6 THE LIVING PLANT 



take the place of those which have been neutralized, and so on 

 until all have been satisfied. 



In practice it is the ionized hydrogen only which is respons- 

 ible for the acidity of a solution at any given moment and so 

 it comes about that the hydrogen ion concentration for a solution 

 is, for biochemical purposes, a much more valuable criterion of 

 the actual conditions prevailing in any given circumstances than 

 is the potential alkali neutralizing power. 



The concentration of hydrogen ions may be expressed as 

 follows. In a decinormal solution of hydrochloric acid there 

 would be O'l gram in 1000 c.c., presuming it to be completely 

 ionized. In actual fact, however, a decinormal solution of 

 hydrochloric acid is only ionized to the extent of 97 per cent., 

 consequently the concentration is only cr I x 97 or 97 x io~ 2 . 

 This concentration is more conveniently expressed as a 

 logarithm: Iog 10 97 = -9868, wherefore 97 x io~ 2 = io' 9868 ~ 2 

 = icr 1 ' 01 . It has been agreed to express hydrogen ion con- 

 centration as the exponent to the base 10 of the concentration 

 with the negative sign omitted, and this is represented by the 

 symbol P H . Hence the hydrogen ion concentration of the 

 above N/io hydrochloric acid would be P H = I'Oi ; if com- 

 pletely ionized it would be P H = I. 



On this principle the following are synonymous methods 

 of expression : 



N = . , = P o - ?! - = N x 10-6 = P 6 



io u 1,000,000 



= N x lo- 1 = P H i - 5 - = N x io-7 = p 7 

 10 10,000,000 



= N x io- 2 = P 2 - - - = N x io- 8 = P 8 



100 H 100,000,000 H 



= N x io-3 = p 3 - N - 



1000 H J 1,000,000,000 



N x io- 4 = P 4 - - - = N x io- 10 = P io 



^ 



10,000 H^ 10,000,000,000 



N 



= N x io- 



100,000 



It will be seen from the above that the greater the value of 

 PH, the lower is the actual hydrion concentration. Moreover, 

 it is an established fact that the product of the concentrations 

 of the hydrogen and hydroxl ions in any given solution, is a 

 constant, namely 



C H X COH = 10- 14 ' 14 



