JACQUES LOEB 687 



gives the observed p.d. in millivolts. The agreement between 

 observed and calculated p.d. is sufficiently close. 



VI. Hydrogen Ion and Chlorine Ion Potentials. 



The equation for the equilibrium condition between gelatin chloride 

 solution and water is as stated above, 



x^ = y {y + z) 



where x is the concentration of H and CI ions in the outside solution, 

 y the concentration of the H and CI ions of the free HCl inside the 

 gelatin chloride solution, and z the concentration of the CI ions held 

 by the gelatin. 



If we write this equation in the form 



y X 



X y + z 



- is the ratio of hydrogen ion concentration inside over the hydrogen 



ion concentration outside; and is the ratio of the concentration 



y + z 



of the chlorine solution outside over the chlorine solution inside. 

 Since 



y 



log — = pH inside minus pH outside 



X 



and 



X 



log = pCl outside minus pCl inside 



y + z 



it follows that 



pH inside minus pH outside = pCl outside minus pCl inside 



It was easy to put this consequence of Donnan's theory to a test 

 and some of the experiments described in the preceding chapter were 

 selected for this purpose. Inside the collodion bag was a 1 per cent 

 solution of gelatin chloride of different pH, outside water. After 

 18 hours equilibrium was established between inside and outside 

 solutions and the pH as well as the pCl were ascertained. The pCl 



