Acetic Acid — Chambers's Encyclopaedia

Acetic Acid, the sour principle in vinegar, is the most common of the vegetable acids. If alcohol, diluted with water, be mixed with a ferment such as yeast, and exposed to the air at or a little above its ordinary temperature, it is rapidly converted into vinegar or acetic acid. The views held by Liebig regarding the part that wood-shavings, sand, ash, &c. play in condensing oxygen, and transmitting it to the alcohol, are now supplanted by those of Pasteur, who maintains that the true acetylating matter is a very minute mycoderma—a special vegetable organised being. It is impossible to conceive a more simple form of vegetation, consisting of extremely minute spores arranged in chains; each spore having a mean diameter not exceeding $\frac{1}{100}$ th of an inch, and the length being about twice as great. The rapidity of the development of the spores, under favourable circumstances, is almost inconceivable; and the power which they possess in fixing the oxygen of the air, and of transmitting it to the alcohol, and of establishing an incomplete combustion of the latter, is no less wonderful. A surface of a square yard covered with this plant, is able, in the course of 24 hours, to fix the oxygen of more than 1000 quarts of air. The temperature of the surface of the fluid at which this slow combustion is proceeding is considerably raised, and often remains for several days at $21^\circ$ or $25^\circ F.$ ( $12^\circ$ or $14^\circ C.$ ) above that of the surrounding air. The process which has just been described bears a very close analogy to the respiratory process, the oxygen of the air being in one case fixed by minute vegetable cells, and in the other by the blood corpuscles. The change is accompanied by the absorption of oxygen, one atom of which combines with two of hydrogen to form water, aldehyde being left. Further oxidation then takes place, acetic acid being formed thus:

Alcohol. Aldehyde. Water. Aldehyde. Acetic Acid.
$C_2H_6O + O = C_2H_4O + H_2O$ $C_2H_4O + O = C_2H_4O_2$ .
From the mode in which acetic acid combines with bases to form salts, it is evident that one atom of the hydrogen differs from the other atoms in being replaceable by a metal or an alcohol radical (as ethyl, $C_2H_5$ ), and on this account acetic acid is called a monatomic acid, and its formula is usually represented as $HC_2H_3O_2$ ; that of acetate of potash being $KC_2H_3O_2$ , and of acetate of ethyl, $C_2H_5C_2H_3O_2$ . A striking experiment may be made illustrating the mode in which alcohol is converted into acetic acid. If slightly diluted alcohol be dropped upon platinum-black, the oxygen condensed in that substance acts with great energy on the spirit, and acetic acid is evolved in vapour. Here the whole office of the platinum is to determine the oxygen of the air and the hydrogen of the alcohol to unite. In the commercial processes for manufacturing vinegar, some vegetable substance containing nitrogen (one of the albuminous principles) takes the place of the platinum-black, and determines the same change. Pure acetic acid is a crystalline solid at ordinary temperatures. It is obtained by distilling dry acetate of potassium and sulphuric acid:

2KC_2H_3O_2 + H_2SO_4 = 2HC_2H_3O_2 + K_2SO_4.

The anhydride of acetic acid (see ANHYDRIDES) is formed by the action of chloride of acetyl on acetate of potassium. It has the composition $(C_2H_3O)_2O$ , and unites with water to form acetic acid. The salts of acetic acid, called ACETATES, are numerous and important in the arts. The most important is acetate or sugar of lead (see LEAD). For the commercial processes of manufacturing acetic acid, see VINEGAR.