ELECTROCHEMICAL THERMODYNAMICS 731 



(2) The Activity Coefficients of Weak Electrolytes in Salt Solutions 



(a) The Ionic Activity Coefficient of Water in Salt Solutions. 

 We have described a cell by means of which the activity coeffi- 

 cient of hydrochloric acid may be obtained in a chloride solution. 

 Suppose we maintain (mi + ^22) constant and measure 7 in the 

 solutions of varying acid and salt concentration. It is found 

 that 7 varies with the acid concentration according to the law* 



log 7 = ami + log 70. (48) 



Thus at constant total molality 7 extrapolates to 70 at zero con- 

 centration of acid, whence we know 7hTci in the salt solution 

 which is free from acid. In a similar manner from measure- 

 ments of the cells containing sodium hydroxide in the sodium 



chloride solutions, we may obtain ^^ ^^ in the hydroxide-free 



salt solution. Also, from measurements of the cells containing 

 sodium chloride, we know 7Na7ci ^-t the concentration (wi + nh). 



Therefore, if we multiply 7h7ci by '^^^^^^ and divide by 



TNa7ci> we obtain the ionic activity coefficient product of water, 



ThToh^ at this concentration of salt. Obviously, by this method, 



may be obtained at other salt concentrations. 



flHiO 



7hToh 



ajiiO 

 The primary dissociation of water is represented by 



H2O ;=± H+ -f OH- 



and the thermodynamic dissociation constant, K, is given 

 exactly by 



^ ^ OhOoh ^ 7H70H ^^^^^ (49) 



OHjO CLRiO 



Since we may determine in the salt solutions, the classical 



CtHiO 



• Earned, /. Am. Chem. Soc, 48, 326 (1926). Guntelberg, Z. physik. 

 Chem., 123, 199 (1926). 



