IN ISO-ELECTRIC REACTION 53 



experimental error, at a point representing the same excess of 

 acid and the same hydrion concentration. 



Sorensen has made a large number of such determinations 

 with egg albumin, and deduces as a mean value from twenty 

 sets of results, 1574 X io~ 6 as the iso-electric point of egg 

 albumin. These determinations are undeniably the most 

 accurate investigations of the iso-electric point so far carried 

 out. 



From our point of view the following points must be noted : 



This work of Sorensen is another investigation carried out 

 with the use of a weak acid. For, by the presence of ammonium 

 sulphate in the relatively high concentration used in the 

 research quoted above, the strength of sulphuric acid is reduced 

 to one-eighth of its ordinary value. Sorensen himself regards 

 such solutions as mixtures of the acid H[NH 4 S0 4 ] with the salt 

 NH 4 [NH 4 S0 4 ]. 



This work is, therefore, a valuable proof of the fact that if a 

 sufficiently weak acid is used, the iso-electric point is largely 

 independent of the ampholyte concentration. This had already 

 been demonstrated for acetic acid acetate mixtures by 

 Michaelis, using the method of electrophoresis, and by Pauli 

 by the use of alcohol precipitation and viscornetry. The 

 behaviour of proteins when strong acids are employed in the 

 absence of their salts comprises, however, a complex of pheno- 

 mena quite by itself, as has been previously emphasised. 



In pure albumin, as in weak acids and bases in general, a 

 notable dissociation by steps is not to be expected. Conse- 

 quently it is enough to assume that a single K a and K & functions 

 in the buffer solutions. The action of one of the strongest of 

 the acidic and basic groups is sufficient in the first instance to 

 decide the acidic and basic behaviour of the substance. It is easy 

 to understand, in these circumstances, that all differences in 

 constitution do not find expression in the values of K a and K 6 , 

 as has already been seen in the case of the simplest amino- 

 acids and dipeptides. Consequently the iso-electric reaction 

 of various proteins with buffer solutions is only partly affected 

 by differences in structure. Consistent with this view is the 

 scarcely surprising fact that proteins of such widely differing 



