56 THEORY OF COLLOIDAL BEHAVIOR 



ascertained from the pH of the gelatin bromide solution by 

 preparing a solution of HBr of the same pH in water, without 

 gelatin, and determining the amount of Br in this solution free 

 from gelatin. By deducting this value from the total Br it can 

 be found how much HBr is combined with the gelatin. Table 

 IV gives the results of such an experiment. 1 Row 1 gives the 

 number of cubic centimeters of 0.01 N free hydrobromic acid 

 originally contained in 100 c.c. of the 0.8 per cent solution of orig- 

 inally isoelectric gelatin. Row 2 gives the pH of each gelatin- 

 bromide solution after equilibrium is established ; Row 3 the total 

 amount of 0.01 N Br found in 10 c.c. of the solution; and Row 



4 gives the amount of Br actually in combination with gelatin 

 after deducting the amount of Br in the free HBr (not in com- 

 bination with gelatin) from the total amount of Br found. 



There is a second method of ascertaining the amount of HBr in 

 combination with a given mass of gelatin, namely by titrating for 

 acid with NaOH. 1 In this case the number of cubic centimeters 

 0.01 N NaOH required to bring 10 c.c. of the gelatin-bromide 

 solution to a pH of 7.0 must be determined. This gives the 

 total acid, from which the value for free acid not in combination 

 with gelatin is to be deducted. This value is obtained by titrat- 

 ing a solution of HBr (free from gelatin) of the same pH as the 

 gelatin-bromide solution with NaOH. A second correction, 

 however, must be made ; namely, the quantity of NaOH required 

 to bring 10 c.c. of an 0.8 per cent solution of isoelectric gelatin to a 

 pH of 7.0 must be ascertained. This value was found to be about 

 1.8 c.c. 0.01 NaOH for lOc.c. of a 0.8 per cent solution of isoelectric 

 gelatin. This value must also be deducted, and if these two 

 deductions are made, approximately the same figures are reached 

 as by direct titration for Br. This is shown by Table IV. Row 



5 gives the number of cubic centimeters 0.01 N NaOH required to 

 bring 10 c.c. of gelatin solution to pH of 7.0. Row 6 gives the 

 corrected NaOH values, i.e., after the two deductions just 

 mentioned are made from the values in Row 5. A comparison 

 of the values of Row 6 with those of Row 4 shows that they are 

 identical within the limits of the accuracy of our methods. 



This method of titrating with NaOH allows us, therefore, to 



1 LOEB, J., J. Gen. PhysioL, vol. 1, p. 559, 1918-19. 



