96 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



ACETIC ACID. CH 3 .COOH. 



Preparation. 



Acetic acid is one of the few products made commercially by 

 biological methods, i.e. by the oxidation of dilute alcohol by means of 

 the micro-organism Mycoderma aceti, or " mother of vinegar". Me- 

 chanical contrivances are used in order to expose a large surface of 

 the alcoholic liquid to the air so that the acetification is as rapid 

 as possible. 



Wine, red and white, cider, beer and malt, and sugar prepared 

 from starch are the materials from which the vinegar is made. Be- 

 sides acetic acid vinegar contains other organic acids, sugar, dextrin 

 and colouring matters which were present in the original material. 

 The amount of acetic acid in the solution varies from about 3-12 

 per cent. , the average quantity being about 5 per cent. 



A large quantity of acetic acid is produced by the dry distillation 

 of wood, the crude material obtained in this way being termed pyro- 

 ligneous acid (p. 63). Tarry matter separates out on adding hydro- 

 chloric acid to the solution which has been neutralised with lime 

 and distilled to remove methyl alcohol and acetone. The clear liquid 

 is neutralised and evaporated to dryness and the dry residue heated 

 to decompose the empyreumatic products. Comparatively pure acetic 

 acid is obtained on distilling the residue with hydrochloric acid. Pure 

 acetic acid is prepared by distilling with potassium bichromate, or 

 neutralising with soda and distilling the sodium salt, which has been 

 heated to destroy tarry matter, with sulphuric or hydrochloric acid. 



Preparation by the Oxidation of Alcohol with Permanganate. 



14 gm. of potassium permanganate are dissolved in about 200 

 c.c. of water in a litre flask and 8 c.c. of concentrated sulphuric acid are 

 added. The flask is fitted with a reflux condenser and through the 

 condenser a mixture of 5 c.c. of alcohol and 50 c.c. of water is slowly 

 added. The reaction must be kept moderate and, after all the alcohol 

 has been added, the mixture is boiled for about 15 minutes. The 

 acetic acid is separated by distilling over about three-fourths of the 

 liquid. The distillate will contain the acetic acid which may be tested 

 for as described on p. 97. 



Properties. 



Acetic acid is a colourless liquid with a characteristic pungent 

 smell. The pure acid boils at 119 and distils without decom- 

 position ; on cooling it crystallises in plates which melt at 17 and 

 hence is termed glacial acetic acid ; its sp. gr. at 16-5 is 1052. 



