16 BACTERIOLOGICAL CHEMISTRY 



demands and thus preventing any considerable change of 

 pH. As the name implies, the buffer sohition acts as a 

 shock absorber, or like a sponge, to take up excess acid 

 or alkali. 



Buffer solutions are, in general, mixtures of a weak 

 acid with one of its salts, the particular acid being chosen 

 to suit the ^^H range desired. A weak acid, such as 

 acetic acid, in solution is only ionised or dissociated to a 

 small extent, so that most of it is present as undissociated 

 neutral acetic acid molecules : — 



HAc^=^H+ + Ac-. (1) 



The Law of Mass Action applies to this dissociation, 

 giving the equation 



[H+]x[Ac-] = A:[HAc], or [H+] = ^^^ . . . (2) 



On the other hand, the salt, sodium acetate, is strongly 

 dissociated to give sodium and acetate ions. Now in a 

 mixture of the acid and the salt practically all the acetate 

 ions will be derived from the highly dissociated salt and 

 only very few from the acetic acid, and consequently, 

 since the number of hydrogen ions cannot be greater 

 than the number of acetate ions derived from acetic acid, 

 the hydrogen ion concentration will be lowered. As the 

 acetate ions in such a mixture come almost entirely from 

 the salt their concentration wall be a x [NaAc], where a 

 is the fraction of the salt which is ionised. So that we 

 can write the last equation as 



That is, the hydrogen ion concentration in such a mixture 

 depends on the ratio of free acid to salt ; the higher the 

 salt content the lower the hydrogen ion concentration. 

 It will be seen that dilution of such a solution will have 

 very little effect on the pH value, as both [HAc] and 

 [NaAc] are altered to the same extent, the ratio being the 



