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Simple, fast, and theoretically substantiated experimental method for determination of improved selectivity coefficients is proposed. The method is based on the well-known fact that low selectivity coefficients determined by the separate solution method (SSM) are time-dependent and, upon our finding, this dependence is a well-defined linear function of time raised to the certain negative power. In particular, the selectivity coefficients obtained for equally charged primary and foreign ions by SSM linearly depend on time to the minus one-fourth. It was found that extrapolation of experimental data using this function to the intersection with Y axes gives reliable values of rather low selectivity coefficients (down to n × 10(-7)), which strongly differ from those measured using SSM and correspond well with the values obtained using the modified separate solution method (MSSM) proposed by Bakker. At the same time, the new method is free of one very essential limitation inherent to MSSM, namely, it is applicable after the conditioning of electrodes in the primary ion solution and can be repeated many times.

By its nature, a traditional potentiometric cell composed of an Ag/AgCl-based reference electrode and a solid-contact indicating electrode is not symmetric. This results in undesirable potential drifts in response to a common perturbation such as a temperature change of the sample. We propose here an approach to restore symmetry by constructing a cell with two identical solid-contact ISEs used as reference and indicating electrodes. In this arrangement, the reference electrode is immersed in a compartment containing a constant background of an ion of interest, while the indicating electrode is directly immersed in the sample solution. This approach was successfully demonstrated for a cell composed of nitrate-selective electrodes with the hydrophobic derivative of poly(3,4-ethylenedioxythiophene) as a transducer layer. In particular, the symmetric setup is shown to lower by 4−5 times the observed potential drift resulting from temperature changes between +25 and +5 °C.

The design of solid-state reference electrodes without liquid junction is of great importance to allow for miniature and costeffective electrochemical sensors in environmental and biomedical applications. To address this, we propose here a pulse control protocol using an Ag/AgI element as reliable solid-state reference electrode. It involves the local release of iodide by a cathodic current that is immediately followed by an electromotive force (EMF) measurement that serves as the reference potential. The recapture of iodide ions is achieved by potentiostatic control. This results in intermittent potential values that are reproducible to less than one millivolt (SD = 0.27 mV, n = 50). The ionic strength is shown to influence the activity coefficient of released iodide in accordance with the Debye-Hückel equation, resulting in a predictable change of the potential reading. The principle is applied to potentiometric potassium detection with a valinomycin-based ion-selective electrode (ISE), demonstrating a completely solid-state sensor configuration. The resulting calibration curves compare quantitatively to a commercial liquid junction-based reference electrode, even in spiked artificial sweat samples. This approach offers a promising strategy to the design of all-solid-state electrochemical sensing probes.