ACETIC ACID (Pr., Acide acetique ; Ger., Essigsdure) Also known as purified pyroligneous acid. Molecular weight, 6o. There are three kinds of acetic acid:—(r) glacial, con taining about 99 per cent. of acid and r per cent. of water (sp.g., 1.065) ; glacial acetic acid is the most widely used for photographic purposes, and receives its name from the fact that it solidifies and freezes into long ice-like crystals at com paratively low temperatures ; (2) commercial strong,' about one-third the strength of the glacial variety, and containing about 33 per cent. of acid, sometimes known as Beaufoy's acetic add (sp.g., 1.044) ; (3) dilute acetic acid, made by mixing r part of the " strong " acid with 7 parts of water (4} per cent.), and sold as " distilled white vinegar ' (sp.g., Poo6). Acetic is the oldest of acids, and is given in old diction aries as " acetous acid." Its impurities may be hydrochloric, sulphuric and sulphurous acids, but most samples sold by chemists are quite pure enough for photographic purposes. Acetic acid readily dissolves in water, alcohol, and ether ; it is a strong escharotic, causing painful blisters if allowed to remain on the skin, but the application of a solution of soda or any other alkali will at once neutralise it. It is extremely
volatile, and should be kept in a glass-stoppered bottle and in a cool place. It has many uses in photography, and in the early days, when it cost as much as 8d. per ounce, was largely used as a constituent of the developer for wet plates. Nowadays, it is used for clearing the iron out of bromide prints after development with ferrous oxalate, to assist uranium toning, and, on rare occasions, as a restrainer when developing with hydroquinone. Acetic acid is a solvent for celluloid, gelatine, and pyroxyline.
In process work, acetic acid is used in the iron developer for wet plates. The amount required increases as the working temperature increases ; at 60° F. oz. of glacial acetic acid to 20 oz. of developer is a suitable proportion. The acid retards the action of the ferrous sulphate. A mixture of acetic acid and salt is used for clean ing up the copper plates during half-tone etching to enable the etcher to see the image better when proceeding to re-etch. It is also used for remov ing the magnesia that is rubbed into the etched plate to make the image visible.