Correction: A submersible probe with in-line calibration and a symmetrical reference element for continuous direct nitrate concentration measurements.

Correction for 'A submersible probe with in-line calibration and a symmetrical reference element for continuous direct nitrate concentration measurements' by Tara Forrest et al., Environ. Sci.: Processes Impacts, 2023, 25, 519-530, https://doi.org/10.1039/D2EM00341D.

Symmetric cell for improving solid-contact pH electrodes.

Traditional pH glass electrodes are designed in a symmetrical manner to guarantee the most reliable and reproducible potentiometric measurements possible. Solid-contact and other pH probes not based on glass membranes are desirable because they allow for new types of applications, may be mass fabricated and less prone to breakage. Unfortunately, however, they introduce electrochemical asymmetry because the reference element used in the reference electrode compartment is now different. This work shows how symmetry may be restored with solid-contact pH probes, using a H+-selective ionophore-based polymeric membrane deposited on top of a conductive polymer (PEDOT-C14) as a transducer layer. The new cell implements a reference element that is composed of a similarly formulated pH probe immersed into a buffer solution and an Ag/AgCl element directly connected to a single-junction Ag/AgCl/3.0 M KCl reference electrode that is placed in contact with the sample solution. By implementing this design, the zero point of the solid-contact pH sensing system may be shifted to the conventional value of pH 7.0. The value of the zero point was experimentally confirmed as 6.96 ± 0.02 pH units at three different temperatures in the range from 5 to 25 °C. This symmetric solid-contact potentiometric cell gave a long-term potential drift of 48 ± 16 µV h-1, comparable to that of a combination pH glass electrode.

A submersible probe with in-line calibration and a symmetrical reference element for continuous direct nitrate concentration measurements.

Current methods to monitor nitrate levels in freshwater systems are outdated because they require expensive equipment and manpower. Punctual sampling on the field or at a fixed measuring station is still the accepted monitoring procedure and fails to provide real-time estimation of nitrate levels. Continuous information is of crucial importance to evaluate the health of natural aquatic systems, which can strongly suffer from a nitrogen imbalance. We present here a nitrate-selective potentiometric probe to measure the analyte continuously without requiring maintenance or high-power consumption. Owing to a simple design where the sensors are located directly in contact with the sample, the need for constant pump usage is eliminated, requiring just 0.7 mW power per day instead of 184 mW per day and per pump. It is estimated that with this power consumption, the setup can easily run for more than 97 h on four simple Li-ion batteries. A simple in-line one-point calibration step was implemented to allow for drift correction. At the same time, a symmetrical design was used involving a second nitrate probe as a reference electrode placed in the calibrant compartment. This, combined with an in situ calibration step, allows one to quantify nitrate ion concentrations directly, instead of yielding activities. The dependence on ion activity was removed by using the analysed sample spiked with nitrate as the calibrant. This results in essentially the same activity coefficients and additionally reduces junction potentials to a fraction of a millivolt. In addition, a symmetrical reference element served to compensate for fluctuations caused by environmental factors (temperature, convection, etc.) to achieve improved stability and signal reproducibility compared to a traditional Ag/AgCl based reference electrode. The final prototype was deployed in the Arve River in Geneva for 75 h without requiring any intervention. The nitrate levels measured using the symmetrical reference element over this period were estimated at 44.0 ± 3.5 M and agreed well with the values obtained with ion chromatography (38.2 ± 2.1 µM) used as the reference method. Thanks to a modular sensing head the potentiometric sensors can be easily exchanged, making it possible to quantify other types of analytes and leading the way to a new monitoring strategy.

Unconditioned Symmetric Solid-Contact Electrodes for Potentiometric Sensing.

