Study of the interconversion of polyaniline oxidation states by optical absorption spectroscopy

The polyaniline (PANI) bases can be classified in three distinct oxidation states: leucoemeraldine (LB), the totally reduced form, emeraldine (EB), the half oxidized form, and pernigraniline (PB), the totally oxidized form. The oxidation state varies continuosly from the leucoemeraldine to the pernigraniline base with addition of oxidant agent, and vice-versa with addition of reductor. In this work, we studied the polyaniline oxidation state interconversion using ultraviolet/visible (UV–vis) optical absorption spectroscopy. We utilized PANI and a derived form of it, poly( o-methoxyaniline) (POMA). For oxidation, we used ammonium peroxydisulfate and, for reduction, ascorbic acid, both in solutions of N-methyl-2-pyrrolidinone (NMP). We observed isosbestic points in the spectra when a base converted to another, and this indicates that one base converts directly to another in one step at molecular level, without passing through any intermediate forms. Using UV–vis optical absorption curves, we obtained a method simpler than others cited in literature to estimate the oxidation states. The method consisted in obtaining calibration curves which give the unknown oxidation states of polymeric samples (PANI and POMA).