8 BACTERIOLOGICAL CHEMISTRY 



number of ions present to carry the current, or by the 

 depression of the freezing point of the sokition, which 

 depends on the total number of ions and molecules present. 

 A tenth normal solution of hydrochloric acid is 91 per 

 cent, ionised so that the concentration of hydrogen ions 

 in such a solution will be 



0-1 X 91 



— — — — = 0-091 = 9-1 X 10~2 grams per litre. 



Acetic acid is a weak acid and is only 1-3 per cent, ionised 

 in a tenth normal solution, and such a solution will, 

 therefore, contain only 



— --- — = 0-0013 = 1-3 X 10~^ grams per litre of hydrogen ions. 



This method of expressing the hydrogen ion concentra- 

 tion and correspondingly the acidity or alkalinity of a 

 solution is somewhat cumbersome particularly in the 

 range of values near neutrality, from about 10^® to 10-^ 

 grams of hydrogen ions per litre, in which the great 

 majority of biological phenomena occur. The adoption 

 of the exponential or pH method of expression suggested 

 by S^rensen in 1909, and which is now universally used, 

 has greatly simplified the statement and comprehension 

 of such values. S</>rensen defined the pH value of a 

 solution as the negative logarithm of its hydrogen ion 

 concentration. That is 



pn = - log (C.H+) or (C.H+) = 10-^H 



For neutral water containing lO-*^ grams per litre of 

 hydrogen ions 



^H = - log 10-' = log 10' = 7 



For the example quoted above of 0-lN-hydrochloric acid 

 containing 9-1 x 10-^ grams per litre of hydrogen ions, 



pU = - log (9-1 X 10-2) = 2 - log 9-1 - 2 - 0-959 = 1-041 



and for 0-lN-acetic acid, 



2)H = - log (1-3 X 10-3) = 3 - log 1-3 = 3 - 0-114 = 2-880 



