720 ACTIVITY COEFFICIENT OF THE HYDROGEN ION 



as that obtained with the formol titration. The formaldehyde 

 evidently merely makes the acid groups stronger so that it is not 

 necessary to titrate to such a high pH. 



The figures in the last column show that 1 per cent gelatin solu- 

 tion is about 0.004 normal wdth respect to free NH2 groups, 0.006 

 normal with respect to total acid groups, and 0.012 normal with 

 respect to total basic groups. That is, there must be approximately 

 twice as many basic as carboxyl groups, and three times as many 

 total basic groups as NH2 groups. These figures agree approximately 

 with those found by Wintgen and Kriiger,^ 



Determination of Hydrogen Ion Concentration.— TYie. pH of the so- 

 lutions was determined before and after hydrolysis. In the case 

 of the buffered solutions no significant change was noted. The 

 determination was made by the e.m.f. method (except in the so- 

 lutions containing HgClo) using the rocking electrodes as described 

 by Clark^ in a constant temperature bath. A saturated KCl calomel 

 electrode was used. The e.m.f. of this electrode was determined 

 before and after every series of determinations against a 1.0 n 

 solution of hydrochloric acid, prepared according to Hulett^ and 

 checked by conductivity measurements. The pH of this 1.0 n 

 HCl was calculated from the activity coefficient as given by Noyes 

 and Maclnnes;^ i. e., 0.082 at 25°C. This standard was chosen since 

 Fales and Vosburgh^^ have shown that there is no diffusion potential 

 in such a cell and since the strength of the acid can be checked by an 

 independent method; i.e., the conductivity. The value for the sat- 

 urated KCl electrode found in this way averaged 244 millivolts. 

 It varied about 1 millivolt on either side of this value, from time 

 to time, but was constant within 1 millivolt during the course of any 

 one series of determinations. 



Influence of Temperature on the pH. — Since the experiments were 

 carried out at 40°C. and most of the pH determinations were made 

 at 25°C. it was necessary to know what effect this change of tempera- 



^ Wintgen, R., and Kruger, K., Koll. Z., 1921, xxviii, 81. 

 ^ Clark, W. M., Determination of hydrogen ions, Baltimore, 1920. 

 8 Hulett, G. A., and Bonner, W. D., /. Am. Chem. Soc, 1909, xxxi, 390. 

 ^ Noyes, A. A., and Maclnnes, P. A., /. ylm. Chem. Soc, 1920, xlii, 239. 

 10 Fales, H. A., and Vosburgh, W. C, /. Am. Chem. Soc, 1918, xl, 1291. 



