Applicability of the phase boundary potential model to the mechanistic understanding of solvent polymeric membrane-based ion-selective electrodes

Improvement of the Upper Detection Limit of Ionophore-Based H+-Selective Electrodes: Explanation and Elimination of Apparently Super-Nernstian Responses

The response range of an ion-selective electrode (ISE) has been described by counterion interference at the lower and Donnan failure at the upper detection limit. This approach fails when the potentiometric response at the upper detection limit exhibits an apparently super-Nernstian response, as has been reported repeatedly for H+-selective electrodes. While also observed when samples contain other anions, super-Nernstian responses at low pH are a problem in particular for samples that contain phthalate, a common component of commercial pH calibration solutions. This work shows that coextraction of H+ and a sample anion into the sensing membrane alone does not explain these super-Nernstian responses, even when membrane-internal diffusion potentials are taken into account. Instead, these super-Nernstian responses are explained by the formation of complexes between that anion and at least two protonated ionophore molecules. As demonstrated by experiments and explained with quantitative phase boundary models, the apparently super-Nernstian responses at low pH can be eliminated by restricting the molecular ratio of ionophore and ionic sites. Notably, this conclusion results in recommendations for the optimization of sensing membranes that, in some instances, will conflict with previously reported recommendations from the ionic site theory for the optimization of the lower detection limit. This mechanistic insight is key to maximizing the response range of these ionophore-based ISEs.

Toward Understanding the Formation Mechanism and OER Catalytic Mechanism of Hydroxides by In Situ and Operando Techniques

Developing efficient and affordable electrocatalysts for the sluggish oxygen evolution reaction (OER) remains a significant barrier that needs to be overcome for the practical applications of hydrogen production via water electrolysis, transforming CO2 to value-added chemicals, and metal-air batteries. Recently, hydroxides have shown promise as electrocatalysts for OER. In situ or operando techniques are particularly indispensable for monitoring the key intermediates together with understanding the reaction process, which is extremely important for revealing the formation/OER catalytic mechanism of hydroxides and preparing cost-effective electrocatalysts for OER. However, there is a lack of comprehensive discussion on the current status and challenges of studying these mechanisms using in situ or operando techniques, which hinders our ability to identify and address the obstacles present in this field. This review offers an overview of in situ or operando techniques, outlining their capabilities, advantages, and disadvantages. Recent findings related to the formation mechanism and OER catalytic mechanism of hydroxides revealed by in situ or operando techniques are also discussed in detail. Additionally, some current challenges in this field are concluded and appropriate solution strategies are provided.

History of Cobaltabis(dicarbollide) in Potentiometry, No Need for Ionophores to Get an Excellent Selectivity

This work is a mini-review highlighting the relevance of the θ metallabis(dicarbollide) [3,3'-Co(1,2-C₂B₉H₁₁)₂]^- with its peculiar and differentiating characteristics, among them the capacity to generate hydrogen and dihydrogen bonds, to generate micelles and vesicles, to be able to be dissolved in water or benzene, to have a wide range of redox reversible couples and many more, and to use these properties, in this case, for producing potentiometric membrane sensors to monitor amine-containing drugs or other nitrogen-containing molecules. Sensors have been produced with this monoanionic cluster [3,3'-Co(1,2-C₂B₉H₁₁)₂]^-. Other monoanionic boron clusters are also discussed, but they are much fewer. It is noteworthy that most of the electrochemical sensor species incorporate an ammonium cation and that this cation is the species to be detected. Alternatively, the detection of the borate anion itself has also been studied, but with significantly fewer examples. The functions of the borate anion in the membrane are different, even as a doping agent for polypyrrole which was the conductive ground on which the PVC membrane was deposited. Apart from these cases related to closo borates, the bulk of the work has been devoted to sensors in which the θ metallabis (dicarbollide) [3,3'-Co(1,2-C₂B₉H₁₁)₂]^- is the key element. The metallabis (dicarbollide) anion, [3,3'-Co(1,2-C₂B₉H₁₁)₂]^-, has many applications; one of these is as new material used to prepare an ion-pair complex with bioactive protonable nitrogen containing compounds, [YH]_x[3,3'-Co(1,2-C₂B₉H₁₁)₂]_y as an active part of PVC membrane potentiometric sensors. The developed electrodes have Nernstian responses for target analytes, i.e., antibiotics, amino acids, neurotransmitters, analgesics, for some decades of concentrations, with a short response time, around 5 s, a good stability of membrane over 45 days, and an optimal selectivity, even for optical isomers, to be used also for real sample analysis and environmental, clinical, pharmaceutical and food analysis.

Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends

This article presents a comprehensive overview of recent progress in the design and applications of solid-contact ion-selective electrodes (SC-ISEs).

General description of the simultaneous response of potentiometric ionophore-based sensors to ions of different charge

The response of ion-selective membrane electrodes is usually still described with the semiempirical Nicolskii-Eisenman equation although it cannot correctly reproduce experimental data if ions of different charges are involved. The recently published self-consistent model that was derived for two ions of any charges is now extended to any number of ions of any charge. One single explicit equation is here given for the first time for any number of monovalent, divalent, and trivalent ions. Deviations relative to the Nicolskii-Eisenman equation are shown to be especially high at low interferences and bias calibrations if done in a mixed solution of a target sample as well as multivariate calibrations with sensor arrays.

Ion-selective electrodes with unusual response functions: simultaneous formation of ionophore-primary ion complexes with different stoichiometries

It is well known that the selectivity of an ion-selective electrode (ISE) depends on the stoichiometry of the complexes between its ionophore and the target and interfering ions. It is all the more surprising that the possibility for the simultaneous occurrence of multiple target ion complexes with different complex stoichiometries was mostly ignored in the past. Here, we report on the simultaneous formation of 1:1 and 1:2 complexes of a fluorophilic crown ether in fluorous ISE membranes and how this results in what looks like super-Nernstian responses. These increased response slopes are not caused by mass transfer limitations and can be readily explained with a phase boundary model, a finding that is supported by experimentally determined complex formation constants and excellent fits of response curves. Not only Cs(+) but also the smaller ions Li(+), Na(+), K(+), and NH(4)(+) form 1:1 and 1:2 complexes with the fluorophilic crown ether, with cumulative formation constants of up to 10(15.0) and 10(21.0) for of the 1:1 and 1:2 complexes, respectively. Super-Nernstian responses of the type observed with these electrodes are probably not particularly rare but have lacked in the past an adequate discussion in the literature, remaining ignored or misinterpreted. Preliminary calculations also predict sub-Nernstian responses and potential dips of a similar origin. The proper understanding of such phenomena will facilitate the development of new ISEs based on ionophores that form complexes of higher stoichiometries.

Generalized Selectivity Description for Polymeric Ion-Selective Electrodes Based on the Phase Boundary Potential Model

A generalized description of the response behavior of potentiometric polymer membrane ion-selective electrodes is presented on the basis of ion-exchange equilibrium considerations at the sample-membrane interface. This paper includes and extends on previously reported theoretical advances in a more compact yet more comprehensive form. Specifically, the phase boundary potential model is used to derive the origin of the Nernstian response behavior in a single expression, which is valid for a membrane containing any charge type and complex stoichiometry of ionophore and ion-exchanger. This forms the basis for a generalized expression of the selectivity coefficient, which may be used for the selectivity optimization of ion-selective membranes containing electrically charged and neutral ionophores of any desired stoichiometry. It is shown to reduce to expressions published previously for specialized cases, and may be effectively applied to problems relevant in modern potentiometry. The treatment is extended to mixed ion solutions, offering a comprehensive yet formally compact derivation of the response behavior of ion-selective electrodes to a mixture of ions of any desired charge. It is compared to predictions by the less accurate Nicolsky-Eisenman equation. The influence of ion fluxes or any form of electrochemical excitation is not considered here, but may be readily incorporated if an ion-exchange equilibrium at the interface may be assumed in these cases.

