INVESTIGATION OF A ZIRCONIUM ELECTRODE AS A SENSOR FOR FLUORIDE IONS

The effect of fluoride ions on the electrochemical behaviour of a zirconium metal electrode was investigated. Since zirconium metal is always covered by a layer of oxide, the anodic characteristic of a Zr/ZrO2 electrode depends highly on the electrolyte composition, Only in hydrochloric and perchloric acid media was a direct proportionality between fluoride concentration and anodic current density found. In other electrolytes the fluoride ion-induced dissolution of the zirconium metal leads to an increase in ZrO2 film thickness, and as a consequence, the mass transport of fluoride through the oxide layer is hindered. The mechanism of the zirconium electrode dissolution at low concentrations of fluoride (below 1 X 10(-3) M) seems to be different from that at high concentrations. The dependence of the equilibrium potential on the fluoride concentration is far from the Nernstian one, and a zirconium electrode could be successfully applied in the potentiometric mode of operation only as an indicator electrode in titrations. The proportionality between the anodic current density and the fluoride ion concentration in perchloric acid media could be exploited for the amperometric detection of fluoride. With a thin-layer cell and a zirconium metal based indicator electrode, nanogram amounts of fluoride could be detected by a simple flow injection system.