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 O=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 : 



= K++ 



This causes an alkalinity equal to that of potassium hydrate. 

 The alkalinity of solutions of sodium carbonate is due to the 



reactions, 



Na 2 CO 3 =2Na+ + CO 3 - 



H 2 O =H+ + OH- 



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

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

 H 2 CO 3 ^H 2 O +CO 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 + + 



^ 



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



E.g. 



NHX1 = NH.+ 



H 9 O = 



HO- 



=NH d OH + HC1. 



