DONNAN EQUILIBRIUM AND THE PHYSICAL PROPERTIES 



OF PROTEINS. 



II. Osmotic Pressure. 



By JACQUES LOEB. 

 {From the Laboratories of The Rockefeller Institute for Medical Research.) 



(Received for publication, March 23, 1921.) 

 I. INTRODUCTION. 



It was pointed out in two preceding papers^ that the curves repre- 

 senting the influence of electrolytes on the membrane potentials 

 between gelatin solutions and water at the point of equilibrium show 

 a marked similarity to the curves representing the influence of the 

 same electrolytes on osmotic pressure, swelling, and viscosity of gela- 

 tin. There was no doubt left that the Donnan equilibrium accounted 

 quantitatively for these variations in the case of membrane poten- 

 tials^ and the question therefore arose whether the same equilibrium 

 condition can account also for the corresponding variations of the 

 osmotic pressure of protein solutions. An attempt was, therefore, 

 made to calculate the osmotic pressures on the basis of the Donnan 

 equilibrium in order to find out whether the curves for the calculated 

 values would show the characteristics of the curves representing the 

 observed values for osmotic pressures. The depressing effect of the 

 addition of a neutral salt on the osmotic pressure of colloidal solu- 

 tions was predicted by Donnan and offered no theoretical difficulty.^ 

 The real test was whether the Donnan equilibrium would be able to 

 account for the peculiar curves obtained when the observed osmotic 

 pressures of 1 per cent solutions of gelatin-acid salts are plotted as 

 a function of the pH. 



1 Loeb, J., /. Gen. Physiol, 1920-21, iii, 557, 667. 



2 Loeb, J., /. Gen. Physiol, 1920-21, iii, 667. 



^ Donnan, F. G., Z. Elektrochem., 1911, xvii, 572. Donnan, F. G., and Harris, 

 A. B., /. Chem. Sac, 1911, xcix, 1554. Donnan, F. G., and Garner, W. E., /. 

 Chem. Sac, 1919, cxv, 1313. 



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