Raman based pH measurements with polyaniline

The reversible acid–base equilibrium between the emeraldine salt (ES) and the emeraldine base (EB) form of thin electropolymerized films of polyaniline (PANI) has been studied between pH 2 and 9 with in situ Raman spectroscopy using the 514, 633 and 780 nm laser excitation wavelengths. The electropolymerization of PANI was done with cyclic voltammetry in a flow cell from a solution consisting of 50 mM aniline and 1.0 M HCl. It is shown that the Raman signal of the CN stretching vibration at 1439 cm⁻¹, originating from the quinoid units of the EB form, can be used for pH measurements between pH 3 and 6 with the 633 nm laser. This vibration is strongly resonance enhanced by the 633 nm laser and its intensity grows therefore considerably in the pH interval where the ES–EB transition mainly takes place (pH 3–6). The influence of the film conditioning pH was also studied as well as the reproducibility of the Raman spectra when pH was changed several times from pH 6 to 3.