Acetic Acid (Lat. acetum, vinegar, of which it constitutes about 6 per cent.) has been known in a dilute form from the remotest antiquity. It can be prepared in two conditions: acetic anhydride, or anhydrous acetic acid, and acetic acid. Anhydrous acetic acid, as obtained by Gerhardt, is a colorless, very mobile liquid, of high refracting power, having a very pungent smell and emitting a vapor which is extremely irritating to the eyes. It gradually absorbs moisture from the air, and becomes converted into the common acid. Acetic acid can be made in a great number of ways: by treating aldehyde, alcohol, and ethylic ethers with oxidizing agents; by fusing sugar, starch, oxalic acid, tartaric acid, or citric acid with potash; by submitting wood, sugar, and gums to dry distillation; by distilling gelatine, caseine, or fibrine with a mixture of sulphuric acid and manganese dioxide. It has been made synthetically by Wanklyn, by passing a current of carbonic acid into a solution of sodium methyl, and appears to exist ready formed in the juices of certain plants, such as the sap of the oak, and in some animal fluids.
The pro-duet of the fermentation of wine and other spirituous liquids is vinegar, formed essentially of acetic acid diluted with water. (See Vinegar.) The acetic acid employed in commerce is chiefly derived from the dry distillation of wood. The process, as described by the late William Allen Miller, is substantially as follows: Harder kinds of wood, particularly the oak, beech, birch, and ash, are subjected to destructive distillation in iron retorts by means of a heat gradually raised to low redness. The wood is usually placed in these retorts in loose iron cases, by which means the charge can be rapidly introduced without loss while the retort is still hot, and the charcoal can be withdrawn when the distillation is complete. The quantity of acid obtained varies from 1 1/2 to 3 1/2 per cent., and in the crude state is called pyro-ligneous acid, in allusion to the mode of its formation (Gr. fire, and Lat. lignum, wood). During the operation a large quantity of tarry matter comes over, accompanied also by volatile and inflammable bodies, among which wood spirit, methyl acetate, and acetone predominate. These bodies are condensed in suitable receivers, while, in addition to carbonic anhydride, a considerable quantity of combustible gases, composed chiefly of hydrogen and carbonic oxide, is directed into the furnace, where they serve as fuel, and aid in heating the retorts. In about 24 hours, or as soon as the gases cease to escape, the loose iron cylinders containing the wood are withdrawn, and immediately closed with an airtight cover, so as to allow the charcoal to cool excluded from the atmosphere. The crude acid liquid which has been collected in the condenser is decanted from the tar, and, when submitted to distillation, furnishes wood naphtha, which constitutes the more volatile portions; afterward the acetic acid is collected. The latter, however, is always accompanied by tarry matters.
In order to get rid of these, the liquid is neutralized by the addition of the milk of lime or of sodic carbonate; a quantity of tar rises to the surface of the liquid on standing; this is skimmed off, and the solution of crude acid thus obtained is evaporated, and the dry residue, if the sodium salt be used, cautiously roasted at a temperature of about 500° F. (260° C.) to expel the tarry matters. It is afterward redissolved in water, decanted from the carbonaceous particles, which are allowed to subside, then recrystallized, and submitted to distillation with sulphuric or with hydrochloric acid, the sulphuric being preferable when sodic acetate is employed, while hydrochloric acid answers best when calcic acetate is used.- Properties of Acetic Acid, Normal acetic acid, C2 H4 O2, is liquid at temperatures above 62.6° F. (17° C.); below this point it crystallizes in radiating tufts of plates, and is called glacial acetic acid. The concentrated acid has a sharp aromatic taste and a peculiar pungent odor; it blisters the skin if applied to it for a sufficient length of time. It boils at 242° F. (117° C), and may be distilled unchanged. Its maximum density is 1.073, corresponding to a mixture of 77.2 per cent, acid and 22.8 per cent. water.
The vapor of acetic acid is inflammable, burning with a blue flame and producing by its combustion water and carbonic acid.