THE EFFECTS OF DOPING ON THE SELECTIVITY AND RESPONSE-TIME OF FLUORIDE ION-SELECTIVE ELECTRODES

We report the effect of doping on the response time and hydroxide ion interference of fluoride ion-selective electrodes made by using membra nes consisting of single crystals of CeF3, LaF3 and NdF3, both undoped and doped with divalent cations, and of mixed crystals of neodymium a nd samarium fluorides (Nd0.95Sm0.05F3 and Nd0.86Sm0.14F3). Doping not only increases the bulk conductivity of the membrane, but also reduces both the response time and the hydroxide ion interference of the elec trode. This behaviour is due to an increase in the number of Frenkel d efects in the crystal membrane, an effect which increases the mobility of the fluoride ion. It has also been established that doped single c rystals of either LaF3 or NdF3 offer the best performance in terms of selectivity.