Phase Boundary Potential Model Research Articles

Titanophosphate glasses as lithium-free nonsilicate pH-responsive glasses—Compatibility between pH responsivity and self-cleaning properties

Lithium silicate-based glasses have been widely used as commercially available pH glass electrodes. It was revealed that Ti3+-containing titanophosphate (TiO2–P2O5, TP) glasses are pH-responsive as lithium-free nonsilicate glasses for the first time. The absorption coefficient at 532nm, α532 as a measure of Ti3+ content in TP glasses increased with increasing melting temperature. TP glasses with large α532 tended to give low electrical resistivity, high pH sensitivity and the short pH response time. The first post-annealing (oxidation of Ti3+) of TP glasses at 600–620°C for 60–240h resulted in the occurrence of the photo-induced hydrophilicity along with the disappearance of pH responsivity and the increase of electrical resistivity. The second post-annealing (reduction of Ti4+) of the first post-annealed TP glasses at 600–620°C for 48h under vacuum recovered both pH responsivity and electrical resistivity to the level of the as-prepared TP glasses with maintaining the photo-induced hydrophilicity. Moreover, the second post-annealed TP glasses had photocatalytic activity for methylene blue (MB) comparable to commercially available self-cleaning glass. Thus, TP glasses with the compatibility between pH responsivity and self-cleaning properties were obtained by the sequential post-annealing (oxidation and reduction) of as-prepared glasses. From some circumstantial evidences, pH response of TP glasses was explained in terms of phase boundary potential model related to hopping conduction of electron from Ti3+ to Ti4+ via O2− ion in TP glasses rather than diffusion potential model.

Revisiting the Response Mechanism of Polymeric Membrane Based Heparin Electrodes

Potentiometric membrane electrodes that respond to heparin and other polyanions were introduced in the early 1990s. Herein, the mechanism of polymer membrane electrode type heparin sensors is revisited. The extraction/diffusion of heparin is studied via both potentiometric and impedance spectroscopic techniques using a pre-fractionated heparin preparation that contains polyanionic species > 10000 Daltons. The reversal in EMF response using this heparin preparation indicates diffusion of higher MW heparin fragments to the backside of the membrane. Diffusion coefficients are calculated using a novel formula derived from the phase boundary potential model and Fick's second law of diffusion. Impedance spectroscopy is also employed to show that high MW heparin species are extracted and diffuse across the PVC membranes.

Potentiometric determination of coextraction constants of potassium salts in ion-selective electrodes utilizing a nitrobenzene liquid membrane phase

A theoretical treatment of potentiometric data is applied to calculate coextraction constants ( K IA) for three potassium salts from water into a liquid nitrobenzene phase. The experiment involves treating nitrobenzene as a membrane and contacting it with two aqueous solutions of different ion activities. In the presence of either a cation or anion exchanger, the ratio of activities of ions in the two aqueous phases gives rise to a potential difference across the membrane that depends upon the nature and charge of the counter ion of the ion-exchanger in excess. Here, the cation exchanger was chosen to be potassium tetrakis(4-chlorophenyl)borate (KTpClPB) and the anion exchanger was tetradodecylammonium chloride (TDDACl). TDDACl was incrementally added to the nitrobenzene phase containing a fixed concentration of KTpClPB, and the corresponding emf was recorded as a function of concentration of TDDACl. The membrane changes from one with cation exchanger properties (excess KTpClPB) to one with anion exchanger properties (excess TDDACl). The potential difference and shape of the titration curve can be predicted by theory based on the phase boundary potential model. Log( K IA) values calculated for KCl, KNO 3 and KClO 4 in nitrobenzene were found as: −10.53 (± 0.09), −8.16 (± 0.05) and −5.63 (± 0.03) respectively, in accordance with the Hofmeister series of lipophilicity, and similar to those observed in PVC membranes containing other plasticizers. The method presented here offers the advantage over other methods to calculate K IA, in that it is relatively experimentally simple without compromising the accuracy of the calculated coextraction constants. The ability to titrate directly into the liquid membrane phase affords a higher precision compared to the preparation of a series of PVC/plasticizer membranes with different compositions.

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.

