AXTISEPTICS 157 



VII. INCREASED EFFICIENCY OF WEAK ACIDS 

 BY INCREASED ACIDITY 



It has been known for a long time that salicylic and 

 benzoic acids are good antiseptics only in acid solution 

 (see e.g., Cruess and Richert, 1929; Cruess and Irish, 

 1932). Sulfur dioxide, the standard disinfectant of the 

 yeast industries, has the same property. Recently, pro- 

 pionic acid has been added to this group. Boric acid 

 might also belong in this class, though no records could 

 be found in the literature to bear out this possibility. 

 The effect of pH on antiseptic efficiency has been 

 ascribed by Bittenbender and associates (1940) to a 

 ^ ' specific hydrogen ion effect ' ', while Hoffman, Schweitz- 

 er and Dalby (1941) explain it by the "relation of polar 

 and non-polar groups" of the disinfecting acid. 



A much simpler explanation has been offered by Rahn 

 and Conn (1944) who assumed that only the undissoci- 

 ated acid acts as antiseptic. In neutral solutions, the 

 acids and their salts are completely dissociated while an 

 increase of acidity produced by the addition of buffers 

 of low pH, such as citric acid and acid phosphates, will 

 decrease the dissociation of the acid in proportion to the 

 dissociation constant. For benzoic acid, this relation is 

 given by the equation 



[H] [Benzoate ions] ^ ^ ^ ^ ^^^ ^ [H] [a] 



[undissociated acid] ' [1 - Q^] 



where a represents the "degree of dissociation". The 

 fraction (1-a), multiplied by the concentration of total 

 benzoic acid gives the concentration of undissociated acid 

 at any pH. 



The highest concentration still permitting slow growth, 

 and the lowest concentration preventing growth have 

 been determined by Rahn and Conn (1944) for yeast. 



