Progress in the Development of Solid-State Sulfate Detectors for Sulfur Oxides

The purpose of this article is to describe and analyze the performance of four different reference electrodes adapted for use in sol id-state sulfate de-tectors for sulfur oxides: an electrode in which a cal ibrated gas circulates, an Ag/Ag + electrode, an oxygen electrode using air as a reference and, finally, a thermal-decomposit ion type of electrode using an MSO4-MO couple. The article also contains a description of a quartz detector for hot-gas analysis of SO~-SOs mixtures and shows how the fact of replacing the p lat inum at the electrode by a less catalytic metal such as gold can lead to the specific detec-tion of SOs in the presence of SO2. Previous work at IREQ has a lready shown that gaseous oxides, the major pol lutants result ing from fossil-fuel combustion, can be measured potentiometr i-cally using electrolytes containing oxyanions (1-3). Efforts so far have focused on the detection of the sul-fur oxides SO2 and SO8. The reaction taking place at the tr iple-contact electrode SOs (g), 02 (g), Pt/SO4 = has been shown to correspond to the half-cel l SO8-5 1/2 02-4- 2e- ~ SO4 = [1] Furthermore, with platinum, it has been observed that equi l ibr ium is achieved rapid ly between SO2, 02, and SOs in the vicinity of the electrode. The emf of the total sulfur oxide detector is given by the following equatio