ALBUMIN : THE ISO-ELECTRIC REACTION 25 



brought out clearly by solving the dissociation equation as 

 follows : 



C H . = K 



(2) 



In an acetate acetic acid mixture, the ionisation of the acetic 

 acid, which is low to begin with, is so far suppressed that the 

 acetate ions in solution can be regarded in practice as derived 

 solely from the acetate. As the acetate is ionised to more than 

 90 per cent, in solutions below 0*01 N, the concentration of 

 the acetate can be written instead of that of the ion. Further, 

 the concentration of the un-ionised molecules of acetic acid is 

 practically identical with that of the acetic acid actually used. 

 So that 



^ = R Caceticacid [j^ = ^ x IO -5] > 



^acetate 



and similarly for the base, 



C OH = K' ^ ammonia - [K' = 177 X io- 5 ]. 



^ammon.chloride 



It is easy to see that the mere proportion of acid (or base) 

 and salt determines the concentration of H- or OH' ions ; and 

 as this proportion is independent of dilution over a considerable 

 range, mixtures of weak acids and bases with highly dissociated 

 salts provide a simple means of preparing in practice media of 

 known H* or OH' concentration. Such solutions are readily 

 reproducible, and proof against reasonable impurities or 

 alterations in volume. They are called regulators or buffer 

 solutions. The table on p. 26 gives an example of the ion 

 concentrations in such solutions. 



This relation between weak and strong electrolytes with a 

 common ion is of importance in the determination of the 

 iso-electric reaction of albumin by means of regulators (L. 

 Michaelis). Further, such solutions are useful for finding out 

 how much of a weak acid or base is combined with albumin, 

 and for estimating the hydrolysis of the salt-like protein 

 compounds formed in this way (Wo. Pauli and Hirschfeld, see 

 Chapter V.). 



