10 THE BIOLOGY OF FLOWERING PLANTS 



to a high concentration of hydrogen ions in a solution. It 

 is the concentration of hydrogen ions present which is the 

 important thing, irrespective of the actual amount of acid 

 present. This concentration depends not only on the 

 amount and nature of the acids, but also on the presence 

 of other substances, such as salts, which may keep the 

 hydrogen ion concentration from changing much over a 

 wide range of concentration of the acid — a so-called 

 " buffer action." 



The expression of hydrogen ion concentration as a 

 fraction of normality is attended with a certain incon- 

 venience, for the amounts are very small. The concentration 

 in a nearly neutral soil might be for example, 0*000000109 



I'OQ _ 



or . This may be written I'oq X io~'', or still 



10,000,000 



more simply as 10""^'^^. 



It is possible, therefore, to express all hydrogen ion con- 

 centrations as negative powers of 10. In practice the 

 exponent alone is used and is written simply as a positive 

 integer. This is called the p^ value of the medium. The 

 acidity, as hydrogen ion concentration, in the example 

 taken, would be v/ritten p^ — 6'96. In using /)" values 

 two points must be remembered ; (i) that the acidity 

 or hydrogen ion concentration increases as the p^ value 

 decreases ; (2) that for every decrease by unity the con- 

 centration of the hydrogen ions has increased tenfold, 

 i.e. ^" = 5'o means ten times as high a concentration as 

 p^ =r= 6*o. The neutral point lies at 7*07, and values above 

 this represent increasing alkalinity, the reason being that the 

 hydroxyl ions (of the constitution OH) which determine 

 alkalinity are now increasingly preponderant. 



Hydrogen ion concentration may be determined by 

 electrometric methods, which yield absolute values of great 

 accuracy. The apparatus required is expensive and the 

 determination somewhat troublesome. It is also possible 

 to use the colour changes of certain indicators added to the 

 solution. The shade of colour produced in the solution 

 to be tested, by an appropriate indicator, is compared with a 