Current-polarized ion-selective membranes: The influence of plasticizer and lipophilic background electrolyte on concentration profiles, resistance, and voltage transients

Lipophilic background electrolytes consisting of a lipophilic cation and a lipophilic anion, such as tetradodecylammonium tetrakis(4-chlorophenyl) borate (ETH 500), or bis(triphenylphosphoranylidene) ammonium tetrakis[3,5bis(trifluoromethyl) phenyl] borate (BTPPATFPB) are incorporated into the membranes of ion-selective electrodes (ISEs) to improve the detection limit and selectivity of the electrodes and decrease the resistance of the sensing membrane. In this work, spectroelectrochemical microscopy (SpECM) is used in conjunction with chronopotentiometry to quantify the effects of a lipophilic background electrolyte on the concentration profiles induced inside current-polarized membranes and on the measured voltage transients in chronopotentiometric experiments. In agreement with the theoretical model, the lipophilic background electrolyte incorporated into o-NPOE or DOS plasticized membranes decreases the membrane resistance and thus the contribution of migration in the overall transport across ion-selective membranes. Consequently, it has a significant influence on the changing concentration profiles of the ion-ionophore complex during chronopotentiometric experiments.

Pulsed Galvanostatic Control of Ionophore-Based Polymeric Ion Sensors

This paper describes a pulsed galvanostatic technique to interrogate ion-selective electrodes (ISEs) with no intrinsic ion-exchange properties. Each applied current pulse is followed by a longer baseline potential pulse to regenerate the phase boundary region of the ion-selective membrane. The applied current fully controls the magnitude and sign of the ion flux into the membrane, thus offering instrumental control over an effect that has become very important in ion-selective electrode research in recent years. The resulting chronopotentiometric response curves essentially mimic traditional ISE behavior, with apparently Nernstian response slopes and selectivities that can be described with the Nicolsky equation. Additionally, the magnitude and sign of the current pulse may be used to tune sensor selectivity. Perhaps most important, however, appears to be the finding that the extent of concentration polarization near the membrane surface can be accurately controlled by this technique. A growing number of potentiometric techniques are starting to make use of nonequilibrium principles, and the method introduced here may prove to be very useful to advance these areas of research. The basic characteristics of this pulsed galvanostatic technique are here evaluated with plasticized poly(vinyl chloride) membranes containing the sodium-selective ionophore tert-butyl calix[4]arene tetramethyl ester and a lipophilic inert salt.

A Generalized Model for Apparently “Non-Nernstian” Equilibrium Responses of Ionophore-Based Ion-Selective Electrodes. 1. Independent Complexation of the Ionophore with Primary and Secondary Ions

A generalized model that describes apparently "non-Nernstian" equilibrium responses of ionophore-based ion-selective electrodes (ISEs) is presented. It is formulated for primary and secondary ions of any charges that enter the membrane phase and independently form complexes with the ionophore, respectively. Equations for the phase boundary potential model were solved numerically to obtain whole response curves as a function of the sample activity of the primary ion, and analytical solutions could be obtained for apparently non-Nernstian response sections in these response curves. Ionophore-based ISEs can give three types of apparently non-Nernstian equilibrium responses, i.e., apparently "super-Nernstian", "inverted-Nernstian", and "sub-Nernstian" responses. The values of the response slopes depend on the charge numbers of the primary and secondary ions and on the stoichiometries of their complexes with the ionophore. The theoretical predictions for super-Nernstian responses agree well with the experimental results obtained with ISEs based on acidic ionophores or metalloporphyrin ionophores. Also, theoretical response curves with inverted-Nernstian slopes were found to be similar in character to the pH responses of Ca2+-selective electrodes based on organophosphate ionophores, which have been known to exhibit a so-called "potential dip". The quantitative understanding of apparently non-Nernstian response slopes presented here provides an insight into ionophore-analyte complexation processes in ISE membranes and should be helpful for the design of new ionophores.

Rational design of potentiometric trace level ion sensors. A Ag+-selective electrode with a 100 ppt detection limit

Submicromolar to picomolar lower detection limits have recently been obtained with various polymer membrane ion-selective electrodes by minimizing biases due to ion fluxes through the membrane. For the best performance, the compositions of the membrane and inner solution should be optimized for each application. Given the number of parameters to be adjusted, it has been difficult to find the best parameters for a target sample. In this paper, a much simplified and more practical steady-state model of zero-current ion fluxes is derived, which is based on measurable parameters. The model allows one to predict achievable lower detection limits for a membrane with given selectivities. It can also be used to predict the optimal composition of the inner filling solution for the measurement of samples with a known, typical ionic background. Selectivity coefficients of monovalent and divalent analyte ions required for desired detection limits in drinking water are calculated. As an application of the proposed general recipe, a silver-selective electrode is developed on the basis of the ionophore O,O''-bis[2-(methylthio)ethyl]-tert-butylcalix[4]arene. With the predicted optimal composition of the inner electrolyte, its lower detection limit is found to be 10(-9) M or 100 ppt Ag+ with an ionic background of 10(-5) M LiNO3, which is very close to the expected value.

