692 



DONNAN EQUILIBRIUM AND OSMOTIC PRESSURE 



Fig. 1 shows a set of such curves where the abscissae are the pH 

 of the gelatin solution at equilibrium, and the ordinates the observed 

 osmotic pressures. There are two outstanding peculiarities in these 

 curves, namely, first, that they all rise from a minimum at pH 4.7 

 (the isoelectric point of gelatin) until they reach a maximum at pH 



450 

 425 

 4D0 

 375 

 0) 350 



h 



g 325 

 0) 300 



f^275 

 o 



•^ 250 

 o 



g 225 

 to 

 O 200 



O 175 

 S 150 



S 125 

 O 

 100 



75 



50 



25 



pli 1.4 L6 IB 2j0 22 ^4 2J5 2B 3.0 32 34 3j6 3fl 40 42 44 46 45 



Fig. 1. Curves representing the influence of pH and valency of anion on osmotic 

 pressure of solutions of gelatin-acid salts containing 1 gm. of originally isoelectric 

 gelatin in 100 cc. solution. The curves for gelatin chloride and gelatin phosphate 

 are identical since the anions, CI and H2PO4, of these two gelatin salts are mono- 

 valent. The curve for gelatin sulfate is less than half as high as the curve for 

 the two other salts because the anion of gelatin sulfate is bivalent. Both curves 

 rise from the isoelectric point at 4.7 to a maximum at pH about 3.4 or 3.5, and 

 then drop rapidly again. 



