420 SCIENCE PROGRESS 



titration. In the latter case hydrogen ions are continuously 

 removed by the alkah used in the titration, thus 



BOH 5 B- + 0H7^' + ^^' ^ ^"0 



The equihbrium is continually being shifted and as con- 

 tinuously readjusted by dissociation of more acid until all the 

 acid has been dissociated and the hydrogen-ions removed by 

 the alkali until [H*] = i x io~7-°7^ which concentration corre- 

 sponds roughly with the end-point of the titration. The 

 " acidity " measured by titration methods refers to the total 

 quantity of hydrogen-ions that can be produced from the acid 

 when the ionic equilibrium is continually shifted by the intro- 

 duction of hydroxy 1-ions, and is often called true or total acidity, 

 but would be more accurately described as potential or 

 iitratable acidity. The potential acidity of such a system as an 

 acid soil may consist partly of undissociated acid dissolved in 

 the soil water, or of acid derived from partly hydrolised soluble 

 salts such as Al2(S04)3, or slightly soluble mineral acid or 

 organic acid or even of " adsorbed " acid, such as phosphoric. 

 The presence of " adsorptively unsaturated " compounds, 

 however, such as colloids, in soils affects the hydrogen-ion 

 concentration of the soil solution without necessarily affecting 

 the titratable acidity and the apparently anomalous behaviour 

 of soil extracts made with neutral salt solutions is often attri- 

 buted to " selective adsorption." 



Nature of Buffer Action 



All properties of a solution due to acidity depend really on 



the particular [H'] at the moment, which, however, may be 



influenced by various factors. Thus, the colloids in the soil 



act as buffers, or reaction regulators in presence of which the 



soil solution has a strong tendency to maintain its [H'] 



unchanged, so that a given amount of acid or base would produce 



much less alteration in the [H"] in presence of a soil than it 



N 

 would in pure aqueous solution. Thus i c.c. of — HCl added 



lOO 



to one litre of water will decrease the Ph from 7 to 5, the resulting 

 solution being strongly toxic to many bacteria. If, on the 

 other hand, the same amount of acid be added to a litre of 

 water containing 50 or 100 g. of soil of Ph = 7 the resulting 

 change in Ph is hardly appreciable. This effect is known as 

 buffer action, the substances causing the action being known as 

 buffers, and the solutions themselves as buffer solutions. In 

 the case of pure aqueous solutions, the mechanism of buffer 



