THE ELECTRICAL CHARGES OF MICELLA 159 



the acid side of the isoelectric point. We shall see that the meas- 

 urements of the P.D. between micellae and surrounding solution 

 are sufficiently accurate to leave no doubt that the Donnan equi- 

 librium alone determines the charge of the micellae and that the 

 cation of the salt does not increase the charge of the micellae of 

 gelatin chloride. 



In order to get accurate measurements it was necessary to use 

 micellae of gelatin chloride of a pH sufficiently far from the iso- 

 electric point to avoid the errors of the measurements which 

 occur near that point. We weighed out doses of 1 gm. of 

 powdered gelatin of a pH of near 7.0 and made them isoelectric by 

 treatment with M/128 acetic acid and subsequent washing as 

 described in Chap. II. In this process some gelatin was dis- 

 solved and lost (probably about 25 per cent). The isoelectric 

 powdered gelatin was put into 200 c.c. of H 2 or a solution of 

 different concentrations of a salt NaCl, CaCl 2 , BaCl 2 , CeCl 3 , 

 or Na 2 SO 4 and containing 16 c.c. of 0.1 N HC1. This brought 

 the pH of the micellae down to 2.8 or less, as Tables XXVI to 

 XXX show. The powdered gelatin was left in these acid-salt 

 solutions for two hours at 20C., with frequent stirring. Then 

 the supernatant liquid was separated from the powdered particles 

 of gelatin by filtration and the P.D. between the micellae and the 

 surrounding liquid (filtrate) measured with the Compton electro- 

 meter using the electrodes described in Fig. 42. After this the 

 value (pH inside pH outside) was obtained with the aid of the 

 hydrogen electrode at 24C. and this value multiplied by 59 is 

 called the calculated P.D. Tables XXVI to XXX give the 

 results. The uppermost row gives the nature and concentration 

 of the salt. The next row gives the relative volume of the gel 

 of gelatin, and the depressing influence of the salt on the swelling; 

 then follow the values for pH inside and outside measured with 

 the hydrogen electrode and then the values pH inside minus 

 pH outside. The last two columns give the calculated P.D. , i.e., 

 the value 59 (pH inside minus pH outside), and the P.D. observed 

 with the Compton electrometer and the indifferent electrodes 

 described in Fig. 42. 



The fact in common to all the experiments is the satisfactory 

 agreement between the observed and calculated P.D. except that 

 the calculated P.D. is on the average about 3 millivolts higher 



