Electrochemical determination of the sodium activity in tin melts by means of the solid electrolyte method

Abstract

A sensor with β"-Al2O3 as solid electrolyte was used in the tin melt for the determination of sodium activity. The chemical, microstructural and functional properties of the home-made β"-alumina were determined. The properties of the home-made ceramics do not much differ from those of commercially obtained materials, which have been used for the purpose of comparison. The behaviour of probes with the reference electrodes sodium, tin-sodium alloys and sodium borosilicate glass was investigated within the scope of emf measurements. No longtime-stable measurements could be carried out with the use of the sensors with sodium as well as tin-sodium reference electrodes. The observed drift of the emf signal is explained by a partial electronic conductivity of the solid electrolyte. Nernst behaviour was found while using a probe with sodium reference electrolyte, i.e. the emf shows a linear dependence on the logarithm of the sodium content in the melt. A proportionally stable behaviour of the emf signal was indicated by measurements with a sodium glass reference system. With the alteration of the sodium concentration the probe reacted spontaneously and reversibly. Besides, an exact Nernst behaviour is indicated. Applying several probes of the same type a displacement of Nernst straight lines is observed, which diverge up to about 400 mV. The family of the Nernst straight lines at multiple use of a single probe moves within a relatively narrow range of only about 25 mV. This permits comparatively precise measurements (error is dependent on the position in the concentration decade). Each probe of the type developed here has to be tested and calibrated by repeated application. A probe delivery with individual characteristic is proposed. The determination of the sodium contents in melts of the basic material and of tin from the float glass process yielded in both cases about 2 wt ppm. These values show that the sodium contents of the tin melt do not differ before and after use.