Lanthanum fluoride single crystals with or without europium fluoride, and their response to fluoride ion

Lanthanum fluoride single crystals with or without europium fluoride were prepared and observed using an energy dispersive X-ray fluorescence spectrometer. Fluoride ion-selective electrodes were then prepared from these single crystals. Only lanthanum peaks appeared on the electron probe X-ray spectra of crystals with europium fluoride (Eu content was 0.5 wt. % or 1 wt. %). Specific resistance at 25 0C was 2.8 -- 3.4 x 106 ohm .cm for crystals without europium fluoride, and for those with europium fluoride was 4.2 -- 5.4 x 105 ohm .cm (Eu content was 0.5 wt. %) and 2.3 -- 3.6 x 105 ohm .cm (Eu content was 1 wt. %). All of the electrodes prepared responded to fluoride ion. The pH range suited for the use of the electrode which membrane contained 0.5 wt. % Eu was pH 4 -- 7 in 10-5 mol dm-3F-, pH 4 -- 9 in 10-3 mol dm-3F- and pH 4 -- 11 in 10-1 mol dm-3F-. The response time of the electrodes was also investigated. (author)