ACETIC ACID, or Acetotes Acid, is the sour part of vinegar, and that to which its peculiar and valuable properties are owing. It is pro cured, first, by the fermentation of saccharine or sugary matter,—secondly, by the action of heat upon wood; the product of the former constituting vinegar, and of the latter pyro ligneous acid.
1. Vinegar. When certain vegetable juices which contain much sugar, such as that of the grape, are fermented, the sugar undergoes the vinous fermentation [FArtmENTArtox], by which alcohol is produced ; and if this process be carried beyond a certain stage, a further fermentation called the acaous fermentation ensues, by which vinegar is produced: so that, in effect, sugar becomes alcohol, and then al cohol becomes vinegar. How this is managed in practice is explained under VIrttnoAa, where also the peculiar arrangements of the great vinegar-factories are described. Vinegar is,, in practice, made from four sources,—wine, malt, sugar, and wood.
Vinegar (or rather Acetio Acid) possesses the usual property of acids to redden vegeta ble blue colours; it combines with the alkalis, earths, and metallic oxides to form the salts which are termed acetates. Vinegar is purified from the sulphuric acid and colouring matter which it contains by distillation ; but its smell and taste are then less agreeable, and it is weaker than acetic acid otherwise procured. When vinegar is exposed to a low tempera ture, it is principally the watery part which freezes.
2. Proligneons Acid. The second method obtaining acetic acid is by heating wood, as the dried branches of trees, in hollow iron I cylinders, Itith a proper arrangement of cool ers, or condensers, and receivers. Birch and beech are the best woods for yielding it. The pyroligneous acid thus procured is of a dark brown colour, has a strong burnt acid smell, is very sour to the taste, and acts strongly on vegetable blue colours. It contains a quantity of tar and oily matter ; from these it is puri fied, in a considerable degree, by redistilla tion ; and a further purification with lime con verts it into a pyrolignite of lime. This pyre • lignite, by the chemical action of dilute sulphate of soda, produces acetate of soda, which is a crystalline salt. From these crys tals pure acetic acid is produced, by exposing them to the action of sulphuric acid, which combines with the soda of the acetate, and leaves the acetic acid free.
Pyroligneous acid is employed to preserve meat, and to impart to it the smoky flavour usually obtained by drying. Pure acetic acid is used in chemical researches, and especially for preparing various acetates. In a less pure state it is employed in the arts, for preparing acetate or sugar of lead, acetate of copper, or verdigris, and acetate of alumina, which is largely used by calico-printers as a mordant.