Ion-selective liquid electrodes: Problems of description and experimental determination of selectivity

Current state of selectivity theory for liquid-membrane ion-selective electrodes was considered. Analytical expressions for the selectivity coefficients of electrodes based on liquid ion exchangers, neutral carriers, and charged carriers were obtained using phase boundary potential model. The reasons responsible for the dependence of experimentally determined selectivity coefficients on the determination conditions are discussed. The main ways for the electrode design optimization providing dramatic improvement of the selectivity are considered.

Potentiometric and impedance studies of membranes based on anion-exchanger and lipophilic inert electrolyte ETH 500

Solvent polymeric membranes based on quaternary ammonium salt tridodecylmethylammonium chloride (TDDMACl) with variable concentration of lipophilic inert electrolyte (ETH 500), plasticized with solvents of different nature were investigated by potentiometry and electrochemical impedance spectroscopy. DOS-plasticized membranes exhibited increased selectivity to divalent anions in agreement with the phase-boundary potential model. Conductivity mechanism in such membranes was proposed on the basis of the data on conductivity, activation energy of conductivity and pre-exponential factor and assuming the formation of electrolyte-containing water droplets in the membrane, facilitated by ETH 500.

Investigation of ion-selective electrodes with neutral ionophores and ionic sites by EIS. I. Theory

Models of an ion selective electrode involving an ionophore and mobile sites in a membrane are proposed. The first model, called the phase boundary potential model, supposed thermodynamic equilibrium; it allows the concentrations of the various species to be calculated. Then, a kinetic model, which takes into account the ionic transfer at the membrane|solution interfaces, was derived. The impedance of the membrane was calculated. It shows that a membrane with nernstian behavior shows only one capacitive loop in the impedance diagram, which is related to the conductivity and dielectric properties of the material of the membrane. Non-nernstian behavior is related to slow ionic transfer at the membrane|solution interfaces or/and transport limitation of the species in the membrane. Finite rate constants of the ionic transfer lead to a capacitive loop in the middle frequency range, whereas finite rate transport leads to a diffusional impedance in the low frequency range.

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.

Cationic or anionic sites? Selectivity optimization of ion-selective electrodes based on charged ionophores

The influence of ionic sites on the selectivities of ionophore-based ion-selective electrodes (ISEs) is described on the basis of a phase boundary potential model. The discussion presented here is significantly more general than previous ones. It is formulated for primary and interfering ions of any charges and it is valid for ISEs based on electrically charged or neutral ionophores. Furthermore, it also applies to membranes that contain more than one type of complex of the primary or interfering ion. It has been believed thus far that only ionic sites of the same charge sign as the primary ion improve the selectivities of ISEs based on charged ionophores. However, it is shown here that the charge sign of the ionic sites that give the highest potentiometric selectivities depends on the charge number of the primary and interfering ions and on the stoichiometry of their complexes with the ionophore. The validity of our model was confirmed experimentally with three ISEs based on different charged ionophores. ISEs based on lasalocid or 1-(N,N-dicyclohexylcarbamoyl)-2- (N,N-dioctadecylcarbamoyl)ethylphosphonic acid monomethyl ester (ETH 5639) as the ionophore responded selectively to Sr2+ or Mg2+, respectively, and discriminated well against other alkaline earth cations when their membranes contained anionic sites. These two electrodes are the first examples of ISEs based on charged ionophores for which maximum selectivities are obtained with membranes containing ionic sites with a charge sign opposite to that of the primary ion. On the other hand, the experimental F- selectivities of membranes based on oxo(5,10,15,20-tetraphenylporphyrinato)molybdenum-(V) improved gradually when the concentration of anionic sites was increased from 0 to 75 mol%. The selectivity-modifying influence of ionic sites for these three types of ISEs can be explained by considering the different stabilities of the 1:2 ion-ionophore complexes of the primary and of the interfering ions.

