726 HARMED 



ART. M 



where anaci is the activity product of the ions as defined by 



Lewis,* and 7, or -^, is the activity coefficient. Hydrochloric 



m 



acid is a uni-univalent electrolyte and, consequently, the reaction 

 of this cell represents the transfer of one gram ion of hydrogen 

 ion and one gram ion of chloride ion. The modifications 

 necessary for the general treatment of electrolytes of different 

 valence types can easily be made. Consider any strong electro- 

 lyte at a molal concentration, m, which dissociates according 

 to the scheme 



C,+Ay_ = v+C + v-A, 

 and let 



a2 = a+''+ aJ'-, 



where a+ and a_ are the activities of the cation and anion, 

 respectively, and az, defined by the above equation, may be 

 regarded as the activity of the electrolyte, and 



a± = (a+''+ aJ'-)'. 



Then equation (41) may be written in general 



n = Rt log a2 + I = vRt log a± + J, (42) 



which serves to define the activity. 7 is a function of the pressure 

 and temperature, but not of the concentrations of the solute 

 epecies. Further, we define the activity coefficient of any elec- 

 trolyte by 



'^ = 7~7^ Zv. ' (43) 



and always measure it in reference to a value of unity when m 

 equals zero. 



By means of cell measurements we obtain y. in reference to an 



* Lewis, Troc. Am. Acad., 37, 45 (1901); 43, 259 (1907). 



