490 



AMPHOTERIC COLLOIDS. IV 



pressure (and the other physical properties) of gelatin are identical, 

 if we use the pH as abscissae. This result contradicts the statements 

 current in colloid chemistry according to which these four cations have 

 a different effect. The colloid chemists who make such statements 

 have failed to measure the hydrogen ion concentration of their solu- 

 tions. Our experiments show that the effects of the same cation on 



325 



300 



275 



£50 



225 



200 



175 



150 



125 



100 



75 



50 



25 



pH 4.9 5.2 5.5 5.8 6.1 6.4 6.7 7.0 73 76 79 8.2 85 

 KOH 



Fig. 4. See explanation under Fig. 2. 



gelatin differ for different pH and hence we cannot be sure that ap- 

 parent differences in the effect of two cations on a protein are the ex- 

 pression of differences in the structure of the two cations, unless we 

 are certain that the pH is the same in both cases. The colloid chem- 

 ists have, moreover, compared the effects of neutral salts of Li, Na, K, 

 and NH4, in the presence of an excess of these salts, which introduces 

 a second error. 



