Solid Contact Ion-Selective Electrodes with a Well-Controlled Co(II)/Co(III) Redox Buffer Layer

Abstract

Solid contact ion-selective electrodes (ISEs) typically have an intermediate layer between the ion-selective membrane and the underlying solid electron conductor that is designed to reduce the irreproducibility and instability of the measured electromotive force (emf). Nevertheless, the electrode-to-electrode reproducibility of the emf of current solid contact ISEs is widely considered to be unsatisfactory. To address this problem, we report here a new method of constructing this intermediate layer based on the lipophilic redox buffer consisting of the Co(III) and Co(II) complexes of 1,10-phenanthroline ([Co(phen)3](3+/2+)) paired with tetrakis(pentafluorophenyl)borate as counterion. The resulting electrodes exhibit emf values with an electrode-to-electrode standard deviation as low as 1.7 mV after conditioning of freshly prepared electrodes for 1 h. While many prior examples of solid contact ISEs also used intermediate layers that contained redox active species, the selection of a balanced ratio of the reduced and oxidized species has typically been difficult and was often ignored, contributing to the emf irreproducibility. The ease of the control of the [Co(phen)3](3+)/[Co(phen)3](2+) ratio explains the high emf reproducibility, as confirmed by the emf decrease of 58 mV per 10-fold increase in the ratio of the reduced and oxidized redox buffer species. Use of a gold electrode modified with a self-assembled 1-hexanethiol monolayer as underlying electron conductor suppresses the formation of a water layer and results in an electrode-to-electrode standard deviation of E° of 1.0 mV after 2 weeks of exposure to KCl solution.

The complexing properties of oxalodihydrazide, acethydrazide and formic hydrazide with Cu(II) in aqueous solution

“…Measurements were accomplished in the concentration of pCu range 1.80-3.88 where a linear relationship was found between the EMF values of the electrode and pCu. This was in accordance with the Nernst equation [32] and allowed to determine the standard values of EMF (E°) and slope of response curve (s), used afterwards in Hyperquad 2008.…”

Intercalation Compounds as Inner Reference Electrodes for Reproducible and Robust Solid‐Contact Ion‐Selective Electrodes

“…Conducting polymers,s uch as polypyrrole [8] or PEDOT, [9] offer ah igher redox capacity than carbon since the target ions can be inserted in the polymer upon reduction of the polymer backbone.N evertheless,c onducting polymers are characterized by aw ide range of redox potentials which renders the prediction and long-term stability of the standard potential difficult. [11,12] Nevertheless,t his approach requires the composition of the ISM to be altered. [10] Another approach is adding ah ydrophobic redox couple as a" redox buffer" into the ISM, resulting in quite remarkable initial reproducibility.…”

Evidence of double layer/capacitive charging in carbon nanomaterial-based solid contact polymeric ion-selective electrodes

“…Based on a similar principle, Buhlmann and subsequently Schuhmann showed that a metallic redox couple may also buffer the interfacial inner potential. 6 Different to the steady-state behavior in potentiometry, it was recently shown that reversible ion-transfer through an ionophore-based membrane can be established by oxidation/reduction of the CP situated underneath of the membrane (Scheme 1c, Fig S1). 7 Clearly, research in the field over many years has alluded to a CP transducing mechanism relying on the ionic buffer capacity mediated by a doping agent resulting in a high redox capacitance.…”