52 CYTOLOGICAL TECHNIQUE 



though unfixed, cannot escape through the nuclear membrane; it 

 distributes itself at random within the nucleus. The colouring of it 

 by basic dyes is therefore not informative. This is an unsuitable 

 fixative for studies of the nucleus and chromosomes (unless 

 acidified). 



ACETIC ACID 



Standard concentration. 5% v/v aqueous solution. 



Formula. H3C.COOH. 



Description. A colourless liquid with a pungent smell, miscible 

 with water and ethanol in all proportions. The crystals produced 

 by cooling melt at 16-6'' C. The undiluted acid is often called 

 'glacial', because it is so easily frozen. 



Ionization. A very weak acid. The pH at the standard con- 

 centration is about 2-3.^°® 



Oxidation-potential. Can oxidize by being reduced to acetalde- 

 hyde (o.p. 0-77 volt). 



Reactions with proteins. It neither coagulates nor gels most 

 proteins; non-additive. It extracts hist one from the tissues. A 

 thick precipitate is formed when acetic acid is added to a solution 

 of nucleoprotein. This is attributed to the action of the acetate 

 ion in splitting off" DNA from protein. 



Reactions with nucleic acids. Precipitates DNA from solution. 



Reactions with lipids. In writings on fixation it is often said 

 loosely that lipids are dissolved by acetic acid. It is true that some 

 of them (cholesterol and sphingomyelin, for instance) are soluble 

 in glacial acetic acid; but at the concentrations at which it is used 

 in fixation, acetic acid is not a lipid-solvent. It has, however, no 

 fixative effect on lipids. 



Reactions with carbohydrates. Neither fixes nor destroys them. 



Rate of penetration. In experiments to determine this, it is 

 necessary to substitute gelatine-nucleoprotein gel for gelatine- 

 albumin, for acetic acid leaves no visible mark to indicate its 

 progress into the latter. It penetrates rapidly {K = 2-75). 



Shrinkage or swelling. Its capacity to swell protein gels and 

 tissues is the most striking character of acetic acid, and the main 



