THEORIES RELATING TO ELECTROLYTIC SOLUTIONS 331 



of the experimental methods, by means of which the data were secured. 

 The practical application of thermodynamic principles to electrolytic 

 solutions is largely due to G. N. Lewis. 12 In recent years numerous 

 other writers have occupied themselves with this subject. 13 The writers 

 on this subject have commonly employed the activity function of Lewis, 1 * 

 which is defined by the equation: 



where a is the activity and io is a function independent of the concentra- 

 tion of the constituent in question. The ratio of the activity of a sub- 

 stance to its concentration is termed its activity coefficient and is thus 

 defined by tfie equation: 



(110) a=. 



In a solution of an electrolyte we have an equilibrium of the type: 

 n + AS + n-A z ~ = A', 



where A' represents a molecule of substance which dissociates into n + 

 positively charged ions A^ and ri~ negatively charged ions A 2 ~. The 

 number of charges on the ions is not indicated. Introducing the values 

 of M from Equation 109 in Equation 104 we may at once derive the 

 expression: 



(111) log - =K, - 



a u 



where a + , cr, and a u denote the activities of the positive and negative 



ions and the un-ionized molecules, respectively, and K is a function inde- 

 pendent of concentration. For the change in the potential of the electro- 

 lyte between any two concentrations of the system, we have the equa- 

 tions: 



(112) (2n'M') 6 (2n'M') a = RT log ^, 



u a 



(113) (2nM) 6 (2nM) a = RT log 



"Lewis, J. Am. Chem. Soc. S}, 1631 (1912). 



"Bronsted, J. Am. Chem. Soc. 42, 761 (1920) ; Bjerrum, Ztschr. f. Elektrochemie 24, 

 321 (1918) ; Ztschr. f. Anorg. Chem. 169, 275 (1920); Harned, J. Am. Chem. Soc. & 



loOo (1920), 



oVHVA?^ roc - Am - Acad ' * 3 > 259 (1907) ; Zt8c * r - / P h V*- C^em. 61, 129 (1907) ; ibid., 

 70, U (1909). 



