578 



AMPHOTERIC COLLOIDS. V 



15 cc. 0.01 N HCl + cc. 0.01 n H3PO4; 13.5 cc. 0.01 n HCl + 4.0 

 cc. 0.01 N H3PO4; 12 cc. 0.01 N HCl + 8 cc. 0.01 n H3PO4 to 3 cc. 

 0.01 N HCl + 32 cc. 0.01 N H3PO4 and cc. 0.01 n HCl + 40 cc. 

 0.01 N H3PO4 were added each to 1 gm. of isoelectric gelatin and the 

 solution was made up to 100 cc, the osmotic pressure of which was 

 determined against 400 cc. of a pure acid solution of the same con- 

 stitution and concentration as that added to gelatin. The pH was 

 the same in all gelatin solutions, namely about 3.5, and the osmotic 

 pressure was also the same, about 300 mm. (Fig. 11). 



3Z5 

 300 

 275 

 Z50 

 225 

 200 

 175 

 150 

 125 

 100 



Fig. 11. Showing that H3PO4 has no antagonistic effect on the influence of 

 HCl upon the osmotic pressure of gelatin. The curve of osmotic pressure of 

 different mixtures of the two acids for the same pH is a straight line. 



There is a sHght antagonistic effect when we mix HCl and oxalic 

 acid (Fig. 12). This is due to the fact that the amount of oxalic 

 acid in combination with a given mass of gelatin is less than twice 

 the amount of HCl (or HBr) in combination with the same mass of 

 gelatin. It is possible that a small fraction of the oxalic acid acts 

 like a dibasic acid on gelatin while the greater part acts like a mono- 

 basic acid. 



