78 THEORY OF COLLOIDAL BEHAVIOR 



selected for the experiment. Doses of 1 gm. each were weighed 

 out and each was put for an hour into 100 c.c. M/128 acetic 

 acid at 10C. to bring the gelatin to the isoelectric point. The 

 powdered mass was then put on a filter and washed five times 

 with 25 c.c. of distilled water of 5C. In the acetic acid solution 

 and during the washing on the filter the powdered gelatin is 

 stirred constantly. In this washing about 20 per cent of the 

 gelatin were lost, so that the mass of gelatin in the following 

 experiments was only about 0.8 gm. each. 



Each dose of originally 1 gm. of dry powder which had mean- 

 while absorbed a certain quantity of liquid (which was about the 

 same for each dose of isoelectric powder) was then put for 1 hour 

 at about 20 into 100 c.c. of different concentrations of the acid 

 or base whose influence on swelling was to be tested, and the 

 mass was frequently agitated. To measure the relative amount 

 of swelling in different acids or alkalies and at different pH the 

 mass was poured into graduated cylinders of 100 c.c. in which the 

 granules settled very rapidly to the bottom. The cylinders were 

 kept in a water bath at 20 for about 10 to 15 minutes and the 

 volume occupied by the gelatin granules, after settling, was then 

 read. This volume included a certain amount of solution between 

 the granules and, therefore, the real volume of the gelatin was 

 smaller than that read. While therefore, the method cannot be 

 used to measure the absolute amount of swelling it allowed us to 

 determine the relative influence of different acids or bases on the 

 swelling for the same pH. 



The pH inside the gelatin granules and the surrounding 

 solution are quite different, owing to the Donnan equilibrium. 

 It is, therefore, not correct to assume that the pH of the granules 

 of gelatin is that of the supernatant liquid. The pH of the 

 granules of gelatin was determined after the gelatin had been 

 poured on a filter and the acid in the interstices of the granules 

 of gelatin had been allowed to drain off. Traces of this outside 

 acid remained undoubtedly at the surface of the granules. The 

 gelatin was then melted and its volume brought to 100 c.c. by 

 adding enough distilled water of pH 5.6. The pH was determined 

 potentiometrically. This pH was probably a trifle too low on 

 account of some of the acid adhering. 



Figures 19 and 20 give the results of the measurements of swell- 



