Oxidative amperometric flow injection determination of sulphite at an electrochemically pre-treated glassy carbon electrode

A flow injection analysis method for determining sulphite amperometrically by oxidation at an electrochemically pre-treated glassy carbon electrode has been developed. The determination is made in 0.05 M sodium carbonate eluent and the glassy carbon electrode is held at +1.0 V versus S.C.E. The glassy carbon electrode is pre-treated on-line in the same eluent before use by holding it at + 1.75 V for 5 min and then at –1.00 V for 1 min. Calibration graphs were rectilinear from 0.1 to 50 µg ml–1 of anhydrous sodium sulphite. A much smaller and much less reproducible signal was obtained when the electrode was not pre-treated. EDTA, which can be used to stabilise sulphite solutions from oxidation by molecular oxygen, interferes by giving a signal about one tenth the size of that of sulphite for an equimolar solution. The sulphite oxidation reaction on the unpre-treated electrode, but not that on the pre-treated electrode, was catalysed by chloride ion.