228 HYDROGEN ION CONCENTRATION 



potential of between 1.3 and 12.2 millivolts. The latter value would 

 represent the highest imaginable. However, such ideal conditions 

 as: a 0.1 molar protein concentration; a complete molecular disper- 

 sion and ionization of the protein solution; absence of protein in the 

 surrounding fluid, can barely be found in the living organism. There- 

 fore it is quite questionable whether Donnan's theory of ionic dis- 

 tribution and of formation of potentials can, in this form, play an 

 important role in the living organism. On the other hand, for cer- 

 tain experimental conditions in colloidal chemistry the theory is of 

 the greatest importance, especially in the following respects. 



It is not unconditionally requisite that the colloidal and the col- 

 loid-free solutions be separated by an actual membrane. Procter^ 

 was the first to dem.onstrate that a piece of solid gelatin, immersed 

 in an aqueous solution and permitted to assume a definite state of 

 swelling, behaves in exactly the same manner. A solution is enclosed 

 within the meshes of the gel, and the gel itself represents, according 

 to the pH and because of its amphoteric character, a non-diffusible 

 acid or base. Consequently, when a piece of gelatin swells in a dilute 

 HCl solution, at equilibrium the H+-concentration must be greater 

 in the outer solution than within the gel, and this difference becomes 

 less upon the addition of NaCl. This observation was verified 

 experim.entally by Jacques Loeb,* and we shall return to this topic in 

 our chapter on colloids. 



63. The relation of membrane potentials to phase boundary 



potentials 



If it appears that the biological significance of these potentials is 

 not ver3^ great for the living cell, it is yet possible that membrane 

 potentials originating from another source may be of importance for 

 the living organism. The investigations of many physiologists have 

 led to the idea that the membranes of various cells show a specifically 

 selective permeability for different ions. R. Hober, especially, 

 advanced the thsor^y that cell membranes possess a general difference 

 in permeabiHty for cations and anions. Ever since the fundamental 



' H. R. Procter, Journ. of the Chem. Soc. 105, 313 (1914) ; H. R. Procter and 

 J. A. Wilson, Journ. of the Chem. Soc. 109, 307 (1916). 



4 Jacques Loeb, Journ. of Gen. Physiol. 3, 667; 3, 691; 3, 827; 4, 33; 4, 97 

 (1921); also J. Loeb. Proteins and the Theory of Colloidal Behavior. 2nd 

 ed. New York, 1924. 



