668 DONNAN EQUILIBRIUM AND MEMBRANE POTENTIALS 



when a neutral salt was added or when the hydrogen ion concentra- 

 tion was altered or when the valency of the ion in combination with 

 the protein changed. This in itself would have meant only the addi- 

 tion of another property varying in the same characteristic way as 

 osmotic pressure, swelling, or viscosity if it had not been for the fact 

 that it was possible to correlate the origin of this new property with 

 Donnan's membrane equilibrium. Donnan's membrane equilibrium^ 

 is established when a membrane separates two solutions of electrolytes 

 one of which has one ion for which the membrane is impermeable, 

 while all the other ions can diffuse through the membrane. It is not 

 necessary that this non-diifusible ion should be a colloid, it may just as 

 well be a crystalloid ; all that is required is that it be impossible for 

 this ion to diffuse through the membrane. The protein ions generally 

 satisfy this condition and a collodion membrane properly prepared 

 is a membrane impermeable to a protein ion. 



When equilibrium is established in such a system the distribution 

 of the ions is not the same on both sides of the membrane and from 

 thermodynamic considerations Donnan was able to develop the equa- 

 tions for the relative concentration of the different ions on opposite 

 sides of the membrane at equilibrium. When a collodion bag 

 filled with a 1 per cent gelatin chloride solution is dipped into a beaker 

 containing a solution of HCl (without gelatin) of the same pH as 

 that of the gelatin solution, the concentration of the hydrochloric 

 acid becomes greater outside than inside the gelatin solution. The 

 Donnan equilibrium demands that free acid be expelled from the 

 gelatin solution into the outside solution and this actually occurs.^ 

 Procter^ has proposed an osmotic theory of the swelling of gelatin 

 chloride on the assumption that the swelHng is a purely osmotic 

 phenomenon. He deduces from Donnan's theory that at the point 



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

 A. B., J. Chem. Soc, 1911, xcix, 1554. Donnan, F. G., and Garner, W. E., J. 

 Chem. Soc, 1919, cxv, 1313. 



^ This is possible on account of the fact that a gelatin chloride solution always 

 contains free HCl and that there seems to be a chemical equilibrium inside the 

 gelatin chloride solution between the free HCl, the gelatin chloride, and the non- 

 ionogenic gelatin. 



^ Procter, H. R., J. Chem. Soc, 1914, cv, 313. Procter, H. R., and Wilson, J. 

 A., /. Chem. Soc, 1916, cix, 307. 



