetic acid is prepared by subjecting alcohol to the powerful oxidizing action of spongy platinum hung above it in abundance of air. The platinum absorbs oxygen and alcohol vapor at the same time, and these combine to form acetic acid and water. Acetic acid has also been produced by oxidizing alcoholic liquors by blowing ozone into them. The greater part of the acetic acid of commerce is obtained by the destructive distillation of wood, acetate of lime being a by-product in the manufacture of wood-alcohol. (See Wood-Alcohol, under Alcohol). The acetate of lime so obtained may be decomposed by the addition of sulphuric acid, when acetic acid is liberated, or it may be treated in any one of a number of other ways for the recovery of the acetic acid. One of the best methods consists in mixing the commercial acetate of lime with calcium chloride, and concentrating the solution until the compound known as calcium aceto-chloride (CaC2H3O3Cl.5H2O) crystallizes out. The crystals so formed are then dissolved in water, the solution is filtered through animal charcoal, 10 per cent more calcium chloride is added, and the operation is repeated to obtain a new crop of purer crystals. These crystals are finally distilled with moderately strong sulphuric acid, when a very pure acetic acid is given off. This is concentrated to the required degree by distilling.
By heating dry acetate of sodium with an equivalent quantity of concentrated sulphuric acid it is possible to obtain acetic acid in a state free from water. The acid so obtained is a colorless liquid boiling at 244° F., and solidifying in prismatic or tabular crystals at about 63° F., into an ice-like mass; from this property the anhydrous acid has been called glacial acetic acid.
Acetic acid is uninflammable in its liquid state, but its vapor burns with a fine blue flame, with the production of water and carbonic acid gas. It has a stinging sour taste, and when strong it blisters the skin. It mixes with alcohol and with ether in all proportions and is used as a solvent for resins, essential oils, gelatin, albumen, fibrin and other organic substances, being a useful substitute for alcohol in certain cases on account of its relative cheapness.
The commercial acid is generally very impure, containing more or less of sulphuric acid and sulphates; sulphurous acid; hydrochloric acid and chlorides; and the salts of copper, lead zinc, tin and arsenic.
Acetic acid is used in the production of the acetates of lead, copper, aluminum and iron; in calico printing; in the manufacture of varnishes; and in domestic economy as vinegar. It has also some medicinal use as an outward application. Its extended solvent properties have already been noted.
Lead acetate (or sugar of lead) and copper acetate (or verdigris) are the most important compounds of acetic acid with the heavy metals. Aluminum acetate and the iron acetates are much used in dyeing. The acetates of lead, potassium and ammonia are also largely used in medicine.
Acetic acid may be formed synthetically by exposing a mixture of one volume of acetylene (q. v.) and two volumes of air to daylight, in the presence of a weak solution of caustic potash. The acetylene is slowly oxidized, combining simultaneously with the caustic potash to form acetate of potash, according to the formula
C2H3 + O + KOH = CH3.COOH
Acetylene, Caustic potash, Acetate of potash
From the acetate of potash so formed the acetic acid can readily be obtained. This mode of formation is of no practical value, but it has a theoretical interest.
The relations of acetic acid with the organic radicals are too numerous and complicated to receive general treatment in the present article. The more important ones are noticed elsewhere. See Aldehyde; Alcohol; Ether; Vinegar; etc.