Polymeric Membrane pH Electrodes Based on Electrically Charged Ionophores

Solvent polymeric membranes containing electrically charged H+ carriers show dramatic variations in their pH response behavior depending on the charge type of lipophilic ionic sites present in the membrane. These shifts are predicted with a simplified phase boundary potential model. Membranes without ion-exchanger sites are expected to show half-Nernstian electrode slopes with extended measuring ranges if the ionophore has acidic as well as basic properties. Relevant experiments with membrane electrodes on the basis of the electrically charged carriers 4‘,5‘-dibromofluorescein octadecyl ester (ETH 7075) and 5-(octadecanoyloxy)-2-(4-nitrophenylazo)phenol (ETH 2412), lipophilic indicators formerly used in optode films, confirm these predictions. It is shown that the incorporation of tridodecylmethylammonium chloride into NPOE−PVC membranes with ETH 7075 induces a dynamic range from 4 to 10.4 in 0.1 M KCl while membranes with ETH 2412 show a dynamic range that is shifted by ∼4 orders of magnitude toward high...

A Phase Boundary Potential Model for Apparently “Twice-Nernstian” Responses of Liquid Membrane Ion-Selective Electrodes

A model that describes divalent cation responses of liquid membrane ion-selective electrodes based on acidic ionophores and ionic sites is presented. Response slopes for membranes with ionophore and anionic sites are predicted to change from Nernstian to apparently "twice-Nernstian" and then back to Nernstian again as the pH of the sample solution decreases. A maximum measuring range for apparently "twice-Nernstian" responses is expected for membranes with 50 mol % anionic sites relative to the ionophore. On the other hand, membranes with ionophore and cationic sites are expected to give only Nernstian responses, either to divalent cations at high pH or to H(+) at low pH. The validity of the present model has been confirmed experimentally with the two Ba(2+)-selective carboxylate ionophores monensin and lasalocid and the Ca(2+)-selective organophosphate ionophore bis(2-heptylundecyl) phosphate. Addition of anionic sites gave apparently "twice-Nernstian" slopes for monensin at pH 7.0 (56.6 mV/decade), for lasalocid at pH 4.0 (53.3 mV/decade), and for bis(2-heptylundecyl) phosphate at pH 3.5 (53.6 mV/decade). Membranes with cationic sites showed only pH responses at the respective pH. The apparently "twice-Nernstian" responses as discussed here are the first examples of super-Nernstian responses that can be explained with a quantitative model based on thermodynamic equilibria.

Lipophilic Ionic Sites for Solvent Polymeric Membrane pH Electrodes Based on 4′,5′‐Dibromofluorescein Octadecylester as Electrically Charged Carrier

The effect of salt additives on the potentiometric response of solvent polymeric membrane pH electrodes based on the electrically charged carrier 4′,5′‐dibromofluorescein octadecylester (ETH 7075) is evaluated. The incorporation of potassium tetrakis(p‐chlorophenyl)borate as anionic site forces the carrier to reveal its weakly basic properties, only showing a Nernstian pH response below pH 2.5 in 0.1 M KCl. In contrast, the presence of tridodecylmethylammonium chloride as cationic sites induces an electrode function that reflects the analytically useful acidic characteristics of the carrier. The latter system shows a pH measuring range between pH 3.5 and 10 and a selectivity coefficient over potassium of log . The differences in response behavior are explained qualitatively on the basis of the phase boundary potential model.

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

AbstractRecent experimental evidence suggests that the equilibrium partitioning of sample ions at the sample/membrane interface is the main parameter governing the potentiometric response of polymeric membrane‐based ion‐selective electrodes (ISEs). It is shown that the response of a neutral‐carrier‐based H+‐selective electrode can be fully predicted on the basis of equilibrium concentrations measured optically within a thin organic film having the same composition as the ISE membrane. Consequently, using this simple and powerful phase boundary potential model together with mass balances and observed complex formation constants, the response and selectivity of various ISE membranes may be described. In this contribution, the most recent applications of the model are reviewed including: (1) a novel and general selectivity description, which is related to the so‐called matched potential method and clearly shows the limitations of the extended Nicolsky–Eisenman equation if ions of different charge are considered; (2) the measuring range of neutral‐carrier‐based H+‐selective ISEs, showing that previous experimental findings can now be explained by theory; (3) prediction of the influence of anionic and cationic sites on the selectivity of charged‐carrier‐based ISEs demonstrating that such ISE membranes need the incorporation of sites of the same charge type as the analyte ion to induce optimum potentiometric selectivity; and (4) the determination of the concentration of anionic impurities in poly(viny1 chloride) by measuring the influence of the anionic site concentration on the divalent/monovalent ion selectivity of a neutral‐carrier‐based ISE.