64 THEORY OF COLLOIDAL BEHAVIOR 



rest exists as non-ionogenic protein. Only if enough acid (or 

 alkali) is added, is all the protein transformed into a salt. The 

 combination curves show that at the same pH the same fraction 

 of the protein present exists in the form of a protein salt. If it is 

 desired to compare the relative efficiency of different ions in 

 combination with a protein, one must be sure that the concen- 

 tration of originally isoelectric protein is the same in both solu- 

 tions and that the fraction of protein which has combined with 

 the two ions is the same. This is only true when the solutions of 

 the protein salts to be compared have not only the same concen- 

 tration of originally isoelectric protein but also the same pH. 



There existed no reason for comparing the effects of different 

 ions at the same pH as long as the titration curves given in this 

 chapter were not known. But the knowledge of these curves 

 forces the experimenter to change his methods and to base all the 

 comparisons of the influence of ions on the physical prop- 

 erties of proteins on protein solutions of equal hydrogen ion 

 concentrations. 