Spectroscopic method for the determination of the ionic site concentration in solvent polymeric membranes and membrane plasticizers

The built-in site density of either fixed sites or mobile hydrophobic ion sites determines whether a membrane is permselective for cations or anions of the sample. The molar ratio of the ionophore to the intrinsic or added ionic sites in an ion-selective membrane significantly influences the potentiometric response of ionophore-based electrodes. Consequently, full knowledge of the "site inventory" in an ion-selective membrane maybe essential when new, uncharacterized polymers or plasticizers are implemented for ion-selective electrode fabrication. A simple spectroscopic method was developed for the fast and accurate determination of the ionic site concentration (covalently attached functionalized groups or impurities) in plasticized polymeric membranes and membrane plasticizers. The method is based on the determination of the degree of protonation of hydrogen ion-selective chromoionophores incorporated into these membranes or dissolved in the membrane plasticizers. In electroneutral membranes, the concentration of the positively charged, protonated ionophore and the total concentration of negative sites are equal. The method was applied for the determination of ionic sites (both positively and negatively charged) in PVC materials (different purity grade, and bearing various functional groups), polyurethanes (aliphatic, aromatic, and polycarbonate-based), and selected Fluka plasticizers (2-nitrophenyl octyl ether and 2-ethylhexyl sebacate). The technique proved to be appropriate for fast quantification of ionic impurities in hydrophobic, optically transparent materials.

Evaluation of the separate equilibrium processes that dictate the upper detection limit of neutral ionophore-based potentiometric sensors

The upper detection limit of polar ionophore-based ion-selective electrode membranes is predicted by utilizing the coextraction constant of dissociated electrolyte, the stability constant of the ionophore, and the membrane composition. The coextraction constant of dissociated electrolytes into the polar poly(vinyl chloride) membrane plasticized with o-nitrophenyl octyl ether (PVC-NPOE) is here measured by a novel approach. The sandwich membrane technique is utilized, with one membrane segment containing a lipophilic cation exchanger and the other containing an anion exchanger. This yields information about the coextraction constant and the free ion concentrations of the electrolyte in the two segments. Predictions correlate quantitatively with the upper detection limit observed for ion-selective electrodes based on the ionophores valinomycin, tert-butylcalix[4]arene tetraethyl ester, and calcimycin. The difficulties of the prediction of the upper detection limit for nonpolar poly(vinyl chloride) membranes plasticized with bis(2-ethylhexyl sebacate) (PVC-DOS) due to ion association are discussed in detail. A thermodynamic cycle experiment with a series of sandwich membranes shows that the principal processes governing the upper detection limit of PVC-DOS membranes are identical to those for the PVC-NPOE membranes. However, the stability of the ion pairs between the ionophore-metal ion complexes and the extracted anion are different from that of ion pairs formed between the same anion and the lipophilic anion exchanger. This makes it difficult to quantitatively predict the upper detection limit on the basis of simple apparent coextraction and complexation data alone. The approach reported herein is useful not only for mechanistic purposes but also to shed light onto the many cases where coextraction effects need to be understood but are not directly experimentally accessible.

Selectivity of lithium electrodes: correlation with ion-lonophore complex stability constants and with interfacial exchange current densities

Lithium-selective electrodes with solvent polymeric membranes based on two different dicyclohexylamide neutral ionophores are studied systematically. The selectivity of lithium response is studied by means of the ordinary potentiometric experiments. Stability constants of lithium, sodium, and potassium ions with the neutral ionophores are measured by means of the segmented sandwich membrane method. Charge transfer through the membrane bulk and across the membrane/solution interface is studied by means of electrochemical impedance spectroscopy. Well-resolved Faradaic impedance semicircles are obtained, allowing calculation of exchange current densities for lithium, sodium, and potassium. It is clearly demonstrated that the potentiometric selectivity coefficients correlate well with thermodynamic equilibrium parameters. The correlation with exchange current densities also exists, although it is low, and seems rather qualitative than quantitative. The results are treated in favor of equilibrium at the membrane/solution interface. It is also concluded (tentatively) that the kinetic description is equivalent to the equilibrium one, giving evidence that ion-ionophore complexes form directly at the interface.

