Amperometric monitoring of sulphur dioxide in liquid and air samples of low conductivity by electrodes supported on ion-exchange membranes

Abstract

An amperometric sensor is described for the determination of sulphur dioxide in both gaseous atmospheres and solutions of low conductivity. It consists of a porous Pt electrode (facing the sample) supported on one face of an ion-exchange membrane (Nafion 417) which serves as a solid polymer electrolyte. The other side of the membrane faces an internal electrolyte solution (1 mol dm–3 aqueous perchloric acid) containing the counter and reference electrodes. This sensor is inserted into a flow cell in which gaseous or electrolyte-free aqueous samples are fed by a peristaltic pump placed in a closed-loop path and SO2 is oxidized at an applied potential of 0.65 V versus Ag–AgCl. The device is found to be characterized by a high current sensitivity and a short response time, 24 A cm–2 mol–1 dm3 and 1 s respectively for gaseous samples; (0.4 A cm–2 mol–1 dm–3 and 4 s, respectively, for water solutions), and by good stability and low background noise. The dynamic range extends up to 2 × 10–4 mol dm–3(gaseous samples) and 1 × 10–3 mol dm–3(water samples) with good linearity, and detection limits of 8 × 10–9 mol dm–3(gaseous samples) and 4 × 10–7 mol dm–3(water samples) are predicted for a signal-to-noise ratio of 3. The advantages offered by this type of sensor over conventional gas-permeation membrane electrodes are discussed.