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.