CHAPTER X 



OSMOTIC PRESSURE 1 



1. THEORETICAL STATEMENTS 



The characteristic features of colloidal behavior appear also in 

 the case of the osmotic pressure of solutions of protein salts. 

 If the Donnan equilibrium is actually the cause of this behavior, 

 as the experiments on membrane potentials suggest, it must be 

 possible to derive these features of the osmotic pressure quanti- 

 tatively and mathematically from Donnan's equilibrium formula. 

 It is the purpose of this chapter to show that this is possible on 

 the basis of van't Hoff's theory of osmotic pressure. The 

 methods of measuring the osmotic pressure have been described 

 in Chap. V. Collodion bags, of a volume of about 50 c.c., are 

 filled with a protein solution, while the outside solution is 350 c.c. 

 of water into which diffuses some of the free acid of the protein- 

 acid salt solution or some of the free alkali of the metal proteinate 

 solution. In order to hasten the establishment of equilibrium 

 between inside and outside, the pH of the outside solution 

 was usually at the beginning of the experiment brought to the 

 same pH as that of the protein solution by adding the same acid 

 or the same base as that of the protein solution. Equilibrium 

 was established after about 6 hours but the measurements were 

 usually taken after about 20 hours. The solutions were kept at 

 a constant temperature of 24C. throughout the experiment. 



A gelatin chloride solution contains free hydrochloric acid, 

 gelatin chloride (which dissociates electrolytically like any other 

 salt in watery solution), and non-ionogenic protein molecules. 

 A 1 per cent gelatin chloride solution of about pH 3.5 is in equi- 

 librium with a HC1 solution (free from protein) of a pH of about 

 3.0, the solutions being separated by a collodion membrane. 



The terms for the calculation of the osmotic pressure of 

 gelatin solutions are the same as those used by Procter (1914) 

 , J., J. Gen. Physiol, vol. 3, p. 691, 1920-21. 

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