562 AMPHOTERIC COLLOIDS. V 



Table I gives the data of one experiment with a 1 per cent solution 

 of isoelectric gelatin made up in 100 cc. of H2O containing varying 

 quantities of 0.01 N HBr, stated in the upper horizontal row. The 

 second horizontal row of figures gives the pH of the gelatin solution 

 at the end of the experiment, and the third row the figures for the 

 cc. of 0.01 N Br found by Volhard's method, in 10 cc. of the gelatin 

 solution. These latter figures are too high since they include the free 

 HBr in the solution. We can find the value for the free HBr solu- 

 tion from the pH and this value must be deducted from the titration 

 number for Br. This deduction gives us the corrected Br values 

 of the horizontal Row 4. Row 5 gives the number of cc. of 0.01 N 

 HBr found in 10 cc. of the gelatin solution by titrating with NaOH 

 to pH = 7.0. This value demands, as stated, two corrections; first, 

 a deduction of the amount of NaOH needed to bring 10 cc of 1 per 

 cent isoelectric gelatin solution to pH = 7.0. This value is 1.8 cc. 

 of 0.01 N NaOH for 10 cc. of a 1 per cent solution of gelatin. The 

 second correction is that for the free HBr present in the gelatin 

 solution wliich can be measured by titrating free HBr of the same 

 pH as that of the gelatin solution to pH = 7.0. Row 6 gives the 

 titration values for NaOH after the two corrections have been made. 

 These corrected figures should give us the amount of HBr in com- 

 bination with 0.1 gm. of gelatin. The reader will notice that the 

 corrected numbers for the direct titration for Br after Volhard 

 (Row 4), and the corrected numbers for titration with NaOH (Row 

 6), are, within the hmits of the accuracy of our method, identical, as 

 they should be. This identity is of the greatest importance for the 

 subject we intend to discuss in this paper, since in the case of other 

 acids (with the exception of HCl and H3PO4) we can ascertain the 

 amount of acid in combination with 0.1 gm. of gelatin only indirectly 

 by titrating with NaOH. It was, therefore, necessary for us to be 

 sure that this titration gives us the correct amount of acid in combi- 

 nation with 0.1 gm. of gelatin if we make the two corrections men- 

 tioned, and the identity of the titration numbers by the direct Vol- 

 hard method for Br and by the indirect method of titrating with 

 NaOH gives us this certainty. 



Row 7 gives the osmotic pressure in mm. of the height of a column 

 of the 1 per cent ge'atin solution. Row 8 gives us the conductivity 



