126 THEORY OF COLLOIDAL BEHAVIOR 



the observed P.D. if the Donnan equilibrium is the exclusive 

 cause for the P.D. between a gelatin chloride solution and the 

 outside watery solution; neglecting the sign. 1 



Although the value pH inside minus pH outside is an observed 

 value, e.g., observed with the hydrogen electrode, we will call the 

 value 59 (pH inside minus pH outside) the calculated P.D. to 

 distinguish it from the P.D. observed with the indifferent 

 electrodes. 



THE INFLUENCE OF THE HYDROGEN ION CONCENTRATION OF GELATIN 

 SOLUTIONS ON THE P.D. 



Collodion bags of a volume of about 50 c.c. were filled with 1 per 

 cent solutions of gelatin chloride of different pH. The bags were 

 put into beakers containing each 350 c.c. of distilled water. To 

 hasten the establishment of equilibrium between gelatin chloride 

 solution and outside water some HC1 was added to the latter 

 in fact the pH of the gelatin and the outside solutions was 

 generally made equal at the beginning of the experiment. The 

 collodion flasks containing the gelatin solution were closed with 

 rubber stoppers which were perforated by glass tubes serving as 

 manometers to allow the measurement of the osmotic pressure of 

 the solution. After about 6 hours osmotic equilibrium was com- 

 plete but we waited, as a rule, about 18 hours before measuring 

 the P.D. across the membrane. Figure 41 shows that the pH 

 influences the P.D. in a similar way as it influences the osmotic 

 pressure, swelling, etc. Similar experiments were made with 1 

 per cent solutions of gelatin phosphate, -gelatin oxalate, and 

 gelatin sulphate, and the curves are also given in Fig. 41. To 

 demonstrate the similarity between the curves for osmotic 



1 The sign of the observed P.D. was apparently, but not in reality, the 

 reverse of the sign of the calculated P.D. In the "observed" P.D. the 

 membrane (acting as a hydrogen electrode) was between the concentrated 

 and dilute HC1, while in the "calculated" values the P.D. was obtained, 

 from the potentiometric determinations of the pH. In this latter case two 

 hydrogen electrodes were separated by a concentrated and a dilute solu- 

 tion. The "observed" P.D. was hence between two solutions of different 

 concentrations while in the "calculated" values we measured the P.D. 

 between two electrodes. In our tables the apparent (but not real) reversal 

 of sign is corrected. 



