While ion-selective electrodes (ISEs) with inner filling solutions are used widely, solid-contact ISEs are better suited for miniaturization and mass manufacturing. Calibration-free measurements with such electrodes require the reproducible control of the phase boundary potential between the ion-selective membrane and the underlying electron conductor. The most promising approach to achieve this goal is based on redox buffers incorporated into the ion-selective membrane. Here we introduce the theory and present experimental data for Co(III), Co(II), Ru(II), Fe(II), and Os(II) compounds that show quantitatively how the phase boundary potential at a solid contact doped with redox-active compounds is affected by weighing errors, reagent impurities, and redox-active interferents. Perhaps surprisingly, theory predicts that there is only a minimal dependence of the phase boundary potential on the ratio of the concentrations of a pure oxidized and a pure reduced compounds if those two compounds are not a redox couple. However, theory predicts that even small redox-active impurities of those compounds shift the phase boundary potential drastically. Experimentally, a surprisingly good in-batch reproducibility was observed by us and others for solid contacts prepared to contain either only the reduced or only the oxidized species of a redox couple. This can be explained by redox-active impurities and is unlikely to be repeatable when different suppliers of reagents are used or long-term experiments are performed. This work confirms that the preferred approach to calibration-free sensing is based on redox buffers that comprise the reduced and oxidized species of a redox couple in well-controlled concentrations.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation (Grant CHE-1748148). X.V.Z. acknowledges a Lester C. and Joan M. Krogh Fellowship, and C.R.R. acknowledges a Kenneth E. & Marion S. Owens Endowed Fellowship, both from the Department of Chemistry, University of Minnesota.
This work was supported by the National Science Foundation (Grant CHE-1748148). X.V.Z. acknowledges a Lester C. and Joan M. Krogh Fellowship, and C.R.R. acknowledges a Kenneth E. & Marion S. Owens Endowed Fellowship, both from the Department of Chemistry University of Minnesota.
© 2018 American Chemical Society.