Determination of total fluoride content in electroslag refining fluxes using a fluoride ion-selective electrode

A method for a routine, direct analysis for fluoride in electroslag refining (ESR) fluxes, using potentiometry, is described that is faster, more economical, and less operator-dependent than previously known methods of fluoride analysis in these materials. Flux samples were prepared using a rapid sodium carbonate/borate fusion with subsequent digestion in dilute nitric acid prior to chelant addition and analysis by fluoride ion-selective electrode (FISE). Disodium ethylenediaminetetraacetate (EDTA), ethylenediamine, ammonium citrate, ammonium tartrate, and citric acid were used to chelate metal cations in solutions of flux containing calcium fluoride (fluorspar) as a major constituent. It was found that, for optimal recovery of fluoride, pH of the chelant/flux solution must be controlled between 8.0 and 9.0. High concentrations of calcium reduced the ability of the fluoride ion-selective electrode to detect fluoride. It is suspected that high concentrations of magnesium would also contribute to this reduced sensitivity, but such fluxes were not tested. The technique is well-suited for analysis of fluxes when the sample CaF ₂ content is ≤63% by mass. Analysis of several flux formulations showed that the new FISE method is more practical and produces results that are as accurate and precise as conventional methodology.