Measurement of Lewis Affinity

A number of groups (Drago [76, 77], Gutmann [78, 79], Maria and Gal [80], Arnett et al. [81]) have proposed measuring Lewis (Bronsted) basicity from the negative enthalpies of the complexation (protonation) reactions (1.50). In order to follow the lUPAC definition of 

The two most commonly employed techniques for obtaining complexation enthalpies are based on the temperature dependence of equilibrium constants or calorimetric procedures. In the latter, the heat evolved when the acid and base are mixed in the reaction cell of a calorimeter is measured. The molar enthalpy of complexation, AH°, is related to the measured heat output, Q, corrected for the heats of dilution, the equilibrium concentration of the complex, [AB], and the volume of the solution in litres, V, by the relation 

If an undetermined number of moles of AB have been formed, and if the equilibrium constant is known from a previous smdy (for example, by a spectrometric method), [AB] can be calculated by Equation 1.81 obtained by resolving Equation 1.82  

The alternative procedure consists in measuring the equilibrium constant at several temperatures. The temperature dependence of the pressure equilibrium constant for an ideal-gas reaction is given by the van t Hoff Equation 1.83  

Since d(T ) = —T dT, Equation 1.83 can be expressed in a form convenient for plotting  

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