HISTORICAL INTRODUCTION 19 



The writer put this theory to a test by measuring the electrical 

 conductivity of solutions of different gelatin salts at different pH, 

 with the result that the parallelism between the concentration of 

 protein ions and the physical properties of proteins demanded 

 by Pauli's theory could not be demonstrated (see Chap. VII). 

 Lorenz, 1 Born, 2 and other authors have recently reached the 

 conclusion that the idea of a hydration of ions is not tenable in 

 the case of polyatomic ions. 3 



The increase in viscosity of certain protein solutions through 

 the addition of acid or alkali to isoelectric proteins is caused 

 by the ionization of proteins, but the connection is not the di- 

 rect one suggested by Laqueur and Sackur but an indirect one 

 due to the role of protein ions in the establishment of a Donnan 

 equilibrium. 



7. DONNAN'S MEMBRANE EQUILIBRIUM 



With the proof of the stoichiometrical character of the com- 

 bination of proteins with acids and alkalies the explanation of 

 colloidal behavior on the basis of the adsorption theory became 

 untenable and another theoretical basis had to be found. The 

 explanation offered in this volume is based on Donnan's theory 

 of membrane equilibria. 



Donnan 4 has shown that when a membrane separates two 

 solutions of electrolytes one of which contains one ion which 

 cannot diffuse through the membrane while all the other ions 

 can diffuse through the membrane, the result will be an unequal 

 distribution of the diffusible ions on the opposite sides of the 

 membrane. At equilibrium the products of the concentrations 

 of each pair of oppositely charged diffusible ions are the same on 

 the opposite sides of the membrane. This unequal concentration 

 of the crystalloidal ions must give rise to potential differences 



1 LORENZ, R., Z. Elektrochem., vol. 26, p. 424, 1920. 



2 BORN, M., Z. Elektrochem., vol. 26, p. 401, 1920. 



3 The term "hydration" is often used in colloid chemistry in a vague 

 way to designate such phenomena as the swelling of proteins which is a 

 purely osmotic phenomenon. It is obvious that it can only lead to confusion 

 if the term hydration is used for osmotic pressure. In this volume the term 

 hydration is only used in the sense of Kohlrausch and Pauli. 



4 DONNAN, F. G., Z. Elektrochem., vol. 17, p. 572, 1911. 



