An SO_x (x = 2, 3) sensor using β-alumina/Na₂SO₄ couple

Abstract

A solid-state sensor for SO_x (x = 2, 3) species has been designed using β-alumina as the solid electrolyte and Na₂SO₄ as the auxiliary electrode. The measured e.m.f. of the cell Pt, O'₂ + SO'₂ + SO'₃|Na₂SO₄||β-alumina||Na₂SO₄|SO''₃ + SO''₂ + O''₂, PT in the temperature range 700 K to 1150 K agrees well with values calculated using the Nernst equation. The sodium sulphate acts both as a protective covering, preventing direct access of the gaseous SOx species to the β-alumina electrolyte, and as an auxiliary electrode, converting chemical potentials of SO_x species and O₂ into an equivalent sodium potential. The open-circuit e.m.f. varies non-linearly with temperature for fixed composition of inlet gas mixtures containing SO₂, O₂ and Ar. The response time (t_0.99) of the cell varies between 1.9 ks at 750 K and 0.06 ks at 1100 K. The e.m.f. response is faster when the partial pressure of SO_x at the electrode is increased than when it is decreased.