Bilateral triple-ion formation

The experimentally noionic accessible limiting conductivities A — A — A of the triple ion must be estimated with consideration of ion sizes yielding Ai =A0/3 [101,102] or 2A0/3 [103], with preference for the latter value. 

Ion-pair association constants K A determined with the set of conductivity equations (7)—(15) agree with those obtained from Eq. (18) and (19) [100]. Salomon and Uchiyama have shown that it is also possible to extend the directly Fuoss-Hsia equation to include triple-ion formation [104], 

Conductivity curves (A versus c ) of salts in solvents of low-permittivity commonly show a weakly temperature-dependent minimum around 0.02 molL-1 followed by a strongly temperature-dependent maximum at about 1 mol L 1. According to Fuoss and Kraus [101,102] the increase of conductivity behind the minimum is due to the formation of new charge carriers from the ion pairs. They assume that coulombic forces suffice to form bilateral cationic [C+A-C+] and anionic [A C+A ] triple ions in solvents of low-permittivity ( 15) if the ions have approximately equal radii. 

The conductivity functions of such electrolytes can be evaluated at the level of limiting laws with the help of Eq. (18), permitting the determination of the tripleion-constant KT and the ion-pair association constant KA. 

The experimentally nonaccessible limiting conductivity of the triple ion aJ, must 

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In contrast to bilateral triple-ion formation, unilateral triple-ion formation may also occur in solvents of high permittivity, when ion-pair association is increased by noncoulombic specific ion-ion interactions in solvents of low basicity such as PC or AN. Exclusive formation of anionic tripleions [A-C+A-] ", is observed in these solvents when large organic molecular anions A interact with small cations such as Li + or H+. For example, in contrast to lithium acetate in DMSO [97], where ion association is moderate, ion association as well as unilateral triple-ion formation is observed in the solvent PC [105] due to the much lower basicity of this solvent, (see Table 2)... 

Figure 6 shows the dependence on concentration and temperature of the molar conductivity of 1,2-dimethoxyethane solutions of LiBF4 from infinite dilution to saturation. The plots of A versus show a minimum at moderate concentrations and a maximum at high concentrations. Although the minimum is only weakly dependent on temperature, the maximum exhibits a strong displacement. The minimum is a general feature of bilateral triple-ion formation ... 

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