24 THEORY OF COLLOIDAL BEHAVIOR 



side HC1. The relative concentration of free HC1 inside and out- 

 side at the time of equilibrium is determined by the equation for 

 the Donnan equilibrium 



x 2 = y (y + z) (1) 



where x is the concentration of the H and Cl ions in the outside 

 solution, y the concentration of H and Cl ions of the free HC1 

 inside the gel, and z the concentration of Cl ions in combination 

 with the gelatin cation, x and y can be " determined experi- 

 mentally and z can be calculated with the aid of the equation. 

 In other words, the distribution of the H and Cl ions on the 

 opposite sides of a membrane is such that the product of the 

 concentrations of the pair of oppositely charged ions is equal in 

 both phases. 



"The gelatin salt, like other salts, is highly ionised into the anion and 

 a colloid cation, which either from polymerisation or other causes 

 peculiar to the colloid state cannot diffuse and exerts no measurable 

 osmotic pressure, whilst its anion is retained in the jelly by electro- 

 chemical attraction of the colloid ion, but exerts osmotic pressure which, 

 on the one hand, causes the mass to swell with absorption of the external 

 solution, and, on the other, expels a portion of the acid, both anion 

 and hydrion, from this solution absorbed, the result in equilibrium being 

 that the jelly is poorer in hydrion and more concentrated in anion than 

 the external acid solution, the difference of concentration between 

 anion and hydrion in the jelly being, of course, equal to the ionised 

 anion of the gelatin salt, and electrically balanced by the positive 

 gelatin ions; whilst the hydrion concentration in the jelly is less than 

 that of the outer solution by the amount of acid expelled." 1 



By establishing a connection between the volume of the gel 

 and the observed values of x and y, Procter and Wilson were able 

 to calculate the effect of different concentrations of HC1 on the 

 swelling of gelatin, and they could show why little acid increased 

 the swelling until a maximum was reached and why the addition 

 of more acid depressed the swelling. They could further show why 

 the addition of neutral salt caused a depression of the swelling. 



It is of interest to inquire why this theory of swelling was not 

 accepted and only rarely mentioned in the colloidal literature. 



1 PROCTER, H, R. and WILSON, J. A., J, Chem. Soc., vol., 109, pp. 309-310, 

 1916. 



