Galvanic Detection of Sulfur Dioxide in Ambient Air at Trace Levels by Anodic Oxidation

Abstract

A continuous method for the measurement of SO₂ in ambient air at trace levels is described. The principle of detection is based on the anodic oxidation of SO₂ in a galvanic cell. A differential measuring technique with a cell with two anodes and one cathode is used; background and noise current are low and stable. A feature of the measuring system is that the air sample is passed through a porous anode against a head of electrolyte on the other side of the anode. The minimum values for the time constant and the lower detection limit were 3 seconds and about 3µg/m³, respectively. The selectivity of the method was compared with those of other techniques in field testing. Trace concentrations measured by the galvanic method in the range from 12 to 135 µg/m³ were in good agreement with those of the flame photometric and the West-Gaeke method. With a Pt/Ni₂B/graphite anode the mean oxidation efficiency was 94 ± 2% on the basis of 2 faraday/mol of SO₂ within the tested concentration range of 50 to 560 µg/m³. During a field test no decrease in the anode activity was observed over a period of ten months. Examples are given for the galvanic detection of other compounds using porous measuring electrodes.