716 ACTIVITY COEFFICIENT OF THE HYDROGEN ION 



from 2 to 3 per cent. It was practically salt-free. Several different 

 preparations were made during the course of the work. No differ- 

 ence could be detected in the behavior of these different lots. 



The increase in free amino or free carboxyl groups is the most 

 significant quantity as regards the hydrolysis of proteins, since if 

 the accepted views of structure of the proteins are correct, each 

 hydrolytic cleavage results in the liberation of a free amino and a 

 free carboxyl group. Two methods are available for following quan- 

 titatively the course of such reaction; Van Slyke's^ amino nitrogen 

 determination, and Sorensen's^ formol titration which determines 

 the free carboxyl groups. For absolute determinations of the amino- 

 acids Van Slyke's method is more accurate, for comparative experi- 

 ments concerning the changes occurring in gelatin solutions, such 

 as were used in this work, the formol titration is more accurate and 

 also much more rapid. Such a slight hydrolysis of gelatin as that re- 

 quired merely to liquefy the protein may be quite accurately deter- 

 mined by the formol titration whereas the increase in amino nitrogen 

 is so small as to be within the limits of error of the Van Slyke method. 

 The formol titration was used therefore in nearly all the experiments 

 reported in this paper. It is well known that many substances, 

 such as phosphates and carbonates, interfere with this titration. 

 In order to avoid these difficulties the method was slightly modified. 

 It is obvious that the final titration figure will depend on the amount 

 of alkali or acid present in the original solution. In order to get 

 comparable results, therefore, it is always necessary to start the 

 titration from the same pH. This was accomplished by titrating 

 the sample to pH 7,0, using neutral red as an indicator, before adding 

 the formaldehyde and continuing with the final titration. In the 

 case of gelatin itself, this method does not give the correct figure 

 since, in order to get the total acidity, it is necessary to start the titra- 

 tion at the isoelectric point of the gelatin; i.e., pH 4.7. The figures 

 obtained by titrating from pH 7.0 are, therefore, too low by the 

 amount of alkali necessary to titrate the gelatin from pH 4.7 to 

 pH 7.0. Since this is a constant quantity for any given concentra- 

 tion of gelatin, the titration figure could be corrected if necessary. 



3 Van Slyke, D. D., /. Biol. Chcm., 1913-14, xvi, 121. 

 ^ Sorensen, S. P. L., Biochem. Z., 1908, vii, 45. 



