Use of the Nasicon/Na₂SO₄ couple in a solid state sensor for SO_x (x=2,3)

Abstract

The e.m.f. of a concentration cell for SO _x (x=2,3)-O₂ incorporating Nasicon as the main solid electrolyte has been measured in the temperature range 720 to 1080 K. The cell arrangement can be represented as, PT, O'₂ + SO'₂ + SO'₃|Na₂SO₄||Nasicon|| Na₂SO₄|SO"₃ + SO"₂ + O"₂, Pt The Na₂SO₄ acts both as an auxiliary electrode, converting chemical potentials of SO_x and O₂ to equivalent sodium potentials, and as an electrolyte. The presence of Na₂SO₄ provides partial protection of Nasicon from chemical reaction with gas mixtures containing SO_x. The open circuit e.m.f. of the cell is in close agreement with values given by the Nernst equation. For certain fixed inlet gas compositions of SO₂+O₂, the e.m.f. varies non-linearly with temperature. The intrinsic response time of the cell to step changes in gas composition is estimated to vary from ~2.0 ksec at 723K to ~ 0.2 ksec at 1077K. The cell functions well for large differences in partial pressures of SO₃(p"SO ₃/p'SO 3≈ 10⁴) at the electrodes.