JACQUES LOEB 287 



When acid, e.g. HCl, is added to isoelectric gelatin, part of the acid 

 combines with gelatin to form gelatin chloride and part of the acid 

 remains free. There is a chemical equiHbrium between isoelectric 

 gelatin, gelatin chloride, and free hydrochloric acid. As long as the 

 pH of the gelatin chloride solution does not exceed 3.3 (i.e. as long 

 as the pH varies between 4.7 and 3.3) the osmotic pressure of gelatin 

 chloride increases with an increase in the concentration of the acid. 

 As soon, however, as the pH reaches 3.3, the osmotic pressure of 

 gelatin chloride diminishes again with increasing concentration of 

 acid. 



Fig. 10 may serve as an illustration. To each of a series of doses 

 of 1 gm. of isoelectric gelatin were added 0, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 

 6.4, 12.8, and 25.6 cc. of m/4 HCl, water was added, the gelatin melted 

 by heating to about 40°C., and enough water was added to make the 

 volume of the solution 100 cc, so that each solution contained 1 per 

 cent gelatin. This solution was put into collodion bags closed by a 

 rubber stopper perforated with a glass tube serving as a manometer. 

 The collodion bag was put into a beaker containing 350 cc. of the same 

 concentration of acid in water as that originally added to the gelatin. 

 Thus the gelatin solution to which 0.2 cc. of m/4 HCl was added per 

 100 cc. of solution was immersed in a HCl solution containing 0.2 cc. 

 of m/4 HCl in 100 cc. of water. This outside solution was a httle more 

 concentrated than the concentration of the free acid inside, since part of 

 the acid added to the gelatin entered into combination with the latter. 

 This difference was equaUzed by the diffusion of some of the outside 

 acid into the gelatin solution, thus sKghtly lowering the original pH. 



The upper curve (Fig. 10) shows that the osmotic pressure of the 

 solution (measured after 20 hours) rises with an increase of the 

 hydrogen ion concentration until it reaches a maximum at pH about 

 3.4 or 3.3, and that with a further rise in the amount of free acid the 

 osmotic pressure of the solution falls until at pH 1.75 the osmotic 

 pressure of the gelatin solution is almost as low as it is near the 

 isoelectric point. 



The lower curve in Fig. 10 represents the velocity of diffusion of 

 water into the gelatin solution measured by the height of the column 

 of liquid in the manometer after 30 minutes. The maximum and 

 the two minima of the curve coincide with those of the upper curve 



