Reagentless Acid–Base Titration for Alkalinity Detection in Seawater

Abstract

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. et al. 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.

Imaging Sample Acidification Triggered by Electrochemically Activated Polyaniline

“…When PANI is electrochemically oxidized from its reduced basal state, the amine-benzenoid structures of the polymer backbone are converted into quinoids, accompanied by the release of protons. This process activates at a milder potential compared to water splitting. , Sample acidification was confirmed by monitoring the pH via a potentiometric sensor located in front to the PANI proton pump. Optical pH sensing of proton release is indeed also possible, as demonstrated for the water splitting process at carbon electrodes .…”

“…First, we investigated the acidification of solutions comprising different buffer concentrations (1–20 mM phosphate, pH 7.0 measured with the pH-meter). The proton release from the PANI mesh was activated by applying a potential equal to the open-circuit potential (OCP) plus 0.4 V (applied with respect to the Ag/AgCl reference electrode) for 180 s . Simultaneously, images were acquired with either the HPTS or EE optodes, with a higher time resolution at the beginning of the applied pulse.…”

“…The demonstrated acidification levels are in principle suitable to further implement the sensor–actuator concept for combined sample pretreatment and analysis in such demanding applications as alkalinity (formal pH of 4.0, pH or CO 2 optode) and dissolved inorganic carbon (pH 4.0 for >99% of conversion, CO 2 optode) detections in the environmental field, whereas the determination of biomarkers or drugs such as penicillin (pH 4–4.5, other detector), certain immunoassays, and total sulfide in fluids and tissues (pH 4.8 for >99% of conversion, H 2 S optode) are important in the clinical domain, − among others. In any case, the PANI mesh could be optimized in terms of porosity and the amount of deposited PANI to fine-tune the delivered charge of protons according to the application needs.…”

“…This process activates at a milder potential compared to water splitting. 6 , 7 Sample acidification was confirmed by monitoring the pH via a potentiometric sensor located in front to the PANI proton pump. Optical pH sensing of proton release is indeed also possible, as demonstrated for the water splitting process at carbon electrodes.…”

“…First, we investigated the acidification of solutions comprising different buffer concentrations (1–20 mM phosphate, pH 7.0 measured with the pH-meter). The proton release from the PANI mesh was activated by applying a potential equal to the open-circuit potential (OCP) plus 0.4 V (applied with respect to the Ag/AgCl reference electrode) for 180 s. 7 Simultaneously, images were acquired with either the HPTS or EE optodes, with a higher time resolution at the beginning of the applied pulse. While the HPTS optode is known to be fairly accurate covering the pH range from 8.5 to 4.5 (p K a HPTS = 6.9), the EE optode was used to detect pH from 4.5 to 1.1 (p K a EE = 2.3); see Figure S3 in the Supporting Information for the calibration graphs of the optodes.…”