146 



ATOMS, IONS, SALTS, AND SURFACES 



part in the newer work on the ionization of salts in solution as that of the degree of 

 ionization in the older theory. Thus the degree of ionization of sodium chloride in its 

 0.1m aqueous solution was given as 0.86, while the activity coefhcient as given above 

 is 0.798. The activity coefficient is sometimes designated as the "thermodynamic de- 

 gree of dissociation." 



Table I gives the activity coefficients for sodium and potassium chlorides and a 

 few salts of higher types. It may be noted that as the product of the valence of the 

 ions of the salt increases the activity coefficient decreases. 



TABLE I 



Activity Coefficients of Salts in Aqueous Solution .4t 25° C, as 

 Calculated from the Lowering of the Freezing-Point 



THE IONIC STRENGTH 



The activity of ions in a mixture is determined by the concentrations and elec- 

 trical charges of all of the ions present. Lewis and Randall found that if the molality 

 of each ion is multiplied by the square of its valence, the sum of these quantities (di- 

 vided by 2, since both positive and negative ions are included) is an important quan- 

 tity, designated as the ionic strength (fx). 



It is found that in dilute solutions the activity coefficient of a given electrolyte is the 

 same in all solutions of the same ionic strength. 



The activity coefficient (a) in water of a salt composed of two ions is given by an 

 extremely simple equation: 



— logia a = o.5oZi ZjV /i , 

 in which Zi and Z, are the valences of the ions of the salt, and 



(i) 



(2) 



calculation of the activity coefficient from solubility 

 If a solid .1 is in contact witli its saturated solution, the relation may be expressed: 



.1 (solid) l,-'l(ilissolve<l) , 



(3) 



