A polymer-based all-solid-state internal reference system for pH glass electrodes: Performance and charge transfer

An all-solid-state internal reference system for pH glass electrodes employing polypyrrole (PPy)-Nafion(R) composites is described. pH sensors based on this reference system are characterised at 23 - 3 DGC. The observed characteristics of the resulting all-solid-state sensors, i.e. slope and linearity of the calibration plot, response behaviour, and signal stability, are very promising, and the differences with respect to the parameters of conventional liquid filled sensors are negligible. The values of the zero potential point, which differ from those of conventional pH glass electrodes, could be controlled within limits via the composition of the PPy-Nafion composite. The excellent sensor performance observed seems to be the consequence of a reversible charge transfer between the ion-conducting membrane, the mixed conducting PPy-Nafion interlayer, and the metal wire. A first discussion of the mechanism of this charge transfer is given.