ELECTROCHEMICAL THERMODYNAMICS 727 



arbitrary standard state, and, therefore, a^ may also be 

 obtained. Now 7 may be computed if we let m be the molal 

 concentration of the electrolyte. This is purely arbitrary since 

 the molal concentration of the electrolyte tells us nothing 

 regarding the real concentrations of the ions in the solution. 

 The activity coefficient 7, however, acquires an important 

 physical significance if the real ionic concentrations are known. 



According to the classical theory of Arrhenius, 7 was thought 

 to measure the actual degree of dissociation of an electrolyte. 

 Later, it was called by Lewis "the thermodynamic degree of 

 dissociation". If this quantity measures the degree of disso- 

 ciation, then the law of mass action in its classic form should 

 be applicable to all classes of electrolytes. In the case of strong 

 electrolytes, this conclusion was found to be erroneous, and 

 therefore the first suppositions regarding 7 were entirely 

 incorrect. The difficulty resides in the failure of these early 

 theories to take into account the effects of the attractive and 

 repulsive forces between the ions, which for charged particles 

 vary inversely as the square of the distance. The careful con- 

 sideration of these effects constitutes the departure of the recent 

 developments of the theory of solutions from the classical 

 theory. 



The most fruitful advance has come from the assumption 

 that, in moderate concentrations in a solvent of high dielectric 

 constant, the strongest electrolytes are completely dissociated 

 into ions. Thus m in the cases of hydrochloric acid solutions, 

 sodium chloride solutions, etc., is the true ionic concentration. 

 If this is true, 7 acquires a definite physical significance. Fur- 

 ther, if the assumption of complete dissociation is correct, then 

 7 must be calculable from fundamental considerations regarding 

 the forces of attraction and repulsion between the ions. 



The various attempts to solve this problem have culminated 

 in the theory of Debye and Hiickel* By the skillful application 

 of Poisson's equation to a system of charged particles in 

 thermal motion, they have succeeded in proving that in moder- 

 ately dilute solutions 7 is a function of the electrostatic forces. 



♦ Debye and Huckel, Physik. Z., 24, 305 (1923). 



