566 



AMPHOTERIC COLLOIDS. V 



gelatinate. The maxima for both gelatin bromide and gelatin sul- 

 fate are found at the same pH, namely between 3.3 and 3.5. van't 

 Hoff's theory would ascribe the difference in the maximal osmotic 

 pressures of the two acids to a corresponding difference in the num- 

 ber of particles in solution; while the colloid chemists would ascribe 

 the difference in osmotic pressure to a difference in the ''hydratation" 

 due to a difference in the ionization of the two gelatin salts. 



pH23 2.5 Z7 Z9 3,1 3.3 3.5 3.7 39 4.1 43 45 47 



He504 • 

 HBro 



Fig. 1. Relative amount of HBr and H2SO4 in combination with gelatin 

 Abscissae represent pH; ordinates, amount of 0.01 N HBr and H2SO4 bound by 

 0.1 gm. of gelatin. The identity of the curves for the two acids proves that 

 H2SO4 combines with gelatin as a dibasic acid. 



The curves for conductivity of the two gelatin salts (plotted accord- 

 ing to the corrected conductivity values of Table I and Table II) 

 differ but shghtly (Fig. 3). As a matter of fact, if other experi- 

 ments with the two acids are taken into account (one of which will 

 be given later) the curves for the conductivities of the two gelatin 

 salts are practically identical if plotted over pH as abscissae. This 

 identity of the conductivity curves contradicts the hydratation 

 hypothesis of the colloid chemists. 



