Joaquín Angel Ortuño

This review examines the reported research on the potential responses of ion-selective electrodes over time when exposed to sudden changes in the concentration of the primary ion (ion initially present in the ion-selective electrode membrane) and/or foreign interfering ions. Particular attention is given to the responses of liquid- and plasticized polymeric membrane-based ion-selective electrodes to foreign ions. The review provides an in-depth discussion of the theoretical models proposed to describe transient potential signals obtained experimentally with these ion-selective electrodes. In chronological order, the different contributions are presented and commented on in terms of their assumptions and mathematical treatments. The final equations obtained in each case, as well as some stages of their derivations, are presented. Additionally, the various models are classified and critically commented upon. Lastly, the review discusses the analytical applications reported for identifying and quantifying ions using the transient potential signals.

This study investigates the potential of transient potentiometric signals generated by an array of ion-selective electrodes (ISEs) as the basis for a potentiometric electronic tongue capable of identifying and quantifying a range of inorganic and organic cations. Six distinct polymeric membrane ISEs were fabricated, differing in plasticizer type (either NPOE or DEHS), and in the presence or absence of a lipophilic ion exchanger (KTClPB) and/or an ionophore (DB18C6). Transient potential responses were recorded following the exposure of the electrode array to various cations at different concentrations. A total of 810 transient signals were analyzed through visual inspection and principal component analysis (PCA), revealing characteristic dynamic patterns influenced by membrane composition, ion type, and ion concentration. PCA was conducted both on the transient signals from each individual electrode and on the aggregated dataset comprising signals from the full six-electrode array (electronic tongue). The electronic tongue exhibited a markedly enhanced capacity for discriminating and quantifying ion concentrations in comparison to any single electrode.

Low-cost electrochemical methodologies for the determination of L-ascorbic acid (vitamin C) and the analysis of juices are developed based on its electro-oxidation on carbon screen-printed electrodes. A novel chronoamperometric methodology is developed for the quantification of L-ascorbic acid in fruit juices. The proposed method stands out for its simplicity and rapidity, demonstrating its efficacy in determining L-ascorbic acid content in various fruit juices. Notably, the results obtained with this chronoamperometric approach are compared with those yielded by chromatography, with no significant differences between the two methods being found. Additionally, an electronic tongue is developed for the differentiation of juices based on the square wave voltammetric signals.

A nitrate selective electrode was used for real-time in situ potentiometric monitoring of a batch nitrate removal process using activated carbon and ion exchange resin. A plasticized polymeric membrane consisting of polyvinyl chloride, 2-nitrophenyl octyl ether and tridodecyl methyl ammonium chloride was incorporated into an ion-selective electrode body. First, the dynamic potential response of the electrode to nitrate was investigated. Two commercial activated carbons with different physical properties were then tested. Nitrate removal with these carbons was monitored potentiometrically using several nitrate concentrations. The extreme turbidity of the solutions was not a drawback during potentiometric monitoring of the process, which is a clear advantage over other methods such as optical monitoring. The potential versus time recordings were converted into nitrate concentration versus time plots, which were evaluated with different adsorption kinetic models. A pseudo-second order kinetic model for nitrate adsorption on both activated carbons was found to fit the experimental data very well. The values of the kinetic parameters were very different between the two activated carbons. The proposed methodology was also satisfactorily applied to the study of nitrate removal by an ion exchange resin. In this case, the experimental results clearly follow a pseudo-first order kinetic model. Potential applications of the proposed methodology for monitoring nitrate removal in real water samples are discussed.

