SOLID-STATE NA-BETA-ALUMINA POTENTIOMETRIC SENSOR FOR MEASURING GASEOUS ARSENIC OXIDES

Na-beta-alumina solid electrolyte in the form of a closed-end tube was employed as a potentiometric sensor for measuring gaseous arsenic oxides. The behaviour of the sensor was tested by measuring the EMF of two cells: Pt, air \Na+-beta-alumina\P(O2), P(AsH3), Pt (cell I) as a function of P(AsH3) and P(O2); and Pt, air, P(AsH3) \Na+-beta-alumina\P(As4O6), air, Pt (cell II) as a function of P(AsH3) when P(As4O6) = constant, and as a function of P(As4O6) when P(AsH3) = constant. In both cells the measurements were made at 1025 and 1075 K. A linear response to P(AsH3) and to P(As4O6) was observed over the concentration range 5 to 2000 ppm AsH3 and 1 to 180 ppm As4O6. The cells were found to be more sensitive to arsine than to As4O6. The oxidation of As4O6 to pentoxide required an efficient catalyst. The EMF dependence on arsenic concentration for the two cells, as well as the EMF dependence on P(O2) of the cell (I), suggest that at 1025 K the formation of pyroarsenate is the reaction which defines the electrode potential. At 1075 K the results indicate that the controlling reaction is the formation of orthoarsenate.