ACETIC ACID. (Fn., Acido acetique ; GER., ESCi),SiltIrC). Formula of the hydrated acid C21-1402 ; of the anhydl.ous C411,02. Specific gravity of tho hydrated acid 1.064 ; of the anhydrous, according to Gerhardt, 1.073. Boiling points 101° (219' F.) and 137° (278° F.) respectively.
Pure acetic acid is a thin colourless liquid, with a pungent odour, which becomes suffocating without a liberal admixture of air. Tho purest acid solidifies below 15° (60° F.), forming largo colourless crystals of prismatio or tabular forrn. In this, its " glacial " state, it does uot redden litmus, requiring the addition of water for the development of acidity. It may, however, be kept in a closed vessel, if perfectly at rest, down to 12^ in a liquid form, but upon the slightest agitation the whole body of acid immediately solidifies. Its vapour is exceedingly inflammable, burning with a bright blue flame and forming carbonic acid and water. Paesed through a red-hot tube tho greater part of the acid remains unchanged, but a portion ie split up into free carbon and com bustible gases, with acetone, napthalin, benzol, and hydrate of phenyl.
Readily miscible with water iu proportions, the specific gravity of the solution is, however, irrog,ular, and forms only an uncertain test of strength. As will be noted from the following table given by Mohr (' Ann. der Chem. und Phu.' xxxi. 227), the ,density increases with the increased percentage of acid up to a certain point, but upon tho further addition of aeid falls away.
Acetic is one of the most powerful of acids, raising blisters if dropped upon the skin, and blackening organic substances after the manner of sulphuric acid. Owing to a peculiar and complex constitution, the crude acid (pyroligneous or commercial, i. e. the acid obtained by dis tilling acetate of lime with sulphuric or hydrochloric acid) is exceedingly uncertain in its action, a sample rdgistering (say) 6° Tw. often producing as good results as one at 9°. The hydrated acid is a powerful solvent of various organic bodies, camphor, resins, essential oils, phosphorus, &c., and it is this and its ready combination with various bases, forming a series of well-known salts, that are its most valuable properties. These salts are remarkable for being all soluble in water; they may be formed by the direct action of the acid upon an "oxide, or by the indirect means of double decomposition between an acetate and a salt of the base required. It should be noted that on account of its solvent power over copper and lead, acetic acid ought to be carefully tested for these substances, which the vessels used in the various processes of manufacture are liable to con taminate it with.
The anhydrous acid has been but little examined, and is, as yet at least, of comparatively small importance. It is a heavy, mobile liquid, colourless, and strongly refracting, with a powerful ethereal odour. Poured into water it does not readily dissolve, but falls to the bottom in oily drops, and is gradually converted into the hydrated acid.
The manufacture and use of acetic acid, as its name implies (Lat. acetutn = vinegar), are of great antiquity. Moses speaks (Numbers vi. 3) of "vinegar of wine," "vinegar of strong drink," and from the testimony of several ancient writers it is evident that the properties and uses of the acid were well ascertained. Perhaps the oldest record proving this is the noteworthy allusion in the Book of Proverbs to the action of vinegar upon nitre. It is the product of the oxidation or destructive distillation of various organic bodies, and exists in nature in considerable quantities, in the juices of many plants, especially trees, and in animal secretions. Until a comparatively recent date, however, its chief source was the distillation of acetate of copper—verdigris.
Acetic acid in its various forms occupies a very prominent place in the arts, manufactures, and commerce. It is extensively used in the treatment of gums, caoutchouc, and various albuminous substances, in the manufacture of paints and varnishes, and as a drug. In a dilute state, and in its well-known form of vinegar (which is simply a weak solution of the acid contaminated with certain vegetable impurities), it is largely employed in culinary arts and the manufacture of pickles, &c. The crude pyroligneous acid, prepared by the distillation of wood, is, from its ad mixture with creasote and other hydrocarbons, a valuable antiputrescent, and as such is used in the preservation of timber—also flesh. The distilled vinegar (wine or malt vinegar deprived by distillation of colouring and other non-volatile bodies) is used in medicine to relieve nervous head ache, fainting fits, and sickness. Smelling salts are usually sulphate of potassium mixed with a little glacial acetic acid. Finally, it forms a series of salts, or "acetates," of special value in calico printing, dyeing, and other branches of industry.