A'l'KiN.s — Factors affecting Hydrogen Ion Concentration of Soil. 373 



(d) Neutrdity.— It has not been explained as yet what is meant by a 

 neutral solution or neutrality. Pure water dissociates primarily into 

 hydrogen and hydroxyl ions, and the product of the ionic concentrations is 

 a constant at constant temperature, namely, 



[H^] X [OH-]=>."[HO]= K. 



Since the ions are produced by pure water in equal numbers, the concentra- 

 tion is the same for both when reckoned in gram-equivalents. A solution is 

 accordingly termed neutral when the hydrogen and hydroxyl ions are present 

 in equivalent amounts, as in pure water. With an acid solution the hydrogen 

 ion is in excess, and the hydroxyl is correspondingly reduced, since the 

 product is constant. In alkaline solutions hydroxyl ions preponderate ; but 

 as the product is constant, it is possible and convenient to state the reaction 

 of the solution in terms of hydrogen ion concentration rather than in terms of 

 the hydroxyl. One scale, the pH or - log H, is therefore obtained instead of 

 two, based upon the concentrations in pure water, as a starting-point for 

 both. 



(e) The liydrogen ion concentration of pure water. — It is of interest to 

 determine the hydrogen ion concentration of pure water, but this is attended 

 by many experimental difficulties. Those due to the solution of minute 

 traces of glass may be avoided by the use of silica or platinum vessels, but 

 the absorption of carbon dioxide is still a trouble. It is not possible to 

 determine the pH value by the potentiometer on account of the great 

 internal resistance of" the hydrogen electrode half-cell when made up with 

 pure water. The various methods adopted have recently been reviewed 

 by Beans and Oakes (1920). They are as follow :— (1) By deduction from 

 measurements of the electromotive force of concentration cells made up 

 with dilute solutions. (2) By conductivity methods, giving the ionic 

 mobilities of hydrogen and hydroxyl ions in very dilute solutions, which are 

 considered to be valid for pure water ; the conductivity of the latter is then 

 measured, and from it and the ionic mobilities the concentration of the ions is 

 calculated. This is the method used by Kohlrausch and Heydweiller (1894), 

 and is the only one of these methods based iipon an examination of pure 

 water, but even then use is made of data derived from solutions. Tor pure 

 water these workers found the hydrogen ion concentration at 26° C. to be 

 1-10 X 10 -', or pH 6-96. The value varies with the absolute temperature, as 

 may be seen from the equation given previously. Michaelis (1914) gave for 

 pure water at 16' C, pH 7-10 ; at 22° C, pK = 7-00 ; and at 28° C, pH 6-90 

 (3) By methods based on the hydrolytie dissociation of salts. (4) By 



