Automatic determination of sulphur dioxide by a coulometric method: interferences and reliability of measurements

Abstract

The selectivity of filters used in coulometric sulphur dioxide analysers was evaluated for various volatile organic substances that can be present in the atmosphere of an industrial area with different chemical plants. The poor selectivity of chemical filters, mainly with respect of vinyl acetate and styrene, was ascertained by several tests at various concentrations of the interfering substance. It is possible that the interferences arise from reactions with the bromine present in the analysis cell. The magnitude of the interference appeared to be related to the kinetics of addition of the bromine to unsaturated organic interferents. Pre-heating the air sample to 800 °C resulted in complete climination of the vinyl acetate interference; less promising results were obtained with styrene. Practical modifications of air pre-treatment on the analysers were consequently adopted.