MEMBRANE POTENTIALS 139 



The flasks were sealed, communicating with the air only through 

 tubes filled with soda lime, as stated. The collodion bags con- 

 taining the gelatin were closed by a rubber stopper perforated 

 by a glass tube which served as a manometer. The experiment 

 lasted 6 hours at a temperature of 24 C. The results of the 

 experiments are given in Table XVII. The upper horizontal 

 row gives the number of cubic centimeters of 0.1 N NaOH 

 originally in 100 c.c. of the gelatin solution; the second row 

 gives the original concentration of NaOH in the outside aqueous 

 solution free from gelatin ; the third row gives the osmotic pressure 

 in mm. H 2 O after 6 hours. The next row gives the pH inside 

 and the following row the pH outside after the experiment was 

 finished (i.e., after 20 hours), and the sixth row gives the differ- 

 ence pH inside minus pH outside. The reader will notice that 

 this difference is always negative with one exception, which is 

 obviously an error. The last two rows give the P.D. calculated 

 from pH inside minus pH outside, and the P.D. observed. 



It is obvious that there is no quantitative agreement between 

 observed and calculated P.D. near the isoelectric point. As soon 

 as the pH is above 7.0 the agreement between observed and 

 calculated P.D. becomes better, so that we are entitled to say 

 that the difference of potential between a Na gelatinate solution 

 and an outside solution at or near equilibrium is due to the 

 Donnan equilibrium which forces the expulsion of NaOH from 

 the inside into the outside solution. As a consequence the pH 

 inside becomes lower than the pH outside. 



THE INFLUENCE OF NEUTRAL SALTS ON THE P.D. OF GELATIN 

 CHLORIDE SOLUTIONS 



It was shown in Chapter VI that the addition of neutral salts 

 to solutions of protein salts depresses the osmotic pressure or 

 viscosity of these solutions, and that the addition of neutral 

 salts to a gel depresses the swelling of the latter (except when the 

 solutions and gels are at the isoelectric point). It was of interest 

 to find out whether or not the addition of a salt to a protein 

 solution depresses also the P.D. across a collodion membrane, 

 and whether this is also due to a depression of the value of pH 



