The Effects of Doping on the Selectivity and Response Time of Fluoride Ion-Selective Electrodes

Abstract

We report the effect of doping on the response time and hydroxide ion interference of fluoride ion-selective electrodes made by using membranes consisting of single crystals of CeF₃, LaF₃ and NdF₃, both undoped and doped with divalent cations , and of mixed crystals of neodymium and samarium fluorides (Nd_0.95Sm_0.05F₃ and Nd_0.86Sm_0.14F₃). Doping not only increases the bulk conductivity of the membrane, but also reduces both the response time and the hydroxide ion interference of the electrode. This behaviour is due to an increase in the number of Frenkel defects in the crystal membrane, an effect which increases the mobility of the fluoride ion. It has also been established that doped single crystals of either LaF₃ or NdF₃ offer the best performance in terms of selectivity.