172 THEORY OF COLLOIDAL BEHAVIOR 



The equilibrium equation, therefore, assumes in the case of 

 gelatin sulphate the following form: 



From equation (2) it follows that 



9 x ~ y 

 ~~ 



The osmotic pressure of the gelatin sulphate solution should 

 therefore be calculated from the following values (omitting the 

 share of the osmotic pressure due to the gelatin molecules and 

 ions). 



3 , z 3 



2. THE CALCULATED CURVES FOR THE INFLUENCE OF pH AND 



VALENCY 



Solutions containing 1 gm. of originally isoelectric gelatin in 

 100 c.c. and containing different quantities of acid were prepared. 

 Collodion bags cast in the form of Erlenmeyer flasks of 50 c.c. 

 volume were filled with the 1 per cent solutions of a gelatin-acid 

 salt and put into beakers containing 350 c.c. of H 2 O. In order 

 to accelerate the establishment of the equilibrium between inside 

 and outside solutions a certain amount of acid was added to the 

 outside water (e.g., HC1 in the experiments with gelatin chloride, 

 H 3 PO 4 , in the experiments with gelatin phosphate, etc.). Each 

 Erlenmeyer flask was closed with a rubber stopper perforated 

 by a glass tube serving as a manometer. All this was described 

 in more detail in Chap. V. 



In Fig. 43 are plotted the values of the osmotic pressures of 1 

 per cent solutions of gelatin chloride, gelatin phosphate, and 

 gelatin sulphate, calculated on the basis of equations (1) and (2); 

 and Tables XXXI, XXXII, and XXXIII give the data on the 

 basis of which the calculations are made. The abscissae in 

 Fig. 43 are the pH in the inside solution at the point of equilib- 

 rium, the ordinates are the values for osmotic pressure calcu- 

 lated from the equations referred to. Figure 44 gives the 

 actually observed osmotic pressures in the same experiments 

 which furnished the data for the calculated curves in Fig. 43. 

 The reader will notice that the three curves plotted in Fig. 43 



