Effect of temperature on the acid dissociation constants of 1,10-phenanthroline and some related bases in water

Abstract

The acid dissociation constants (K_a) of the 1,10-phenanthrolium ion, 5-nitro-1,10-phenanthrolium ion, 2,9-dimethyl-1,10-phenanthrolium ion and 5,6-dimethyl-1,10-phenanthrolium ion in aqueous solution have been determined at various temperatures between 298 and 523 K. The temperature dependence of the pK_a for each phenanthrolium ion is fitted by an equation of the form pK_a=(A/T)–B+CT. The changes in enthalpy, entropy and free energy at each experimental temperature have been calculated from the temperature dependence of pK_a.