DISINFECTANTS 85 



bacteria will die at pH 2, which means 10 mg. H ions per 

 liter, or 10 ppm., hydrogen ions must be considered very 

 efficient disinfectants.) 



In a few cases, however, such as that of HNO3 and HF, 

 not only the H ions but the anions also act on proteins ; 

 the death rate then is higher than the pH would indi- 

 cate. With weak organic acids such as benzoic and sali- 

 cylic, the undissociated molecule itself is toxic. The same 

 is true for SO, (see p. 159). The strongly dissociated 

 CCI3COOH acts by its anions as well as its cations. 



The effect of pH on the colloidal state of protein is 

 well known, and denaturation by a high concentration 

 of hydrogen ions is to be expected, provided that the 

 acidity of the medium can affect the pH of the cell con- 

 tents. But different proteins have different isoelectric 

 points, and a wide range of different responses to pH 

 changes is not surprising. In fact some yeasts as well 

 as some Aspergilli can multiply at pH 2.0 ; certain sulfur- 

 oxidizing soil bacteria are known to exist even at pH 1 

 (Starkey and Waksman, 1943), while some urea-splitting 

 bacteria grow best at pH 8.5 to 9.0. 



2. Alkalies. Alkalies are commonly used as bac- 

 tericides especially in soaps and washing powders. It 

 has been shown that some compounds like trisodium 

 phosphate, sodium metasilicate, and slaked lime act 

 largely in proportion to their hydroxyl ions (Myers, 

 1928 and 1929, Tilley and Schaffer, 1931). But the tol- 

 erance for these ions varies greatly with the species. The 

 lethal action is usually explained by the prompt denatur- 

 ation of proteins by excessive alkali. 



3. Heavy Metal Compounds. Kronig 

 and Paul (1897) proved that heavy metal salts act pri- 

 marily by their metallic ions. These authors decreased 

 the ionization of HgCL by the addition of NaCl, and 

 showed that, as the concentration of the latter was in- 

 creased, the efficiency of the mercury salt decreased. 



