C2H5OH + 2O = HC2H3O2 + H2O.
The various kinds of vinegars are all made by this process. In the manufacture of cider vinegar the sugar present in the cider first undergoes alcoholic fermentation; the resulting alcohol then undergoes acetic fermentation. The amount of acetic acid present in vinegars varies from 3 to 6%.
2. From the distillation of wood. The liquid obtained by heating wood in the absence of air contains a large amount of acetic acid, and this can be separated readily in a pure state. This is the most economical method for the preparation of the concentrated acid.
Acetic acid is a colorless liquid and has a strong pungent odor. Many of its salts are well-known compounds. Lead acetate (Pb(C2H3O2)2) is the ordinary sugar of lead. Sodium acetate (NaC2H3O2) is a white solid largely used in making chemical analyses. Copper acetate (Cu(C2H3O2)2) is a blue solid. When copper is acted upon by acetic acid in the presence of air a green basic acetate of copper is formed. This is commonly known as verdigris. All acetates are soluble in water.
Butyric acid (H·C4H7O2). Derivatives of butyric acid are present in butter and impart to it its characteristic flavor.
Palmitic and stearic acids. Ordinary fats consist principally of derivatives of palmitic and stearic acids. When the fats are heated with sodium hydroxide the sodium salts of these acids are formed. If hydrochloric acid is added to a solution of the sodium salts, the free palmitic and stearic acids are precipitated. They are white solids, insoluble in water. Stearic acid is often used in making candles.
Acids belonging to other series. In addition to members of the fatty-acid series, mention may be made of the following well-known acids.
Oxalic acid (H2C2O4). This is a white solid which occurs in nature in many plants, such as the sorrels. Its ammonium salt ((NH4)2C2O4) is used as a reagent for the detection of calcium. When added to a solution of a calcium compound the white, insoluble calcium oxalate (CaC2O4) precipitates.
Tartaric acid (H2·C4H4O6). This compound occurs either in a free state or in the form of its salts in many fruits. The potassium acid salt (KHC4H4O6) occurs in the juice of grapes. When the juice ferments in the manufacture of wine, this salt, being insoluble in alcohol, separates out on the sides of the cask and in this form is known as argol. This is more or less colored by the coloring matter of the grape. When purified it forms a white solid and is sold under the name of cream of tartar. The following are also well-known salts of tartaric acid: potassium sodium tartrate (Rochelle salt) (KNaC4H4O6), potassium antimonyl tartrate (tartar emetic) (KSbOC4H4O6).
Cream of tartar baking powders. The so-called cream of tartar baking powders consist of a mixture of cream of tartar, bicarbonate of soda, and some starch or flour. When water is added to this mixture the cream of tartar slowly acts upon the soda present liberating carbon dioxide in accordance with the following equation: