Solvate ions limiting conductances

As mentioned in Section 7.1, if we determine the molar conductivity of an electrolyte as a function of its concentration and analyze the data, we can get the value of limiting molar conductivity A°° and quantitative information about ion association and triple-ion formation. If we determine the limiting molar conductivity of an ion (7 °) by one of the methods described in Section 7.2, we can determine the radius of the solvated ion and calculate the solvation number. It is also possible to judge the applicability of Walden s rule to the ion under study. These are the most basic applications of conductimetry in non-aqueous systems and many studies have been carried out on these problems [1-7]. 

These several assumptions do not lead to the same conclusions. For example, transfer activity coefficients obtained by the tetraphenylarsonium tetraphenyl borate assumption differ in water and polar aprotic solvents by up to 3 log units from those based on the ferrocene assumption. From data compiled by Kratochvil and Yeager on limiting ionic conductivities in many organic solvents, it is clear that no reference salt can serve for a valid comparison of all solvents. For example, the tetraphenylarsonium and tetraphenyl borate ions have limiting conductivities of 55.8 and 58.3 in acetonitrile. Krishnan and Friedman concluded that the solvation enthalpy of... 

Comparisons of the Limiting Conductances (cm (int. moL ) of Hydrogen and Solvate Ions and... 

The story of diffusion of small ions in water is, however, still not complete and the picture given above is over-simplified. For example, theory would predict a rather similar size dependence of conductivity for alkali cations and halide anions. In reality, however, the limiting conductivities of positive (alkali) and negative (halide) ions lie on different curves when plotted against the inverse radius (or radius). The peak appears at a larger radius for the anions. In order to explain this result one needs to consider an accurate interaction potential that differentiates between a cation and an anion of equal size. Such a study was carried out by Rasaiah and Lynden-Bell [18], who indeed found that solvation structures around cations and anions are markedly different for ions of the same size (such as and Cr ions). 

Eor a given solvent, the limiting value of the single ion conductivity, 2 , is independent of the counter-ion present in the solution and characterizes the solvated ion i. Therefore the limiting equivalence conductance of an electrolyte, A°°, can easily be calculated from the ion conductivities, e.g., for a binary electrolyte... 

With the possible exception of (n-CsHrjiNI, there is no evidence in these data for the maintenance of a solvent layer between counter ions in the pair at contact distance. The limiting conductances of the alkali- and tetraalkylammonium bromides provide unequivocal evidence of substantial hydrodynamic transport of solvent by Li+ (39), however. Apparently the solvated free lithium ion is larger than... 

Studies on the solution structure of A3-iodanes are relatively limited. In polar solvents, cryoscopic and conductance measurements have shown extensive dissociation of diaryl-A3-iodanes (Ar2IL L = BF4, Cl, Br, OAc) into the solvated iodo-nium ions (Ar2I+S S = polar solvents such as H20, MeOH, and DMSO) [3,210]. Even in dichloromethane, bis(4-methylphenyl)-A3-iodane (Ar2IBF4 Ar=p-MeC6H4) dissociates into the solvated iodonium ions with dissociation constant Kdissoc=4.7xlO-6M[211]. 

Because of its dipole moment, the ammonia molecule interacts with these ions to form solvates in a manner analogous to the water molecule in aqueous solutions. Solutions in liquid ammonia show significant electrical conductance. Pure ammonia, like water, itself has a conductivity that, although limited, is based on dissociation according to ... 

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