EDWIN JOSEPH COHN 705 



trations the protein is combined with acid, and at smaller with base.^ 

 In either case the solubility is no longer equal to the sum of the un- 

 dissociated protein molecule and its resultant ions, but to the sum of 

 these and the protein acid or basic compound. Equation (10) and 

 not equation (9) then obtains. Moreover, the degree of dissociation 

 of the compound of a protein with either a strong acid or a strong 

 )ase must be very great in comparison with the dissociation of 

 he pure protein, for the same reason that the dissociation 

 li the salt of any weak electrolyte and a strong electrolyte is 

 very much greater than the dissociation of the weak electrolytes 

 (32, 33). As a result, when the protein is combined with even a very 

 small amount of either acid or base, the solubility due to the dissocia- 

 tion of the protein must be considered very small in comparison 

 with the solubility due to the dissociation of the protein compound. 

 But the concentration of the protein compound is not independent, 

 but is a function, of dilution, as these experiments and previous ones 

 have indicated (34, 35). 



These observations, therefore, present a new method of determining 

 the isoelectric point of a protein. This method is free from the errors 

 that often enter into the determination of the minimum in solubility 

 of a protein, though theoretically, and in practise, the two methods 

 are capable of giving the same result. But, since the presence of a 

 foreign protein (unless it has the same solubility and the same iso- 

 electric point), or of multivalent cations or anions, or of a neutral 

 salt, miay shift the point of minimum solubility of a protein in the man- 

 ner and for the reasons that have already been described, the presence 

 of these impurities renders impossible the determination of the true 

 isoelectric point. Upon their removal, however, protein will dissolve 

 in water to a constant and characteristic extent, and will dissociate to 

 a constant and characteristic extent. The hydrogen ion concentration 

 due to this dissociation is also a characteristic of each protein, and 

 has come to be known, as we have seen, as its isoelectric point. ^ 



* Experiments that will he reported in a subsequent communication suggest 

 that a protein can exist in coml)ination with acid and base even at its isoelectric 

 point. 



' In practise the hydrogen ion concentration due to the dissociation of pure 

 protein is determined with great difficulty because of its very small solubility. 