In potentiometric sensing, the preparation of the electrodes preceding a measurement is often the most time-consuming step. Eliminating the conditioning process can significantly speed up the preparation procedure, but it can also compromise the need for proper pre-equilibration of the membrane. We propose here a symmetric setup to address this challenge with an identical indicator and reference elements measured against each other, thereby compensating for potential drift. This strategy allows one to achieve potentiometric measurements using non-conditioned all-solid-state ion-selective electrodes for the detection of nitrate and potassium ions with Nernstian response slopes and detection ranges identical to those of conventional systems. To establish symmetry, a set of solid-contact ion-selective electrodes placed in a reference cell is measured against a set of identical electrodes in a sample cell. By subtracting the potentials between the two cells, potential instabilities not directly relevant to the measuring sample are eliminated, giving minimal potential drifts and stable 5-day potential responses. The E0 value of the nitrate-selective electrodes in the symmetric setup had a standard deviation of just 3 mV for the 5-day period in contrast to 19 mV in the asymmetric system, clearly demonstrating the influence of the conditioning step which is almost eliminated in the former system. During the 20 h potential monitoring experiments, the drift dropped to below 0.3 mV/min in less than 6 min, as opposed to an average time of 35 min for the asymmetric system. The applicability of the proposed setup was successfully demonstrated with the measurement of nitrate in a river water sample, where a potential drift lower than 0.1 mV/min was reached in less than 5 min of first contact with solution.

Solid-Contact Potentiometric Cell with Symmetry.

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.

Ion-to-electron capacitance of single-walled carbon nanotube layers before and after ion-selective membrane deposition.

The capacitance of the ion-to-electron transducer layer helps to maintain a high potential stability of solid-contact ion-selective electrodes (SC-ISEs), and its estimation is therefore an essential step of SC-ISE characterization. The established chronopotentiometric protocol used to evaluate the capacitance of the single-walled carbon nanotube transducer layer was revised in order to obtain more reliable and better reproducible values and also to allow capacitance to be measured before membrane deposition for electrode manufacturing quality control purposes. The capacitance values measured with the revised method increased linearly with the number of deposited carbon nanotube-based transducer layers and were also found to correlate linearly before and after ion-selective membrane deposition, with correlation slopes close to 1 for nitrate-selective electrodes, to 0.7 and to 0.5 for potassium- and calcium-selective electrodes.

Potentiometric Sensor Array with Multi-Nernstian Slope.

The sensitivity of potentiometric sensors functioning in equilibrium mode is limited by the value predicted according to the Nernst equation and inversely proportional to the charge in the analyte ion. Therefore, an increased ion charge results in a dramatic decrease in the sensor sensitivity. We propose an approach to allow one to increase the sensitivity of the potentiometric measurements by using a combined electrochemical cell composed of several identical ion-selective electrodes immersed into separate sample solutions of equal composition. The combination of n electrodes, demonstrating individually a Nernstian slope in one electrochemical cell allows to amplify the signal and associated response slope by n times. The proposed approach is shown to provide a double and triple Nernstian slope for potassium-, calcium-, nitrate-, and carbonate-selective electrodes by combining two or three identical electrodes, correspondingly. Each ion-selective electrode functions in an equilibrium mode, hence, ensuring response stability and reproducibility.

A Solid-State Reference Electrode Based on a Self-Referencing Pulstrode.

The design of solid-state reference electrodes without a liquid junction is important to allow miniature and cost-effective electrochemical sensors. To address this, a pulse control is proposed 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 extended 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.

From Molecular and Emulsified Ion Sensors to Membrane Electrodes: Molecular and Mechanistic Sensor Design.

Selective molecular ion probes are often insoluble in water and require a hydrophobic solvent environment for strong and selective binding, which runs counter to the desire of utilizing them in a homogeneous solution. This Account aims to guide the reader on how such molecules, often coined ionophores, can be harnessed to design exceptionally useful optical and electrochemical sensors. We start here with some historical context on the design of such ionophores and continue with the explanation of the response mechanism of optical and potentiometric sensors and the role of combined components to build a robust ion sensor. This Account is addressed to nonspecialist readers and for this reason avoids extensive use of equations or theoretical considerations. The interested reader should turn to the original literature for further reading. Emulsified optical sensors are introduced as an initial example. Here, multiple reagents are confined in an attoliter sensing nanodroplet of the organic phase, immiscible with the aqueous sample phase. In this case, the ionophore molecules may retain their high affinity and selectivity to the target ion and the aqueous sample phase does not have to be modified. Emulsified optical sensors allow one to achieve the selective chemical sensing of ions, even with optically silent ionophores. Such ionophore-based nanodroplets are also discussed as a useful novel class of complexometric titration reagents and optical end point indicators with unique selectivities. We then turn our attention to potentiometric sensing probes and briefly discuss the unique opportunity of a direct characterization of ion-ionophore complexation properties offered by membrane electrodes. A carbonate-selective membrane electrode containing a highly selective tweezer-type ionophore with trifluoroacetophenone functional groups is then used as an example for the construction of a robust all-solid-state sensor. This potentiometric probe, in combination with a pH electrode, can directly measure PCO2 in freshwater lakes, demonstrating a dramatically improved response time relative to traditional sensors equipped with a gas-permeable membrane. In recent years, new sensing modes and electrode designs have been introduced to expand the application scope of ionophore-based potentiometric sensors. Membrane electrodes containing ionophores are placed under dynamic electrochemistry control to give important progress in the field. We specifically highlight our recent works by membranes that are controlled by chronopotentiometry (controlled current) for speciation analysis, by ion transfer voltammetry on thin sensing films for multianalyte detection, by exhaustive coulometry for potentially calibration-free sensors and with coulometric membrane pumps for the selective delivery of reagents.

