Polymer-based cation-selective electrodes modified with naphthalenesulphonates

Abstract

Polymer-based cation-selective electrodes based on cation exchangers incorporated in electrochemically polymerized pyrrole films deposited on solid electrodes are described. Pyrrole was polymerized anodically on Pt or pyrolytic graphite electrodes from CH₃CN—H₂O mixtures of various proportions together with naphthalenesulphonate compounds as dopants in the polymerized film. The pH was adjusted to 1–2 with HClO₄ and polymerization was accomplished by galvanostatic methods. Hence electrodes selective to monovalent cations were prepared. The composition of the CH₃CN—H₂O solvent was found to be an important factor governing the e.m.f. response of the electrodes. X-ray photoelectron spectroscopic analysis revealed that the amount of naphthalenesulphonate relative to polypyrrole varied with the composition of the CH₃CN—H₂O solvent used for the electrochemical polymerization. A sulphur to nitrogen ratio >0.20 was required for a Nernstian response to cationic activities. The monovalent/divalent selectivity was between –2.8 and –3.4, and was most pronounced for the naphthalenetrisulphonate compound. Although there were small differences in the selectivities of the electrodes towards monovalent cations, it might be possible to obtain specific ion selectivities by modification of the polymer films. The possibility of constructing miniaturized cation-selective electrodes by means of an electrochemical procedure was also demonstrated.