66 



Microbes and You 



Exact determinations have revealed that one hter of pure water 

 (H2O) contains 0.000,000,000,000,01 (10-^^) gram of hydrogen and 

 hydroxyl ions. The hydrogen and the hydroxyl ions are always 

 equal in pure water. It follows, therefore, that the hydrogen ions 

 are found in a concentration of 0.000,000,1 gram per liter. The 

 logarithm of this fraction (0.000,000,1 gram) is minus 7. For con- 

 venience we express this as a positive number and call it pH 7.0. 

 Numbers less than 7.0 on the pH scale, therefore, represent greater 

 acidities. Figures above pH 7.0 indicate lesser hvdrogen ion 

 concentrations, but greater hydroxyl ion concentrations, or 

 alkalinities. The normality of the hydrogen ions times the 

 normality of hydroxyl ions is a constant number, called the dis- 

 sociation constant. This number must always be approximately 

 10"^^, so if we know the concentration of hydrogen ions, we can 

 determine the hydroxyl ion concentration by subtraction. Each 

 whole number on the pH scale represents a ten-fold difference from 

 the number above or below it. A pH of 6.0 would therefore have a 

 hydrogen ion concentration ten times that of pH 7.0. We can 

 express these facts about pH in tabular form. 



