HYDROGEN ION CONC'KXTUATIOX IX S.A. SOILS. 199 



1. The value of pH for a neutral solution is 7'07. 



2. Lower values of pH than this indicate an increased [H-] 



and therefore a greater degree of acidity ; higher values 

 indicate a greater degree of alkalinity. 



3. If the pH is altered by one integer, the hydrogen ion 



concentration is altered 10 times, e.g. — 



pH7=[H-] 10-^ and pH8=[H-] 10-\ 



The hydrogen ion concentration of the second solution is ten 

 times that of the first, though the difference between the pH 

 values is only 1. 



Where only a small range of acidity is likely to be encoun- 

 tered, it is frequently convenient to convert the pH into what is 

 called " specific acidity " or " specific alkalinity. The specific 

 acidity of a solution of pH = 7 is taken as unity, and this is altered 

 10 times whenever the pH is altered bj' one integer, e.g., pH6 

 corresponds with a specific acidity of 10, pHo a specific acidity 

 of 100. For values of pH greater than 7, specific alkalinity is 

 expressed as negative acidity, e.g., pH8 corresponds with a 

 specific acidity of -10, pH9 a specific acidity- of -100. The 

 specific acidity of any solution of given pH is then most easily 

 obtained by plotting the known specific acidities against the 

 integral pH values. From the curve thus obtained the specific 

 acidity of any solution of known pH can be immediately read off. 



An example of the usefulness of this method of expressing 

 results is given by Wherry" as follows: " An AictosiaphyJos 

 association is recorded as having pH4-8 and a CaJluna association 

 pB[4-6, but it is difficult to gain a conception of relative acidities 

 represented by these figures. When translated into terms of 

 the specific acidity, however, they yield 159 and 2ol respectively, 

 and it can then be seen at a glance that the Calluna soil is over 

 1^ times as acid as the Arctostaphylos soil." It should be pointed 

 out, however, that where large variations in ' acidity are being 

 dealt with, the pH form is the only convenient one for the 

 graphical expression of results. 



Buffer Action and the Effect of Dilution. 



Dilute solutions of strong acids, such as hydrochloric acid, are 

 readily affected by traces of alkali in the glass, ammonia from 

 the air, etc. Similarly dilute solutions of strong alkalies ai'e very 

 sensitive to the carbon dioxide of the atmosphere. The hydrion 

 concentration of such solutions is, therefore, very susceptible to 

 change, and solutions of these substances of known pH cannot 

 be prepared and kept for use. Salts of weak acids or weak 

 bases, however, have a low hydrion concentration which is kept 

 constant in the presence of acids or bases by the dissociation 

 of a large reserve of electrolyte. In such solutions the hydrion 

 concentration is only slightly affected by traces of acid or alkali, 

 the large quantity of electrolyte acting as a " buffer " to small 

 additions. An example of this buffer action is given by Evers and 



