134 



FIXATION 



ACETIC ACID 



Standard concentration for fixation. 5% v/v aqueous solution. 



^° 



Formida and formula-weight. H3C.C/ . 6o-o. 



Description. Acetic acid is a colourless liquid with a pungent 

 smell. It boils at US'" C; the crystals formed on cooling melt at 

 1 6-6° C. The ease with which it may be frozen has given rise to the 

 name of 'glacial' acetic acid, which is applicable only when the 

 acid is free of water. The acid is miscible with water and ethanol 

 in all proportions. 



Beyond its use as vinegar and for pickling, acetic acid is impor- 

 tant in the production of cellulose acetate plastics. 



Ionization. Acetic is a moderately weak acid, with ionization 

 constant i-8o x 10"^. The pH of the 5% solution is 2-32 ^^^ (Seki^^^ 

 gives 2-4). 



Oxidation-potential. Acetic acid can act as an oxidizer on reduc- 

 tion to acetaldehyde, or oxidized by strong oxidizers to carbon 

 dioxide and water. The oxidation-potential of the 5% solution 

 is 077 volt.^^^ 



Manufacture. The best culinary vinegar is made by the oxidation 

 of the ethanol in wine or other alcoholic liquors through the 

 action of bacteria (generally Acetohacter spp.). Acetic acid is also 

 made by the destructive distillation of the sawdust of beech and 

 other hard-woods under a pressure of several atmospheres. The 

 distillate is wood-vinegar or pyroligneous acid, which contains 

 acetic acid at about 5%, with creosote and other contaminants. The 

 weak acetic acid made by either of these processes may be treated 

 with lime or soda to make calcium or sodium acetate. The salt is 

 distilled with sulphuric acid, and the distillate fractionally distilled 

 to give the glacial acid. 



Acetic acid is also made by the oxidation of acetaldehyde with 

 atmospheric oxygen in the presence of cobalt acetate as catalyst; 

 the acetaldehyde is prepared from acetylene. 



Introduction as fixative. Vinegar appears to have been used for 

 pickling vegetables from remote times, and it is remarkable that the 

 regular use of acetic acid in microtechnique is scarcely more than 

 a hundred years old. In the eighteenth century Henry Baker ^^ had 

 tried vinegar as a preservative for hydra, but without much success. 

 Quekett, in his book published more than a century later, ^^^ does 