Ion-selective electrodes for tetrachloroaurate(III) have been developed for potentiometric monitoring of the reduction reaction of tetrachloroaurate(III). Three different plasticized polyvinyl chloride membranes containing tridodecymethylammonium chloride as an anion exchanger were investigated. These membranes differ in the plasticizer used, either 2-nitrophenyl octyl ether (NPOE) or tricresyl phosphate (TCP) or bis-(2-ethylhexyl) sebacate (DOS). The potentiometric response of the electrodes to the tetrachloroaurate(III) concentration was studied by two methods. In the first method, commonly used in the calibration of ion-selective electrodes, successive tetrachloroaurate(III) concentration increments were used and the potential was allowed to stabilize after each concentration step. The second method was developed to mimic the tetrachloroaurate(III) reduction reaction in which there is a continuous decrease in the concentration of tetrachloroaurate(III). This was achieved by continuously diluting an initial concentration of tetrachloroaurate(III) by pumping a diluent solution while keeping the sample volume constant. This method gave an excellent linear response to the tetrachloroaurate(III) concentration. The calibrated electrodes were used for the potentiometric monitoring of the kinetics of a newly observed reaction: the reduction of tetrachloroaurate(III) by hydroxylamine catalyzed by iodide. A mechanism for this reaction is proposed on the basis of the experimental results obtained.

A potentiometric electronic tongue (ET) for the analysis of well and ditch irrigation water samples is herein proposed. The sensors’ array is composed of six ion-selective electrodes based on plasticized polymeric membranes with low selectivity profiles, i.e., the membranes do not contain any selective receptor. The sensors differ between them in the type of ion-exchanger (sensors for cations or anions) and the plasticizer used in the membrane composition, while the polymeric matrix and the preparation protocol were maintained. The potentiometric response of each sensor towards the main cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) and anions (HCO₃⁻, Cl⁻, SO₄²⁻, NO₃⁻) expected in irrigation water samples was characterized, revealing a fast response time (2+, Na⁺, and Cl⁻ concentrations and acceptable results for the rest of ions. Overall, the ET is a promising analytical tool for irrigation water quality, exceeding traditional characterization approaches (conductivity, salinity, pH, cations, anions, etc.) in terms of overhead costs, versatility, simplicity, and total time for data provision.

The present paper addresses the development and use of a new potentiometric electronic tongue for both qualitative and quantitative characterization of natural mineral waters. The electronic tongue is particularly related to the conductivity and ion content of/in the water sample. The analytical system is based on six ion-selective electrodes whose membranes are formulated to provide either cationic or anionic response and considering plasticizers with different dielectric constants (bis(2-ethylhexyl) sebacate, 2-nitrophenyl octyl ether or tricresylphosphate), while keeping the polymeric matrix, i.e., poly(vinyl chloride). Notably, the absence of any ionophore in the membrane provides a general response profile, i.e., no selectivity toward any special ion, which is convenient for the realization of an effective electronic tongue. The dynamic response of the tongue toward water samples of different chemical compositions and geographical locations has been obtained. At the optimized experimental conditions, the tongue presents acceptable repeatability and reproducibility (absence of hysteresis). The principal component analysis of the final potential values observed with the six electrodes allows for the differentiation and classification of the samples according to their conductivity, which is somehow related to the mineralization. Moreover, quantitative determination of the six main ions in the water samples (i.e., chloride, nitrate, hydrogen carbonate, sulfate, sodium, calcium, and magnesium) is possible by means of a simple linear calibration (and cross-validation) model.

A study of the transient potential signals obtained with a cation-selective electrode based on an ion-exchanger was carried out for solutions of the following individual cations at different concentrations: H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Mg²⁺, Ca²⁺, choline (Ch⁺), acetylcholine (AcCh⁺), and procaine (Pr⁺). Three different general types of transient signals were distinguished depending on the value of the selectivity coefficient of the corresponding ion. A principal component analysis (PCA) was performed on the signals, finding that the qualitative identification of the corresponding ion from the scores of two principal components is possible. The study was extended to the transient signals of solutions containing an analyte in the presence of an interfering ion. The PCA of the corresponding signal allows for the detection of the presence of interfering ions, thus avoiding biased results in the determination of the analyte. Moreover, the two principal components of the transient signals obtained for each of the ions at different concentrations allow for the construction of calibration graphs for the quantitative determination of the corresponding ion. All the transient signals obtained experimentally in this work can be reconstructed accurately from principal components and their corresponding scores.