Atkins — Factors affecting Hydrogen Ion Concentration of Soil. 371 



times convenient to use C„ values. To r.onvert the - log [H+] or pH values 

 into Ch or [ff] values use may be made of semi-logarithmic paper, as 

 pointed out by Eoaf (1920). The first decimal points of the pH values are 

 marked off from right to left as abscissae, the Oh values from 0"1 to I'O being 

 ordinates against the logarithmic rulings. A diagonal being drawn, the C„ 

 value of any pH value may be read off, the whole number of tlie latter being 

 the negative power of ten, by which the Ch value so read must be multiplied. 

 Thus, for example, with pH 4"50 it is found that O'oO on the abscissa corre- 

 sponds to 0'32 on the ordinate; the 0^ value is then 0"32 x 10"*. Conversion 

 tables are, however, given by Clark, also at shorter intervals by Schmidt and 

 Hoagland (1919). In making a graph for converting pH into Oh values, it 

 should be noted that the logarithmic scale, as on a slide rule, begins at 

 1, not 0. A further reason for using - log [H+] values will be given later on. 



(b) Buffer action, — If alkali is added to acid, after a certain amount has 

 been run in, a neutral solution is obtained. With a strong acid, such as 

 hydrochloric, the addition of the alkali in successive portions results in a 

 progressive diminution in the hydrogen ion concentration, as may be seen 

 from the fact that the strong acid was largely ionized at the start. But 

 with a weak acid the neutralization of the hydrogen ions existing at any 

 instant results in a new equilibrium being attained by the remaining undis- 

 sociated molecules of the acid, which become ionized to the same percentage 

 as before. The alteration in the hydrogen ion concentration is therefore 

 much less over a considerable range. If the results are plotted with pH 

 values as ordinates and cubic centimetres of alkali added as abscissae, the 

 slope of the curve will be less steep for the weak acid than for the strong, 

 except near the neutral point. Curves illustrating this are given by Clark 

 (1920) and by Fisher (1921), and in the older literature. A weak acid thus 

 has a considerable " buffer action " in preventing rapid alterations in the 

 pH vall^es, and the same is true of weak bases. Thus peptone and albumen 

 solutions alter in pH value comparatively slowly when acids and alkalis are 

 added. The same is true of many soil suspensions, such as those of peat and 

 calcareous silt ; and though these are not solutions, yet they may be con- 

 sidered as having a considerable biiffer action, since they yield a continuous 

 supply of a very dilute solution of acid or alkali till exhausted. 



(c) 'The determination of hydrogen ion concentrations. — From theoretical 

 considerations Nernst developed an equation connecting the electromotive 

 force of a concentration cell with the concentration of its ions. Platinum 

 black deposited on platinum and immersed in a solution through which pure 

 hydrogen is bubbled constitutes a hydrogen electrode. On the assumption 

 that in very dilute solutions the ions obey the gas laws, it has been shown 



2x2 



