THE ELECTRICAL CHARGES OF MICELLAE 155 



3. THE INFLUENCE OF pH ON THE CHARGE OF SUSPENDED 



PARTICLES OF POWDERED GELATIN 



One-gram samples of powdered isoelectric gelatin going through 

 mesh 30 but not through mesh 60 were put into 350 c.c. of water 

 containing various quantities of HC1 (see first horizontal row of 

 Table XXIV), and left in these solutions for 24 hours at 20C. 

 The mixtures were occasionally stirred. After 24 hours the 

 relative volume of the particles was measured and they were put 

 on a filter to allow the acid to drain off. The gelatin was then 

 melted by heating to 45C. and poured into glass cylinders which 

 at their lower end had two glass side tubes attached (Fig. 42). 

 The mass was then allowed to solidify by cooling and the P.D. 

 between gelatin and watery solution was ascertained. One of the 

 two glass tubes dipped into a beaker containing the outside HC1 

 solution (the filtrate) with which the gelatin had been in equilib- 

 rium, and the other dipped into a beaker containing a saturated 

 solution of KC1. Each beaker was connected with a calomel 

 electrode (filled with saturated KC1) leading to a Compton 

 electrometer. The last row in Table XXIV gives the observed 

 P.D. in millivolts. 



The pH of the melted gelatin was determined potentiometri- 

 cally. This is called pH inside in Table XXIV. The pH of the 

 outside solutions (filtrate) was also determined. 



While the agreement between the observed P.D. and the value 

 of 59 (pH inside micellae minus pH outside) is not as good as in the 

 experiments with collodion bags, it is at least sufficient to leave 

 no doubt that the charge is caused by the Donnan equilibrium in 

 the way discussed in the preceding chapter. 



4. THE INFLUENCE OF ACID AND ALKALI ON THE SIGN OF CHARGE 



OF MICELLAE 



In Hardy's experiment with white of egg the particles were 

 positively charged on the acid side of the isoelectric point and 

 negatively charged on the alkaline side. It can be shown that 

 this is also true for the charges of the suspended particles of 

 powdered gelatin, and that this change of sign of charge of these 

 particles by going from the acid to the alkaline side of the iso- 



