JOHN H. NORTHROP 7l7 



This correction is small, however, and in the case of comparative 

 experiments cancels out. In the titration of the amino-acids the 

 correction is within the limits of error of the titration. A pH of 

 7.0 was chosen as the standard since the titration curve of gelatin 

 is very flat at this point so that the adjustment of the reaction is 

 accurate to less than 0.05 cc. of 0.1 n NaOH and also since the pre- 

 sence of citrate does not interfere at this pH. 



Adjustment of the pH of Gelatin Solutions. — In strongly acid or alka- 

 line solutions it is not necessary to use buffers since the change in 

 the pH during the hydrolysis of the gelatin is neglible. Between pH 

 1.5 and 11.0, however, it is necessary to use some buffer as other- 

 wise the pH of the solution changes rapidly during the course of the 

 reaction. It was noted above that phosphates and carbonates in- 

 terfere with the titration. Phosphates are difficult to remove and 

 were not used at all in these experiments. It was found that a com- 

 plete series of buffer solutions could be prepared with various mix- 

 tures of trisodium citrate, HCl, and Na2C03. These solutions were 

 therefore used. Since carbonate also interferes with the titration 

 it was removed by bringing the sample to about pH 2.5 with strong 

 acid, boiling out the CO2 and then titrating back to pH 7.0. Con- 

 trol experiments showed that this procedure had no effect whatever 

 on the final titration; that is, the figure obtained for 10 cc. of gelatin 

 solution was identical when the gelatin was titrated to pH 7.0 di- 

 rectly before making the formol titration, and when it was titrated 

 first to say pH 10 with Na2C03, then made acid, the carbonate boiled 

 out, and then titrated to pH 7.0. The total concentration of citrate 

 was not above 0.05 n, since more concentrated solutions caused a 

 slight increase in the titration figure and rendered the adjustment 

 to pH 7.0 more difficult. 



Formalin Solution. — A solution of formalin was used containing 

 30 cc. of a saturated aqueous solution of thymol blue per 300 cc. 

 of solution, and titrated with sufficient alkali, so that when 5 cc. 

 were added to 15 cc. of water, the resulting solution had a pH of 

 8.4. This procedure obviates the necessity of making a correction 

 for the formalin solution. 5 cc. of this solution were used for 10 

 cc. of the sample. 



