COATED GRAPHITE-EPOXY ION-SELECTIVE ELECTRODE FOR THE DETERMINATION OF IRON(III) IN OXALIC MEDIUM

A coated graphite-epoxy iron(III) ion-selective electrode, based on th e ion-pair between [Fe(oxalate)(3)](3-) anion and tricaprylylmethylamm onium cation (Aliquat 336) in a poly(vinylchloride) (PVC) matrix is co nstructed. A thin membrane film of this ion-pair, dibutylphthalate (DB Ph) in PVC was deposited directly onto a Perspex(R) tube containing a graphite-epoxy conductor substrate attached to the end of a glass tube . The effect of membrane composition (ion pair, DBPh and PVC), oxalate concentration, pH and some cations and anions upon the electrode resp onse is investigated. The electrode shows a linear anionic response to E vs. log[Fe3+] in the iron(III) concentration range from 2.9 x 10(-6 ) to 10(-2) mol/L, and a slope of -18.7 +/- 0.5 mV/dec, at pH working range 2-8 and 0.3 mol/L oxalate concentration. Variation in the potent ial of about +/- 2 mV was observed during a working day of 7-8 h. The response time was less than 5 sec and the lifetime of this electrode w as superior to one year (over 1500 determinations by each polymeric me mbrane), with a practical detection limit of 2.1 x 10(-6) mol/L. Appli cation of this electrode for iron(III) determination in biotonico samp le (Brazilian tonic formula) is described.