Electrochemically Switchable Polymeric Membrane Ion-Selective Electrodes.

We present here for the first time a solid contact ion-selective electrode suitable for the simultaneous sensing of cations (tetrabutylammonium) and anions (hexafluorophosphate), achieved by electrochemical switching. The membrane is based on a thin plasticized polyurethane membrane deposited on poly(3-octylthiophene) (POT) and contains a cation exchanger and lipophilic electrolyte (ETH 500). The cation exchanger is initially in excess; the ion-selective electrode exhibits an initial potentiometric response to cations. During an oxidative current pulse, POT is converted into POT+, which results in the expulsion of cations from the membrane followed by the extraction of anions from the sample solution to fulfill the electroneutrality condition. This creates a defined excess of lipophilic cation in the membrane, resulting in a potentiometric anion response. A reductive current pulse restores the original cation response by triggering the conversion of POT+ back into POT, which is accompanied by the expulsion of anions from the membrane and the extraction of cations from the sample solution. Various current pulse magnitudes and durations are explored, and the best results in terms of response slope values and signal stability were observed with an oxidation current pulse of 140 µA cm-2 applied for 8 s and a reduction current pulse of -71 µA cm-2 applied for 8 s.

An Interface Equilibria-Triggered Time-Dependent Diffusion Model of the Boundary Potential and Its Application for the Numerical Simulation of the Ion-Selective Electrode Response in Real Systems.

A simple dynamic model of the phase boundary potential of ion-selective electrodes is presented. The model is based on the calculations of the concentration profiles of the components in membrane and sample solution phases by means of the finite difference method. The fundamental idea behind the discussed model is that the concentration gradients in both membrane and sample solution phases determine only the diffusion of the components inside the corresponding phases but not the transfer across the interface. The transfer of the components across the interface at any time is determined by the corresponding local interphase equilibria. According to the presented model, each new calculation cycle begins with the correction of the components' concentrations in the near-boundary (first) layers of the membrane and solution, based on the constants of the interphase equilibria and the concentrations established at a given time as a result of diffusion. The corrected concentrations of the components in the boundary layers indicate the start of a new cycle every time with respect to the calculations of diffusion processes inside each phase from the first layer to the second one, and so on. In contrast to the well-known Morf's model, the above-mentioned layers do not comprise an imaginary part and are entirely localized in the corresponding phases, and this allows performing the calculations of the equilibrium concentrations by taking into account material balance for each component. The model remains operational for any realistic scenarios of the electrode functioning. The efficiency and predictive ability of the proposed model are confirmed by comparing the results of calculations with the experimental data on the dynamics of the potential change of a picrate-selective electrode in nitrate solutions when determining the selectivity coefficients using the methods recommended by IUPAC.

Time-Dependent Determination of Unbiased Selectivity Coefficients of Ion-Selective Electrodes for Multivalent Ions.

A new method for the determination of unbiased low selectivity coefficients for two of the most prevalent cases of multivalent ions (zi = 2, zj = 1 and zi = 1, zj = 2) was theoretically and experimentally substantiated. The method is based on eliminating the primary ion concentration near the membrane by extrapolating the linearized time dependencies of selectivity coefficients determined by the separate solutions method (KijPot(SSM) as a function of t-1/3 or t-1/6, depending on the charge combination of the two ions, to infinite time. The applicability of the method is demonstrated for ionophore-based Mg2+-, Ca2+-, and Na+-selective electrodes. It is shown that the high level of primary ion impurities in the salts of interfering ions can significantly limit the efficiency of the technique, as demonstrated with salts of different purity levels.

Improved separate solution method for determination of low selectivity coefficients.

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.