4 THEORY OF COLLOIDAL BEHAVIOR 



ments on potential difference and osmotic pressure to be dis- 

 cussed later such an assumption does not lead to the idea that 

 chemical reactions occur only at the surface of the micellae for 

 the simple reason that solid gels of proteins (e.g., of gelatin) are 

 easily permeable to acids, alkalies, and salts or to crystalloids in 

 general. Chemical reactions are, therefore, not restricted to the 

 surface of protein micellse. 



While a number of authors, like Bugarszky and Liebermann, 1 

 Osborne, 2 Robertson, 3 Pauli, 4 and others assumed that the reac- 

 tions of proteins are purely chemical, this assumption could not 

 be proved conclusively until the modern methods of measuring 

 the hydrogen ion concentration of protein solutions were 

 developed by Friedenthal, S^rensen, 5 Michaelis, 6 Clark, 7 and 

 their collaborators. On the basis of these methods it was easy 

 to demonstrate the purely stoichiometrical character of the 

 combination of proteins with acids and alkalies. 



Thus it was proved that gelatin combines with acids only when 

 the hydrogen ion concentration of the solution is above a certain 

 critical point, namely greater than N/50,000 (or pH = 4.7).* 

 At hydrogen ion concentrations above N/50,000, H 3 PO 4 dis- 

 sociates as a monobasic acid. Hence, if gelatin combines stoichio- 

 metrically with acids it should require three times as many 

 cubic centimeters of 0.1 N H 3 PO 4 as it requires cubic centimeters 

 of 0.1 N HC1 or HNO 3 to bring 1 gm. of gelatin in 100 cc. solution 

 from a hydrogen ion concentration of N/50,000 to that of, e.g., 

 N/1,000. The strong acid H 2 SO 4 dissociates, however, in this 



1 BUGARSZKY, S. and LIEBERMANN, L., Arch. ges. PhysioL, vol. 72, p. 51, 

 1898. 



2 OSBORNE, T. B., Die Pflanzenproteine : Ergeb. PhysioL, vol. 10, p. 47, 

 1910. 



3 ROBERTSON, T. B. "The Physical Chemistry of the Proteins," New York, 

 London, Bombay, Calcutta, and Madras, 1918. 



4 PAULI, W., Fortschr. naturwiss. Forschung, vol. 4, p. 223, 1912. "Kol- 

 loidchemie der Eiweisskorper," Dresden and Leipsic, 1920. 



6 S0RENSEN, S. P. L., see Bibliography given in W. M. CLARK, "The 

 Determination of Hydrogen Ions," Baltimore, 1920. 



6 MICHAELIS, L., "Die Wasserstoffionenkonzentration," Berlin, 1914. 



7 CLARK, W. M., "The Determination of Hydrogen Ions," Baltimore, 

 1920. 



8 LOEB, J., J. Gen. PhysioL, vol. 3, p. 85, 1920-21. Science, vol. 52, p. 449, 

 1920. J. chim. physique, vol. 18, p. 283, 1920. 



