FACTORS AFFECTING THE RESISTANCE AND PERFORMANCE OF FLUORIDE ION-SELECTIVE ELECTRODES

Some lanthanide fluoride (LnF(3), Ln = Ce, Nd, Sm) crystals have been grown, and fluoride ion-selective electrodes (F-ISE) have been constru cted using these and some commercial LaF3 single crystal membranes. As pects of crystal growing, particularly the high-temperature dimorphism of SmF3, are addressed, and the effects on the electrode resistance a nd response characteristics of crystal quality, doping and membrane ge ometric dimensions are studied. The results show that, while doping is effective in increasing the membrane conductivity, membrane geometric dimensions also affect the electrical resistance substantially. Undop ed membranes have noise-free Nernstian response to F- solutions, provi ded appropriate geometric dimensions are chosen. The crystal orientati on of the membrane does not have noticeable effect on the performance. The crystal quality also affects the membrane resistance and performa nce. Single crystal membranes with minor and localized visual defects show little difference in conductivity and no difference in performanc e compared with a 'perfect' single crystal membrane, Membranes grown f rom lower grade starting materials do not show poorer performance, alt hough these membranes were slightly cloudy. Membranes with severe visu al defects have much lower conductivity, and deteriorated performance.