An optical redox chemical sensor based on ferroin immobilised in a Nafion® membrane Original Research Article

The redox indicator, Ferroin, tris(1,10-phenanthroline) iron(II) was successfully incorporated into the perfluorosulfonated ionomer membrane Nafion®. The redox properties of the optode membrane were studied under batch and flow conditions using acidified bromate or cerium(IV) solutions as oxidants and ascorbic acid or sodium ascorbate solutions as reductants. The experiments under flow conditions were performed in a computer-controlled sequential flow injection system incorporating a flow-through measuring cell where the optode membrane was mounted. Optical fibre technology was used to monitor the membrane absorbance with time. Some loss of sensitivity was observed after a number of redox cycles of the optode membrane. This was attributed to the dissociation of the Ferroin in the presence of H+ ions that entered the optode membrane from the acidified oxidising solutions and to the formation of a binuclear Fe(III)/1,10-phenanthroline complex when Ferroin was oxidised. Both undesirable effects were suppressed by the addition of excess 1,10-phenanthroline to the membrane composition.