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

The acid dissociation constants (Ka) 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 pKa for each phenanthrolium ion is fitted by an equation of the form pKa=(A/T)–B+CT. The changes in enthalpy, entropy and free energy at each experimental temperature have been calculated from the temperature dependence of pKa.