Reference electrode of simple galvanic cells for developing sodium sensors for use in molten aluminum
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1.
INTRODUCTION
T H E simplest galvanic cell for measuring sodium activity is Na (ref) I] Na ยง conductor II Na (in sample). Two important elements in such a cell are the sodium-ion-conducting electrolyte and the sodium reference electrode. For cells to be used at elevated temperatures, the reference electrode is perhaps a more crucial factor, because sodium/3alumina and Nasicons have been intensively studied as the solid electrolytes. Therefore, this article focuses on the sodium reference electrode. Following a review of the possible sodium references, the laboratory evaluation of selected references and semi-industrial trials on as-prepared sensors in molten aluminum are reported.
II.
BACKGROUND
When selecting a sodium reference electrode, in addition to its reversibility and chemical stability, rapid electrode/electrolyte equilibrium, especially for sensors of short service life, would be advantageous. This leads to the preference of liquidt~J and semiconducting t2] electrodes. Though less essential, it also is preferred to have a sodium activity of the reference electrode close to that of the working electrode, so as to avoid electrolyte polarization. In the laboratory, stringent requirements can be met with complicated experimental setup. A long cell response time is not regarded as a major drawback, and the cost is not a prime concern. Such cells can be very accurate due to complete control over the experimental conditions. One example is the use of (Na2CO 3 + C O 2 + 02); a N a z C O 3 powder is densely packed in a /3-alumina close-end tube and the reference compartment is supplied with a (CO2 + 02) gas mixture of a fixed ratio. [31 Later, Kale and Jacob I4] adapted this arrangement in measuring the partial free energy of
L. ZHANG, Postdoctoral Fellow, formerly with the Department of Mining and Mineral Engineering, University of Leeds, is with the Department of Civil Engineering, University of Sherbrooke, Sherbrooke, PQ, Canada J1K 2R1. D.J. FRAY, formerly Professor of Mineral Engineering, Department of Mining and Mineral Engineering, University of Leeds, Leeds, United Kingdom LS2 9JT, is Professor of Materials Chemistry, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB2 3QZ, United Kingdom. J.C. DEKEYSER and F. DE SCHUTTER, Project Leaders, are with the Viaamse lnstelling Voor Technologische Onderzoek, B-2400 Mol, Belgium. Manuscript submitted May 9, 1995. 794--VOLUME 27B, OCTOBER 1996
sodium oxide in Nasicon solutions by passing an equimolar mixture of (CO2 + 02) over a sintered Na2CO3 pellet. Apparently, the sodium activity is dedicated by 2Na + CO2 + ( 1 / 2 ) O 2 = Na2CO 3.
In industry, sophisticated electrochemical sensors should be simple, rugged, reliable, cost-effective, and rapid in response. Therefore, a cell suitable for thermodynamic studies may not be ideal for industrial applications.
A. Potential Sodium Reference Electrodes 1. Pure sodium Pure liquid sodium has a constant sodium activity (aNa = 1) over a wide temperature range. However, it rarely
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