Conductive epoxy-graphite composite as a solid internal reference in a NASICON-based sodium ion-selective electrode for flow-injection analysis

A solid-state conductive polymer composite is proposed as a substitute for the internal liquid reference in a sodium sensor. Epoxy-graphite is used as the composite and NASICON as a sensitive membrane. The absence of a liquid internal solution allows the straightforward construction of these transducers in various shapes and dimensions. This is possible because of the use of an easily moldable plastic support. The conductivity of the epoxy-graphite composite and the electrochemical behaviour of the NASICON/conductive epoxy interface were characterized by impedance spectroscopy. The stability and reproducibility of the electrode potential were studied. Different compositions of the reference system were tested. An all-solid-state, flow-through, tubular sodium sensor based on a NASICON membrane has been evaluated. The construction of a miniaturized sensor is described and its application to flow-injection analysis is reported.