CHAPTER IV 



QUANTITATIVE PROOF OF THE CORRECTNESS OF THE 

 CHEMICAL VIEWPOINT 



1. The qualitative experiments of the second chapter did 

 not permit us to decide whether ions combine with proteins 

 stoichiometrically (i.e., by the purely chemical forces of primary 

 valency), or according to the empirical rule of adsorption, as is 

 assumed in colloid chemistry. A decision can be rendered by 

 titration experiments. 1 



The titrations required for this proof differ from those usually 

 performed in chemistry. In the usual chemical work titration 

 is carried to the point of neutrality, i.e., pH near 7.0. Proteins, 

 however, are amphoteric electrolytes, the isoelectric point of 

 which is generally different from neutrality. Gelatin and casein 

 act as bases for a pH below 4.7, and if we wish to ascertain how 

 much of a certain acid 1 gm. of isoelectric gelatin can bind we have 

 to titrate to a pH below 4.7. In doing this, we must also remem- 

 ber that at such a high hydrogen ion concentration only strong 

 dibasic acids, like H 2 SO 4 , continue to dissociate both H ions, 

 while weaker dibasic or tribasic acids, e.g., H 3 PO 4 , are only able 

 to split off one H ion, acting therefore like monobasic acids. 



Our solutions contain generally 1 gm. of isoelectric protein in 

 100 c.c., and such solutions will be called 1 per cent protein 

 solutions. When 1 per cent solutions of albumin sulphate or 1 per 

 cent solutions of gelatin chloride are mentioned, this means that 

 1 gm. of originally isoelectric albumin or gelatin was in 100 c.c. of 

 the solution. The concentration of the stock solution of isoelec- 

 tric gelatin, albumin, or casein was determined by measuring the 

 dry weight of the solution. 



When different quantities of 0.1 N acid, e.g., HC1, are added 

 to the same quantity of protein, e.g., I gm. of isoelectric gelatin 

 or crystalline egg albumin, bringing the volume of the solution 



1 LOEB, J., J.Gen. Physiol., vol. 1, p. 559, 1918-19; vol. 3, p. 85, 1920-21. 



40 



