364 ELECTRICAL CHARGES OF COLLOIDAL PARTICLES 



completely by the Donnan equilibrium. There can be no other 

 source for the electrical charge of this model of a protein micella 

 except that due to the membrane equilibrium. 



V. The Electrical Charges of Suspended Particles of Powdered Gelatin. 



It is possible to show that the electrical charges of the powdered 

 particles of gelatin suspended in a watery solution are determined 

 by the fact that acid is forced from the suspended particles into the 

 watery solution when the particles consist of gelatin chloride, and that 

 alkali is forced from the particles int^ the solution when they 

 consist of Na gelatinate. 



Measurements of the p.d. between solid gels of gelatin and the 

 surrounding solution suffer from inaccuracies (especially near the 

 isoelectric point) which we have not been able to eliminate, so that we 

 must be satisfied with only an approximate confirmation of the theory. 

 In order to prove or to make it probable that the p.d. is due to the 

 Donnan equilibrium we must be able to show that there exists a 

 difference of the value of pH inside and pH outside the gel when the 

 suspended particles of gelatin chloride or Na gelatinate are in equilib- 

 rium with the watery solution. 



1 gm. of powdered gelatin of grain size between mesh 30 and 60, 

 and rendered isoelectric was put into each of a series of closed flasks 

 containing 350 cc. of distilled water with varying quantities of 0.1 N 

 HCl or NaOH per 100 cc. (see Table IV). The temperature was 

 20°C. After 4 hours the powdered gelatin was separated from its 

 liquid by filtration, the gelatin was melted and the pH of the melted 

 gelatin and of the outside solution (filtrate) were measured. The 

 gelatin was then solidified and the p. d. between the solid gelatin and 

 the fi.ltrate (outside solution) determined, as will be described a little 

 later. The results of the experiments are given in Table IV. The 

 first row gives the number of cc. of 0.1 N HCl or NaOH contained 

 originally in 100 cc. outside solution. The next row gives the relative 

 volume of the solid mass of gelatin, i.e. the degree of sweUing. The 

 rest of the table needs no explanation. It is obvious that pH inside 

 minus pH outside is positive as long as the pH of the gelatin is on the 

 acid side of the isoelectric point, while it is negative when the gelatin 

 is on the alkaline side of the isoelectric point. The turning point is 



