366 ELECTRICAL CHARGES OF COLLOIDAL PARTICLES 



approximately at the isoelectric point, but the measurements near the 

 isoelectric point are obviously vitiated by experimental errors or by 

 some other factor so that we cannot demonstrate more by the experi- 

 ment than that the suspended particles of solid metal gelatinate have 

 the opposite sign of charge from the gelatin chloride and that this dif- 

 ference is accompanied by a reversal of the sign of the value of pH 

 inside minus pH outside, which is positive in the case of gelatin chlo- 

 ride and negative in the case of Na gelatinate. It may also be pointed 

 out that the minimum of swelling (volume) coincides with the mini- 

 mum of P.D. 



While the experimental errors are rather great in the neighborhood 

 of the isoelectric point and on the alkaline side, they are fortunately 

 less annoying on the acid side when the hydrogen ion concentration is 

 sufficiently large. In this case the agreement between the value pH 

 inside minus pH outside and the p.d. observed is at least sufficient to 

 show that the p.d. is caused by the Donnan equilibrium. 



1 gm. of powdered isoelectric gelatin going through mesh 30 but 

 not through mesh 60 was put into 350 cc. of water containing various 

 quantities of HCl (see first horizontal row of Table V), and left in 

 this solution for 24 hours at 20°C. The flasks were occasionally 

 stirred. After 24 hours the volume of the particles was measured 

 and they were put on a filter to allow the outside solution to drain off. 

 The gelatin was then melted by heating to 45°C. and poured into glass 

 cylinders which at their lower end had two glass side tubes attached. 

 The mass was then allowed to solidify and the p. d. between gelatin 

 and watery solution was ascertained. One of the two glass tubes 

 dipped into a beaker containing the outside HCl solution (the filtrate) 

 with which the gelatin had been in equilibrium, and the other dipped 

 into a beaker containing a saturated solution of KCl. Each beaker 

 was connected with one of the calomel electrodes (filled with saturated 

 KCl) of a Compton electrometer. The last row in Table V gives the 

 observed p.d. in milHvolts. 



The gelatin was then melted and its pH was determined potenti- 

 ometrically. This is called pH inside in Table V. The pH of the out- 

 side solutions (filtrate) was also determined at 24°C. 



While the agreement between the observed p.d. and the values of 

 58 (pH inside minus pH outside) is not as complete as in the experi- 



