Conductivity and the Formation of Triple Ions

In solvents of low permittivity (er 10), most ions are assodated as in Eq. (7.3), even at dilute electrolyte concentrations. Moreover, with increasing electrolyte concentrations, triple ions are formed, as in Eq. (7.8), and sometimes even quadru-poles (or dimers), as in Eq. (7.9)  

In conductimetric studies of triple ion formation, it is often assumed that M2A+ and MA2 have the same formation constants (Kt= Kt+= Kt ) and the same molar conductivities at infinite dilutions (2 = = 2 ). If we denote by a and a, the 

As mentioned in Section 2.6, triple ion formation is not limited to low permittivity solvents. It also occurs in high permittivity solvents, if they are of very weak acidity and basicity for example, Kt for the formation of Li2Cl and LiCl2 in AN has been determined by polarography to be 105 M 2 [19]. Li+ and Cl- in AN are only weakly solvated and tend to be stabilized by forming triple ions. For conductimetric studies of triple ion formation in dipolar protophobic aprotic solvents, see Ref. [20]. 

The problem of triple ion formation has been studied in detail, because it is related to lithium battery technologies [18]. In some cases, however, the occurrence of the minimum in the log A-log c curve, as observed in Fig. 7.2, is not attributed to triple-ion formation but is explained by ion-pair formation only. The increase in log A at high electrolyte concentrations is attributed either to the increase in the distance of closest approach of ions, the increase in the solution permittivity, or the decrease in the activity coefficient of the ion-pairs. Although there is still some controversy, it seems certain that triple ions are actually formed in many cases. 

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