Potentiometric estimation of the stability constants of ion-lonophore complexes in ion-selective membranes by the sandwich membrane method: theory, advantages, and limitations

Segmented sandwich membrane method of studying stoichiometry and stability constants of ion-ionophore complexes in ion-selective membranes is considered in detail. The experimental data (reported earlier in Russian) concerning complexes of various ions with valinomycin, with H+-selective neutral ionophore hexabutyltriamidophosphate, and with anion-binding neutral ionophore p-hexyl trifluoroacetylbenzoate is presented in a compact form. Advantages of titration technique in the sandwich membrane method (the presence of an internal criterion of reliability, and the possibility of direct determination of complex stoichiometry coefficients) are specially addressed. Biases of the estimates of the constants caused by ion-pair formation in real membranes and by diffusion potential are analyzed by means of computer simulations. The possibility of revealing two coexisting complexes with different compositions is also discussed.

Threshold ionic site concentrations required for Nernstian potentiometric responses of neutral ionophore-incorporated ion-selective liquid membranes

An equation that can describe the concentrations of ionic sites required for a Nernstian potentiometric response slope of neutral ionophore-incorporated ion-selective liquid membranes is presented. This equation is derived from a model based on electrical diffuse layers on both the membrane and the aqueous sides of the interface, in which the phase boundary potential is correlated to the surface charge density as well as the salt concentrations in the bulk membrane and aqueous solution. To experimentally and accurately confirm the validity of this equation, response characteristics of field effect transistors covered by neutral ionophore-based liquid membranes with varying concentrations of a derivative of tetraphenylborate as an anionic site but free of ionic impurities were examined. The observed membrane potentials and the response slopes for membranes with various concentrations of anionic sites were in good agreement with the values calculated from the theory presented in this paper with the measured complexation stability constants for the relevant systems. This result indicates that the theoretical prediction based on the proposed equation for the anionic site concentration is accessible for the preparation of neutral ionophore-incorporated ion-selective liquid membranes, which show Nernstian response slopes for the primary ions.

Studies on the matched potential method for determining the selectivity coefficients of ion-selective electrodes based on neutral ionophores: experimental and theoretical verification

A theory is presented that describes the matched potential method (MPM) for the determination of the potentiometric selectivity coefficients (KA,Bpot) of ion-selective electrodes for two ions with any charge. This MPM theory is based on electrical diffuse layers on both the membrane and the aqueous side of the interface, and is therefore independent of the Nicolsky-Eisenman equation. Instead, the Poisson equation is used and a Boltzmann distribution is assumed with respect to all charged species, including primary, interfering and background electrolyte ions located at the diffuse double layers. In this model, the MPM-selectivity coefficients of ions with equal charge (ZA = ZB) are expressed as the ratio of the concentrations of the primary and interfering ions in aqueous solutions at which the same amounts of the primary and interfering ions permselectively extracted into the membrane surface. For ions with unequal charge (ZA not equal to ZB), the selectivity coefficients are expressed as a function not only of the amounts of the primary and interfering ions permeated into the membrane surface, but also of the primary ion concentration in the initial reference solution and the delta EMF value. Using the measured complexation stability constants and single ion distribution coefficients for the relevant systems, the corresponding MPM selectivity coefficients can be calculated from the developed MPM theory. It was found that this MPM theory is capable of accurately and precisely predicting the MPM selectivity coefficients for a series of ion-selective electrodes (ISEs) with representative ionophore systems, which are generally in complete agreement with independently determined MPM selectivity values from the potentiometric measurements. These results also conclude that the assumption for the Boltzmann distribution was in fact valid in the theory. The recent critical papers on MPM have pointed out that because the MPM selectivity coefficients are highly concentration dependent, the determined selectivity should be used not as "coefficient", but as "factor". Contrary to such a criticism, it was shown theoretically and experimentally that the values of the MPM selectivity coefficient for ions with equal charge (ZA = ZB) never vary with the primary and interfering ion concentrations in the sample solutions even when non-Nernstian responses are observed. This paper is the first comprehensive demonstration of an electrostatics-based theory for the MPM and should be of great value theoretically and experimentally for the audience of the fundamental and applied ISE researchers.

