Polymeric membranes with ion-exchange properties have found numerous applications in water treatment, dialysis, energy storage, chemical sensors, and bio-interfaces, among others. Notably, it is common to operate under non-equilibrium conditions while pursuing specific features (e.g., current generation) through an electron-to-ion mechanism. To maximize the final performance, it is crucial to understand the role of each interface within the system, which becomes complex when the device is tailored with several materials and films. This is the case for ion sensors based on thin membranes in backside contact with a redox active conducting polymer. Herein, we investigate such a system operating under a charge transfer mechanism, which features electroneutrality maintenance as the main driving force upon application of a linear sweep potential. This potential is modeled as being unequally distributed among the various system interfaces. Our results demonstrate and quantify the existence of a transient membrane potential at the membrane-electrolyte interface, owing to the implementation of a strategical measurement point on the buried membrane side and connected to a built-in electrometer for the exclusive acquisition of the potential difference at such an interface. The transient membrane potential was found to be <1 % of the total applied potential, meaning that the ion-transfer process at the electrolyte-membrane interface is less energetically costly that the electron transfer and doping processes occurring at the conducting polymer side. This small contribution can be potentiated by increasing the ion-exchange capacity of the membrane, which indirectly enlarges the system current and serves as a strategy for increasing the efficiency of the device.

In this work, we present a model based on the finite element approach to describe the electrochemically controlled release of ions from a redox-active film into a sample confined to a thin-layer spatial domain. The model includes the effect of interfacial charge transfer kinetics and 1D-diffusion treatment for an electron transfer-ion transfer (ET-IT) coupled reaction. More in detail, the oxidation of the redox-active film (ET) involves an ion release to an aqueous phase (IT). The dynamic concentration of the released ion is calculated when the ET-IT reaction proceeds under potentiostatic control, and the effect of the thickness of each phase (i.e., film or aqueous) on the diffusion profile is analyzed. The model is experimentally validated for the particular case in which oxidation of a thin film of polyaniline (PANI, 10 mu m in thickness) is linked to the release of protons from the film into an electrolyte solution. The proton release produces certain pH changes in the electrolyte that are monitored by a pH sensor located at 330 mu m from the PANI film. The charge associated with the proton release is related to the dynamic concentration of protons in the electrolyte through pH-coulograms that agree with the theoretical predictions. Overall, the model can reproduce the general behavior of the experimental proton pump and provides key insights into the functioning mechanism of electrochemical systems where redox and ion transfer reactions are coupled.

Herein, we report on a reagentless electroanalytical methodology for automatized acid–base titrations of water samples that are confined into very thin spatial domains. The concept is based on the recent discovery from our group (Wiorek, A. Anal. Chem. 2019, 91, 14951−14959), in which polyaniline (PANI) films were found to be an excellent material to release a massive charge of protons in a short time, achieving hence the efficient (and controlled) acidification of a sample. We now demonstrate and validate the analytical usefulness of this approach with samples collected from the Baltic Sea: the titration protocol indeed acts as an alkalinity sensor via the calculation of the proton charge needed to reach pH 4.0 in the sample, as per the formal definition of the alkalinity parameter. In essence, the alkalinity sensor is based on the linear relationship found between the released charge from the PANI film and the bicarbonate concentration in the sample (i.e., the way to express alkalinity measurements). The observed alkalinity in the samples presented a good agreement with the values obtained by manual (classical) acid–base titrations (discrepancies <10%). Some crucial advantages of the new methodology are that titrations are completed in less than 1 min (end point), the PANI film can be reused at least 74 times over a 2 week period (<5% of decrease in the released charge), and the utility of the PANI film to even more decrease the final pH of the sample (pH ∼2) toward applications different from alkalinity detection. Furthermore, the acidification can be accomplished in a discrete or continuous mode depending on the application demands. The new methodology is expected to impact the future digitalization of in situ acid–base titrations to obtain high-resolution data on alkalinity in water resources.