198 HYDROGEN ION CONCENTRATIOX IX S.A. SOILS. 



Since the number of undissociated HoO molecules in pure 

 distilled water is relatively enormously large it can be taken as 

 a constant and then 



[H-] X [OH'J=a constant. 



This is known as the " water constant " Kw The numerical 

 value of Kw has been determined by several methods, which 

 need not be discussed here, and from these it is estimated that 

 the value of K^ for one litre of pure distilled water at 18°C 

 is 10~'^'". Since in such water the hydrogen ion concentration 

 must be equal to the hydroxyl ion concentration, for the water 

 is neutral, 



[H-] = [OH'l=10-^-«^ 



The concentrations of H* and OH' present in pure water, 

 although low, are thus quite definite. The concentration of H2O 

 molecules in solutions of ordinary concentration is practically 

 unalterable by any alteration in the hydrolysis. 



A consequence of the constancy of K^ is that all aqueous 

 solutions contain both H- and OH', and if one of these is known 

 the other can be calculated, for example, if the hydrogen ion 

 concentration of a solution is known to be 10~'', then from the 

 above 



[H-]x [OH'J=10-'' 



.-. [OH] =10-'" 



It is unnecessary therefore to state both the acidity and alkalinity 

 of an acid or alkaline solution, but only its acidity or hydrion 

 concentration. 



Neutral solutions then are those in which, as in pure water, 



[H-] = [OH']=10-^-" g. ions per litre. 



Acid solutions have a higher hydrion concentration than this, 

 alkaline solutions a lower hydrion concentration. 



The Expression of Hydrogex Ion Concentration. 



Owing to the large variations possible in the hydrogen ion 

 concentration — from 10° to 10~'' gm. ions per litre — it becomes 

 necessary to introduce a simplification, and this is done by deter- 

 mining the reciprocal of the logarithm of the hydrion concen- 

 tration. An actual example will best illustrate this. If the 

 hydrogen ion concentration of a solution is found to be 0'0000016 

 gm. ions per litre, then [H'] =1'6 x 10— ^ Expressing this as 

 a power of 10, we then get [H']=10— ^•\ It is thus possible to 

 express the hydrogen ion concentration of any solution as a poM'er 

 of 10, and it is then simpler to omit the 10 and express the 

 [H-] by means of minus logarithms, which are merely the 

 reciprocals of the logarithms of the hydrion concentrations, and 

 are generally written — log [H-] or pH or P„. The pH of the 

 solution in the given example is, therefore, 5"8. Where this 

 method of expressing the results is used the following points must 

 be clearly borne in mind : 



