Measurement of Trace Levels of Atmospheric Sulfur Dioxide with a Gold Film Sensor

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Abstract
The mercury translation principle, wherein S(IV) reacts with Hg(l) to produce Hg(ll) and Hg(O), was utilized to measure trace levels of SO2 with a conductometric gold film sensor. One approach involved impregnated glass fiber filters containing mercurous salts. The second and more successful approach utilized a porous membrane reactor where the reaction occurs in the interstitial liquid-gas interface as the sample gas flows inside and a dilute solution of Hg2(NO3)2 flows outside the porous PTFE membrane tube. The first approach attained a LOD of 200 pptv SO2(g), however, batch to batch reproducibility of the filters was poor and the filters could not be stored more than two days. The membrane reactor method provided better translation efficiency and highly reproducible results, with an LOD of 50 pptv SO2(g). An intercomparlson of this method with an independent method relying on completely different chemistry showed good agreement for the measurement of subppbv levels of ambient SO2. Both mercury translation methods were essentially unaffected by H2S or CH3SH.