62 



IONISATION 



solution by this reaction, however, disturbs the equilibrium 

 between H* and OH' and some more water must ionise to maintain 

 the product [H] x[OH] constant. 



Summarising these reactions as follows : 

 A'+H 2 0=HA+OH', 



the net result is the liberation of OH ions. The addition of a salt 

 of a strong base and a weak acid is to make the solution alkaline, 

 i.e. to reduce the hydrogen ion concentration. This is a fact of 

 great physiological importance, as most of the salts of the body 

 are composed of organic acids combined with the strong bases 

 sodium and potassium. 



KCN, a powerful poison, dissociates as follows : 



KCN = K> + CN-j 



H 2 =H+ + OH-j = 



This causes an alkalinity equal to that of potassium hydrate. 

 The alkalinity of solutions of sodium carbonate is due to the 

 reactions, 



H 2 O =H+' + OH- 



CO 3 - + H 2 .= HCO 3 - + OH- 

 HC0 3 -+H 2 O = H 2 CO 3 +OH- 

 H 2 C0 3 ^H 2 +C0 2 



If the CO 2 is allowed to escape, the last reaction will only cease 

 when all the H 2 CO 3 has been decomposed. The total result is 

 an increase in [OH] and, therefore, of alkalinity. 



(b) In the case of a weak base combined with a strong acid, 

 the solutions become acid, as the following equations denote. 



BA = B+ 



H 2 = 



OH- 



and HA is a strong acid and BOH a weak base. 

 E.g. NH 4 C1 - 



H 2 = H+ + HO-J 



NH 4 OH+HC1. 



