HYDROGEN ION CONCENTRATION IN S.A. SOILS. 197 



into positively charged hydrogen ions and negatively charged 

 chlorine ions. Further, it has been clearly established that the 

 characteristic properties of any acid solution, for example, sour 

 taste, reddening of blue litmus, are entirely due to the presence 

 in the solution of these hydrogen ions. Weak acids, that is, those 

 vv^hose acid properties are only slight, are only dissociated to a 

 small extent and consequently there are few hydrogen ions present 

 in their solutions. Strong acids, on the other hand, are almost 

 entirely dissociated in solution, producing a high concentration of 

 hydrogen ions. The strength of an acid solution is therefore 

 dependent, not on the total quantity of acid present in it, but 

 upon the number of hydrogen ions present in a certain volume 

 of the solution, that is, on the hydrogen ion concentration. This 

 is very well shown by Wherry^ in connection with hydrochloric 

 and formic acids. Normal solutions of these two acids each 

 contain 1 gm. of acidic hydrogen per litre — the total quantity 

 of acidic hydrogen is the same in each. The strong acid, how- 

 ever, is 75 per cent, ionised, while the weak acid is only 1 per 

 cent, ionised. The hydrogen ion concentration of the former is 

 therefore 0'75 gms. per litre, while that of the latter is only 0"01. 

 The hydrochloric acid solution contains 75 times as many hydrogen 

 ions as does the formic acid solution. " The situation," says 

 Wherry, '" is analogous to that of two men, both possessing £100, 

 but one having £25 in a savings bank and £75 in his pocket, the 

 other having £99 and £1 in these respective places. The first 

 man can purchase 75 times the amount of any commodity that 

 the second can, even though the total quantity of money they 

 own is the same. Purchasing power, in this illustration, corres- 

 ponds exactly to hydrogen ion concentration ; for the amount of 

 hydrogen which is ionised, not the total amount, determines 

 most of the things an acid can do." 



In the same way as acids depend for their properties on the 

 presence of hydrogen ions, bases depend on the presence of 

 h^ydroxyl ions, and the strength of a base depends on the number 

 of hydroxyl ions present in a given volume of solution. A neutral 

 solution is produced when the h^'drogen ion concentration is equal 

 to the hydroxyl ion concentration ; acidity is due to a rise in 

 the former, alkalinity to a rise in the latter. 



Even pure distilled water is ionised to a slight extent and 

 the water molecules dissociated into hydrogen and hydroxyl ions, 

 these forming an equilibrium mixture, thus: — 



H,0 ^i:^ H+-fOH- 



In such a mixture, according to the laws of mass action, the 

 following relationship must hold. (Concentrations are expressed 

 by means of square brackets). 



[H-] X [OH'] 



= a constant. ^^'rMT*"^^ 



[H,0] X^ s ^ 



.-.[H-] X [OH'] = [H.O] X a constant. /^ o"^^^ ^<^ 



uj L I B R .^ R 



