HYDROGEN ION CONCENTRATIONS 63 



where [H + ] = the molecular concentration of hydrogen ions. 



fOH~] = the molecular concentration of the hydroxyl ions, 

 and [H2O] = the molecular concentration of the undissociated 



water. 



In practice it is found that so little water is dissociated that 

 relative to [H] and [OH], [H.O] is constant. A'[Ho()] is thus 

 constant and equal to K.^^„ which is defined as the dissociation 

 constant of water. 



The value of K^^., the dissociation constant of ivatcr, depends only 

 on the temperature. 



.... (1) 



(2) 

 (3) 

 (4) 



100,000,000,000,000 



To save writing those cumbrous fractions, the index notation is 

 used. Thus fraction 



Since [H+] X [OH"] = i^„„ 



and obviously H+ and 0H~ are produced in equal amounts, 

 therefore [H+] = [OH"] = \/K^- 



Between 22° and 23° C. water has a dissociation constant with 

 which it is convenient to work, and measurements of hydrogen 

 ion concentrations are usually made at this temperature or 

 referred to this temperature ; 



i.e. at 23° C, K^, = lO-i^ ; 



.-. [H+] X [0H-] = 10-". 



[H+] is therefore equal to VlO~" = 10" 7, 



and [OH-] „ „ „ a/10-" = 10-^. 



It is usual to write H- for H+ and OH' for OH". 



Still further to shorten the symbols, S^rensen suggested the 

 use of the logarithm to denote the hydrogen ion concentration. 



