SALTS 61 



acetic acid, for instance, at 23 is dissociated 1 -36 per cent. There- 



1 '36 

 fore its (H+) would be equal to ---- x 1Q- 1 = 1 -36 x 10~ 3 or p H of 



2-86. 



Normal solutions of acid are all equal as regards the amount 

 of alkali they can neutralise. 1 c.c. of any N/10 acid is exactly 

 neutralised by 1 c.c. of any N/10 alkali. That is, they have the 

 same titratable acidity. They differ in their concentration of 

 hydrogen ions. 



As we have seen 



HC1 in water at 23 =p H 1 or C H -1 xlQ- 1 , 



CH 3 COOH =PK 2 -86 or C H -1 -36 xlO~ 3 . 

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



times the amount of hydrogen ions per litre 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 II * 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, H + and A~ ions are present. 

 According to the law of mass action 



[ ^f W - or 



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. The removal of hydrogen ions from the 



