Indirect determination of the protonation constants pKa(B/BH+) of substituted pyrazoles and indazoles from electrochemical results on the corresponding pentacyanoferrates complexes

A linear correlation is found between the oxidation potentials of the pentacyano (L) ferrates(II) complexes, where L refers to various substituted pyrazoles and indazoles, and the protonation constants of the heterocyclic base L. This correlation allows the pK a (L/LH + ) of very weak bases in these series be estimated.