SOIL REACTION 419 



Soil Acidity correlated with Physico-Chemical Concep- 

 tions OF Acidity : Acidity as a Function of [H-] 



At the outset, soil acidity should be correlated with physico- 

 chemical conceptions of acidity ; all phenomena depending on 

 acidity — whether in the soil or in any other medium, homo- 

 geneous or heterogeneous — are determined by the concentration 

 of hydrogen-ions in the continuous liquid phase. " Acidity," 

 " alkahnity," and " neutraUty " have a definite and quantita- 

 tive meaning, and refer to the ratio of the hydrogen-ion and 

 hydroxyl-ion concentrations in the solution. 



Neutrality is the condition when the two concentrations are 

 equal, as in pure water, and in such a case [H'] ' = [OH^] = 10 -'""^ 

 g.-ions per litre at i8°C. Acidity corresponds to a higher 

 hydrogen-ion concentration than this, and " alkahnity " to a 

 lower. 



A simplification can be effected by regarding intensity of 

 acidity as a function, not of the hydrogen-ion concentration, but 

 of the reciprocal of the logarithm of the hydrogen-ion concentra- 

 tion. It is not easy at first sight to compare such a series of 

 hydrogen-ion concentrations as 



IXI0-5J 4-0 = 10-^; i-6xio-^;6-3Xio-'; 

 by a simple conversion, however, we get the following series : 



I xio-5 = io~5 i-6xio-^=io-5-8 



4-0 X 10-^ = IO-5-4 6-3 x 10-7 = lo-^'* 



and the order of the acidities is at once apparent. It thus 

 becomes simpler to omit the 10, which is common to all, and 

 to express the [H'] by means of minus logarithms. These are 

 merely the reciprocals of the logarithms of the hydrogen-ion 

 concentrations and are generally written — log [H*] or Ph. A 

 further advantage of this mode of expression is found in 

 expressing graphically on squared paper large variations in 

 [H*]. It is impossible, for example, to compare graphically on 

 the same sheet of paper [H'] = 10"° and [H'] = 10 -^^ g. per 

 litre or any really large variations between these limits ; but 

 it is perfectly easy to compare the minus logarithms of these 

 values, viz. o and 14 and any variation of any magnitude between 

 these values. 



Distinction between Degree of Acidity and 



TiTRATABLE AciDITY 



The degree of acidity as expressed by the hydrogen-ion 

 concentration is, of course, not the same as that measured by 



1 The square bracket signifies concentration. 