Modeling of divalent/monovalent ion selectivity of ion-exchanger-based solvent polymeric membranes doped with coexchanger

It is shown that the addition of a coexchanger does not always improve the selectivity of an ion-selective electrode. As a model case, the selectivity toward divalent primary ions I2+ in the presence of monovalent interference J+ is numerically simulated for membranes based on ion-exchanger R- and doped with a coexchanger (ionic additive) S+. It is, for the first time, demonstrated that the addition of a coexchanger, together with a high mobility of ion-exchanger sites, may be either beneficial or harmful to selectivity. Accordingly, two distinctive representatives of systems, i.e., membrane/solution are discussed. The theory is illustrated with experimental data obtained from PVC membranes based on calcium bis(2ethylhexyl)-phosphate plasticized with tris(2ethylhexyl)phosphate and doped with tetradecylammonium. Electrodes with these membranes exhibit both types of selectivity behavior. Interference from lithium with a calcium response may be sufficiently decreased by the addition of tetradecylammonium, while the interference from potassium and cesium was increased in the doped membranes.

Determination of complex formation constants of lipophilic neutral ionophores in solvent polymeric membranes with segmented sandwich membranes

A potentiometric method to determine ionophore complex formation constants in solvent polymeric membrane phases, proposed originally by Russian researchers, is critically evaluated and compared to other established methods. It requires membrane potential measurements on two-layer sandwich membranes, where only one side contains the ionophore. The resulting initial membrane potential reflects the ion activity ratio at both aqueous phase--membrane interfaces and can be conveniently used to calculate complex formation constants in situ. This method is potentially useful, since it does not require the use of a reference ion or second ionophore in the measurement. In this paper, the five ionophores valinomycin, BME-44, ETH 2120, tert-butylcalix[4]arene tetraethyl ester, and S,S'-methylenebis(diisobutyldithiocarbamate) are characterized in poly(vinyl chloride) (PVC) plasticized with dioctyl sebacate (DOS) and compared with other established methods. The resulting formation constants correspond well to literature values. The influence of varying membrane concentrations and different anionic site additives is studied and found to be relatively small. Experiments are also performed with and without lipophilic inert electrolytes and with ionophore-free sandwich membranes to illustrate the effect of ion pairing and the membrane internal diffusion potential on the response of such sandwich membranes. These experiments suggest that ions are completely associated in PVC-DOS membranes, but that such ion pairs are rather nonspecific. Diffusion potentials seem to play a minor role with these systems. The results are explained with theory. This work indicates that the characterization of electrically charged ionophores, anion-selective ionophores, and ionophores in membrane matrixes other than PVC plasticized with DOS may now be experimentally accessible.

Voltammetric and amperometric transduction for solvent polymeric membrane ion sensors

This paper describes basic response features of solvent polymeric membrane ion sensors with voltammetric and amperometric transduction. The model systems used here contain no ionophore for simplicity reasons. Reasonable simplifications of the theory are introduced that allow one to understand the response mechanism in view of a practical application of these sensors. It is shown that ion-sensing membranes preferentially contain no ion-exchanger properties in order to function optimally in a voltammetric mode. As with the systems studied by Kihara, both liquid-polymer interfaces of the membrane are preferably polarizable. Specifically, they contain the highly lipophilic electrolyte tetradodecylammonium tetrakis(4-chlorophenyl)borate (ETH 500) in the membrane to improve lifetime, increase the magnitude of the potential window, and prohibit exchange reactions with sample ions. An ohmic behavior that is associated with an assisted electrolyte-transfer process is observed only above a threshold potential which can be quantitatively predicted by theory. The threshold potential depends on the nature and activity of sample anions and cations in the sample and inner filling solution of the membrane electrode. Within the experimental conditions discussed in this paper, these sensors seem to measure sample ion activities, not concentrations, since the rate-limiting step is the diffusion of extracted ions away from the interface into the membrane bulk. Similarly, no effect of sample stirring on the measured current is observed. This contrasts to work done on liquid-liquid electrolyte-transfer reactions, where large diffusion coefficients in the organic phase often lead to substantial sample depletion effects. The detection of anions and cations with the same membrane is demonstrated in a cyclic voltammetric mode. Direct continuous detection of one type of anion is accomplished by pulsed amperometry to ensure a rapid, repetitive renewal of the membrane composition between measurements.