68 ION IS ATI ON 



That is, hydrochloric acid, under the above conditions, has 



10-1 ^ 10-286 == 73.5 



times the amount of liydrogen ions per Htre that acetic acid has. 

 N/10 Hydrochloric acid at 23° C. is therefore 73-5 times as strong 

 as N/10 acetic acid. 



Salts. 



It is very seldom that acids, weak or strong, occur alone or 

 diluted with water in physiological fluids. Salts are always 

 present. In (d) and (e) are mentioned two classes of salts which 

 alter the [H] of water when dissolved in it. They do so directly 

 in virtue of their possession of an additional H- or OH' ion. 



Other salts cause alterations in acidity by upsetting the balance 

 between H- and OH' in water. Their action is indirect. 



(1) The salt of a strong acid and a strong base, e.g. NaCl, causes 

 little or no change in [H]. 



(2) If one of the constituents of a salt be weak, changes occur. 

 (a) If the salt BA of the strong base B . OH and the weak 



acid HA be dissolved in water, it forms BA = B+ + A-. But owing 

 to the ionisation of the solvent there are present H+ and OH- ions 

 and a second change takes place, for H+ and A- ions are present. 

 According to the law of mass action 



[H-l X [A'l 



^HA] = ^^^ ^' H- + A' - HA. 



As no HA is present to balance the reaction, H- will combine 

 with A' to form HA until the point of equilibrium for that dilution 

 has been reached. 



Summarising these reactions as follows : 



A' + H2O = HA + OH', 



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

 of a strong base and a zveak 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- ] ^ 



H2O = H+ + OH ) "^'^ ^ ^ + un . 



This causes an alkalinity equal to that of potassium hydrate. 

 The alkalinity of solutions of sodium carbonate is due to the 

 reactions, 



