Detection and selectivity properties of Li+-ion-selective electrodes based on NASICON-type ceramics

Crystallised fast Li+ conductors have been used as ionic sensitive materials for lithium ion measurements by potentiometry. The compounds are NASICON-type ceramics of general formula Li1+xAlxM2−x(PO4)3 (M = Ti or Ge and 0.3≤x≤0.7). For 0.3≤x≤0.7, the electrodes present a Nernstian response down to a detection limit of 6×10−5 to 2×10−4 mol l−1. The selectivity properties have been examined. It has been shown that alkali metal ion interferences (i.e. Na+ and K+) are governed by steric effects. The most selective NASICON-type skeleton is the one presenting the smallest unit cell (i.e. Li1+xAlxGe2−x(PO4)3), in spite of a lower ionic bulk conductivity than its Ti-based homologous Li1+xAlxTi2−x(PO4)3. The potentiometric interference coefficient KpotLi,Na are 0.65 and 1.6×10−2 for Li1.3Al0.3Ti1.7(PO4)3 and for Li1.3Al0.3Ge1.7(PO4)3 respectively. The KpotLi,j coefficients have been determined by the mixed solutions method using the Nikolskii-Eisenman empirical equation. It has been shown that KpotLi,j depends on Li+ concentration when the primary ion level is fixed and on the j interfering ion concentration when the j level is fixed. Such results indicate clearly that the Nikolskii–Eisenman model is not sufficiently adapted to the characterization of NASICON-type electrodes.