Monitoring of electrochemically inactive compounds by amperometric gas sensors

Abstract

Electrochemical sensors have many useful applications in air and toxic gas monitoring, but they are extremely limited in the gaseous species that they can detect. To extend the applicability of amperometric gas-sensing instruments to electrochemically inactive compounds, gas samples were exposed to a heated platinum or gold filament before being introduced into the sample chambers of several different amperometric sensors. The sensors were of the three-electrode type, having platinum-black reference and counter electrodes and sensing electrodes made of platinum black, powdered gold or vapor-deposited platinum or gold on porous tetrafluoroethylene membranes. All three electrodes were in contact with a 25 – 30 wy.% sulfuric acid solution. The responses of four different sensors to various compounds at 20°C were measured at sensing electrode potentials ranging from 0.9 V to 1.4 V versus RHE (reversible hydrogen electrode in the same electrolyte), with and without a heated platinum filament at the sample inlet. Of 10 compounds tested, only two elicited significant responses without the filament. With the filament heated to about 700 °C, each of the tested compounds elicited a significant response under certain conditions. Moreover, the particular sensors and electrode potentials corresponding to the strongest responses were different for each compound. Qualitatively similar, but quantitatively more pronounced, responses were obtained with the same filament heated to 800 °C or 1050 ± 50 °C, or with a gold filament heated to 950 ± 50 °C. The responses were proportional to concentration in the 0 – 50 ppm range, and usually proportional in the 0 – 200 ppm range.