Over street and Dean 99 



Phase I contains soil particles and ions such as Ca++, Mg++, Na+, 

 and K+ in the adsorbed state. Also it will contain the aforementioned 

 cations and anions such as CI - , HCO.. - , NO., - , H 2 P0 4 - , and S0 4 ~ 

 in the free or unadsorbed state. Phase II is purely an electrolyte solution 

 which contains no soil particles. 



We may now consider the conditions for equilibrium between Phase 

 I and Phase II. According to a law of thermodynamics, the partial molal 

 free energy (F) of any component is the same in all phases of a system 

 of phases at equilibrium. That is, for example, 



(F KC i)i = (Fkci)ii at equilibrium. (t) 



This thermodynamic law is perfectly general in character and applies 

 for the case of charged ions as well as for the case of uncharged com- 

 ponents. For this reason we can also write 



(Fk + )i = (FkOii 



(Fci-)i = (F cl -)„ 



Since we know that the concentrations of individual ions in our sys- 

 tem may be quite different in the two phases, it should be borne in 

 mind that the partial molal free energy of an ion in such a system may 

 be a function of factors other than concentration such as electric poten- 

 tial or interaction with surfaces.* 



In conformity with the original definition of activities, we can define 

 the activity (a) of an individual ion in our system by means of the 

 equation: 



F - F? = RT In a-, 



where F" represents the partial molal free energy of the ion i in an 

 arbitrarily chosen standard state. Consequently, we may write 



*It should be noted that in this approach the partial molal free energy of an 

 individual ion is identified with the "escaping tendency" of the ion. It is identical 

 with the electrochemical potential, fT, of another treatment and nomenclature (for 

 example, see The Physical Chemistry of Electrolytic Solutions by H. S. Harned 

 and B. B. Owen, New York, Reinhold Publishing Company, 1943, page 315). 



