where u+ and u-are the mobility of the cation and anion, respectively; c+ and c-are the concentration of cation and anion, respectively; and z+ and z-are the charges of cation and anion, respectively. Since the concentration of the cation and anion is the same in the solution, then c+=c-=c+/-and z+=z-=z, therefore eq. ( ) could be expressed as follows: