Matched Potential Method Research Articles

Selective electrochemical sensor for copper (II) ion based on chelating ionophores

Plasticized membranes using 3-(2-pyridinyl)-2H-pyrido[1,2,-a]-1,3,5-triazine-2,4(3H)-dithione ( L 1) and acetoacetanilide ( L 2) have been prepared and explored as Cu 2+-selective sensors. Effect of various plasticizers, viz. chloronaphthalene (CN), benzyl acetate (BA), o-nitrophenyloctyl ether ( o-NPOE), and anion excluders, sodium tetraphenylborate (NaTPB) and oleic acid (OA) was studied in detail and improved performance was observed at several instances. Optimum performance was observed with dithione derivative ( L 1) having a membrane composition of L 1 (5):PVC (120): o-NPOE (240):OA (10). The sensor works satisfactorily in the concentration range 5.0 × 10 −8 to 1.0 × 10 −2 M (detection limit 4.0 × 10 −8 M) with a Nernstian slope of 29.5 mV decade −1 of activity. Wide pH range (3.0–9.5), fast response time (12 s), non-aqueous tolerance (up to 20%) and adequate shelf life (4 months) indicate the vital utility of the proposed sensor. The potentiometric selectivity coefficient values as determined by match potential method (MPM) indicate good response for Cu 2+ in presence of interfering ions. The proposed electrode comparatively shows good selectivity with respect to alkali, alkaline earth, transition and some rare earth metals ions. The electrode was used for the determination of copper in different milk powder, water samples and as indicator electrode in potentiometric titration of copper ion with EDTA.

Novel Poly(ethylene glycol)s Bearing Tributyltin Carboxylate End Groups as Ionophores in the Development of Chloride Ion‐Selective Electrodes

AbstractThe potentiometric response characteristics of electrodes based on PVC membranes containing novel polyethylene glycols (PEGs) with tributyltin carboxylate end groups as ionophores for chloride ions were studied in 0.1 M HEPES solution, at the spontaneous pH. The effects of solvent mediator, amount of cationic additive, amount of ionophore and PEG chain length on the behavior of the sensors were investigated. The membranes with the best composition responded to chloride concentration in a linear range from 10−4 to 10−1 M Cl− with a nearly Nernstian slope and a detection limit of 6.5×10−5 M. The sensor showed a short response time (<25 s) in the whole concentration range and an operational lifetime of about one week for the most performing PVC membranes. In comparison with ISEs based on anion exchangers the interferences from the more lipophilic anions were greatly reduced, as inferable by the selectivity coefficients determined with the matched potential method at chloride concentration of 3.0×10−3 M.

A newly synthesized macrocyclic dithioxamide receptor for phosphate sensing

Polyvinyl chloride (PVC) based membranes using macrocyclic dithioxamide receptor ( I) derived from isophthaloyl dichloride and dithioxamide have been prepared and explored as HPO 4 2−-selective sensors. Effect of various plasticizers viz., bis(2-ethylhexyl) sebacate (DOS), dibutylphosphate (DBP), tri- n-butylphosphate (TBP), O-nitrophenyl octyl ether (NPOE), tris(2-ethylhexyl)phosphate (TEHP) and a cation excluder, tridodecylmethylammonium chloride (TDDMACl) was studied in detail and improved performance was observed at several instances. Optimum performance was observed with the membrane having ( I)-PVC–TDDMACl–NPOE in the ratio 2:33:1.5:63.5 (w/w). The sensor works satisfactorily in the concentration range 1.7 × 10 −6 to 1.0 × 10 −2 M (detection limit 0.2 ppm) with Nernstian compliance (29.6 mV/decade of activity) at pH 8.0 with a fast response time of about 8 s. The potentiometric selectivity coefficient values as determined by the matched potential method (MPM) and the fixed interference method (FIM) indicate selective response for HPO 4 2− in presence of interfering ions. The sensor exhibits adequate shelf life (∼2 months) with good reproducibility (S.D. ± 0.4 mV). The sensor was also used successfully in the potentiometric titration of HPO 4 2− with Ba 2+.

A comparative study of Pb 2+ selective sensors based on derivatized tetrapyrazole and calix[4]arene receptors

Two neutral ionophores, 2,12-dimethyl-7,17-diphenyltetrapyrazole ( I) and 5,11-dibromo-25,27-dipropoxycalix[4]arene ( II) have been explored for preparing PVC based membrane sensors selective to Pb 2+. The addition of sodium tetraphenylborate and various plasticizers viz. DOS, TEHP, DBP, DOP and TBP has been found to substantially improve the performance (working concentration range, slope and response time) of the sensors. The best performance was obtained with the sensor having a membrane of composition (w/w) of ( I) (1%):PVC (33%):TBP (65%):NaTPB (1%). The sensor exhibits Nernstian response in the concentration range 2.5 × 10 −6 to 5.0 × 10 −2 M Pb 2+, performs satisfactorily over wide pH range (1.6–6.0) with a fast response time (∼10 s). The sensor was found to work satisfactorily in partially non-aqueous media up to 25% (v/v) content of acetone, methanol or ethanol and could be used over a period of 5 months. Potentiometric selectivity coefficients as determined by match potential method (MPM) indicate excellent selectivity for Pb 2+ ions. The sensors could be used successfully in the estimation of lead in Eveready battery waste and also as an indicator electrode in potentiometric titration.

Selective anion recognition: Charged diaza crown ethers based electrochemical sensors for chromate ions

Attempts have been made to develop a poly(vinyl chloride) (PVC) based sensor for CrO 4 2− ions using positively charged diaza crown ethers (12-crown-4 and 18-crown-6) with dibutylphthalate (DBP), dioctylphthalate (DOP) and chloro naphthalene (CN) and dioctylsebacate (DOS) as plasticizing solvent mediators. These sensors work well over a wide concentration range 5.0 × 10 −6 to 1.0 × 10 −1 M (detection limit up to 0.3 ppm) with Nernstian slope (29.6 mV/decade) between pH 6.5 to 10.0 with a fast response time of <10 s. The selectivity coefficient values as determined by Match Potential Method (MPM) indicate excellent selectivity for Cr(VI) ions over a large number of ions. Interference caused by common anions, viz. Cl − and SO 4 2− has been investigated in simulated mixtures containing high concentration level of interfering ions and the sensor was found to be tolerant against these anions. These sensors exhibit adequate shelf-life (>3 months) with good reproducibility (S.D. ±0.2 mV). Quantification of Cr(VI) in electroplating waste was successfully achieved using the proposed sensor.

Chelating ionophore based membrane sensors for copper(II) ions

Acetylacetone, ethylacetoacetate and salicyldehyde, are reported to form chelates with copper of high stability as compared to other metals. Therefore, PVC based membranes of bis[acetylacetonato] Cu(II) ( A), bis[ethylacetoacetate] Cu(II) ( B) and bis[salicyldehyde] Cu(II) ( C) have been investigated as copper(II) selective sensors. The addition of sodium tetraphenylborate and various plasticizers, viz., DOS, TEHP, DOP, DBP and TBP have been found to substantially improve the performance of the sensors. The membranes of various compositions of the three chelates were investigated and it was found that the best performance was obtained for the membrane of composition A (1%): PVC (33%): TBP (65%): NaTPB (1%). The sensor shows a linear potential response to Cu(II) over wide concentration range 2.0 × 10 −6 to 1.0 × 10 −1 M (detection limit ∼0.1 ppm) with Nernstian compliance (29.3 mV decade −1 of activity) between pH 2.6 and 6.0 with a fast response time of ∼9 s. The potentiometric selectivity coefficient values as determined by match potential method (MPM) indicate excellent selectivity for Cu 2+ ions over interfering cations. The sensor exhibits adequate shelf life (∼3 months) with good reproducibility (S.D. ±0.2 mV). The sensor has been used in the potentiometric titration of Cu 2+ with EDTA. The utility of the sensor has been tested by determining copper in vegetable foliar and multivitamin capsule successfully.

Oligocarbazoles as Ligands for Lead-selective Liquid Membrane Electrodes

Oligocarbazoles have been applied as new ionophores in liquid membrane electrodes (ISEs) destined for lead(II) determination in water samples. The oligocarbazole-containing ISEs demonstrated a close-to-Nernstian potentiometric response towards Pb2+ in the activity range 10(-7)-10(-2) M. The selectivity coefficients measured by the matched potential method (MPM) confirmed their good selectivity against common interfering mono- and doubly charged cations. The oligocarbazole-containing ISEs do not respond towards protons. Their applicability has been checked by performing the recovery test while using a sample of wastewater.

A lipophilic sol-gel matrix for the development of a carbonate-selective electrode.

Organic-inorganic hybrid sol-gel matrixes were used as hosts for trifluoroacetyl-p-decylbenzene (TFADB), a traditional ionophore for carbonate. The sol-gel precursor was prepared by the reaction of (3-isocyanopropyl)triethoxysilane with ethylene glycol. Hexadecyltrimethoxysilane (HDTMOS) was employed as a co-precursor. An appropriate amount of tridodecylmethylammonium chloride (TDMAC) and 2-nitrophenyloctyl ether (NPOE) were used as membrane components. On mixing with an acidic catalyst, the sol-state precursors slowly gelled, yielding a membrane in which the active components, TFADB and TDMAC, were encapsulated. Infrared, (1)H, and (29)Si MAS NMR spectrometers were employed to monitor the sol-gel process and the degree of polymerization. The performances of the sol-gel membrane-based electrodes were compared to those of TFADB-based poly(vinyl chloride) (PVC) membrane electrodes. Membranes with a molar ratio of TFADB:TDMAC (1:0.14) showed extended lifetime and stable baseline potential. The response slope toward carbonate was approximately 27 mV/decade between 10(-)(5) and 10(-)(3.5) M at 18 degrees C. Interestingly, selectivity toward carbonate over salicylate and other lipophilic anions was improved, clearly deviating from the Hofmeister selectivity pattern. Responses toward small inorganic anions including chloride and sulfate were negligible. The selectivity coefficients measured by the matched potential method in 0.1 M tris-sulfuric acid buffer, pH 8.75, were log = -0.3, log = -4.2, and log = -2.5.

Copper(II) selective electrochemical sensor based on Schiff Base complexes

Plasticized membranes using Schiff Base complexes, derived from 2,3-diaminopyridine and o-vanilin have been prepared and explored as Cu 2+-selective sensors. Effect of various plasticizers viz., dibutyl phthalate (DBP), dioctylphthalate (DOP), chloronaphthalene (CN), tri- n-butylphosphate (TBP) etc. and anion excluder, sodium tetraphenylborate (NaTPB) was studied in detail and improved performance was observed at several instances. Optimum performance was observed with Schiff Base (B) having a membrane composition of B(1%):PVC(33%):DOP(65%):NaTPB(1%). The sensor works satisfactorily in the concentration range 5.0×10 −6 to 1.0×10 −1 M (detection limit 0.3 ppm) with a Nernstian slope of 29.6 mV per decade of activity. Wide pH range (1.9–5.2), fast response time (<30 s), high non-aqueous tolerance (up to 20%) and adequate shelf life (>4 months) indicate the vital utility of the proposed sensor. The potentiometric selectivity coefficient values as determined by match potential method (MPM) indicate good response for Cu 2+ in presence of interfering ions. The tolerance level of Hg 2+, which causes serious interference in the determination of Cu 2+ ions ( K Cu 2+Hg 2+ Pot(MPM): 0.45), was determined as a function of Cu 2+ concentration in simulated mixtures. The sensor was also used in the potentiometric titration of Cu 2+ with EDTA.

Influence of anionic additive on Hg 2+ interference on Ag +-ISEs based on [2.2.2] p, p, p-cyclophane as neutral carrier

The influence of the anionic additive on the Hg 2+ interference on Ag +-ion-selective electrodes (Ag +-ISEs) based on [2.2.2] p, p, p-cyclophane as π-coordinating neutral carrier was studied by potentiometric measurements. The weakly coordinating carborane anion (7,8,9,10,11,12-hexabromocarborane, CB 11H 6Br 6 −) was compared with the commonly used tetrakis(4-chlorophenyl borate) (TpClPB −) as the anionic additive in plasticized PVC membrane-based Ag +-ISEs. The Hg 2+ interference was reduced by six orders of magnitude when replacing TpClPB − with CB 11H 6Br 6 − as the anionic additive, resulting in log K Ag, Hg =−3.8±0.1 determined by the matched potential method (MPM). This indicates that weakly coordinating carboranes may be particularly useful as anionic additives in ISEs for soft cations.

Copper Ion Selective Membrane Electrodes Based on Some Schiff Base Derivatives

AbstractA series of Schiff base derivatives were studied to characterize their abilities as a copper ion carrier in PVC membrane electrodes. The electrode based on 2,2′‐[4,4′diphenyl‐methanebis(nitrilomethylidyne)]‐bisphenol exhibits a Nernstian response for copper ions over the activity range 8.0×10−6−1.0×10−1 mol L−1 with detection limit of about μ mol L−1 of copper ion in comparison with two other Schiff bases. The response time, pH effect and other characteristics of the electrodes were studied in a static mode. The effect of the methyl group substitute on Schiff base structure with SO2 bridging group in different positions with respect to OH group was studied. The results show that behavior of the electrodes is not considerably influenced by the position of methyl substitute. The selectivity coefficients were determined with modified fixed interference method (FIM) and matched potential method (MPM). The proposed electrodes comparatively show good selectivity with respect to alkali, alkaline earth and some transition metal ions. The electrodes were used for the determination of copper in black tea, multivitamin and mineral capsule and as an indicator electrode in potentiometric titration of copper ion.

New flow injection potentiometric graphite coated ion-selective electrode for the determination of VO 2+ ions

A new coated ion-selective electrode for the determination of trace vanadyl ions (VO 2+) by flow injection potentiometry (FIP) with a home-made flow cell has been developed. The PVC-based membrane was coated on a graphite electrode with an effective area of 4.90 mm 2. The optimum membrane contains 5 wt.% 1,8-diaminonaphtalene as ionophore, 35 wt.% plasticizer 2-nitrophenyl octyl ether, 55 wt.% PVC and 5 wt.% additive potassium tetrakis ( p-chlorophenyl) borate. The electrode in flow injection potentiometry resulted in well defined peaks for vanadyl ions with a very high sampling rate (180 injections/h). Linear calibration was obtained from 1.14×10 −7 to 1.14×10 −1 M vanadyl ions, with a slope of 28.3±0.3 mV per decade change in vanadyl concentration, and very low detection limit of 1.14×10 −7 M and the electrode can be used for at least 1 months without any considerable change in potential response. Selectivity coefficients for several ions were obtained by the matched potential method with respect VO 2+ ions. The flow cell is simple to construct and free from memory effect problems over long periods of use. The sensor was used for the recovery of trace VO 2+ ions from tap water and the determination of VO 2+ in synthetic sample.

Potentiometric Response of Liquid Membrane Electrode Incorporated with Macrocyclic Polyamine Towards Benzoate

The potential responses of electrodes based on a macrocyclic polyamine incorporated in poly(vinyl chloride) (PVC) liquid membrane, without and with added lipophilic salt tridodecylmethylammonium chlorides (TDDMACl) towards benzoate were studied. These electrodes are very sensitive towards H+, the potentiometric measurements were realized in the presence of 10−2 M MES buffer pH 5.0. They responsed towards benzoate with the detection limit 1.0 × 10−4 M. This level of sensitivity is sufficient for the determination of benzoic acid added to juices as conservative compound in milimolar concentration. The interferences caused by common anions (CH3COO−, citrate, Cl−, , phosphate and ), which are present in food samples were determinated by the matched potential method (MPM). The validation of macrocyclic polyamine electrodes was done by the recovery test of benzoate in the presence of artificial juice matrix. The results obtained pointed out that these sensors might be applied for the direct determination of benzoate in food samples.

Polymeric Acetate-Selective Electrodes Based on meso-(α,α,α,α)-Tetrakis-[(2-arylphenylurea)phenyl]porphyrins: Electormic and pH Effects

Polymeric membrane electrodes for acetate anion based on meso-(<TEX>${\alpha}$</TEX>,<TEX>${\alpha}$</TEX>,<TEX>${\alpha}$</TEX>,<TEX>${\alpha}$</TEX>)-5,10,15,20-tetrakis[2-(penta-fluorophenylurea) phenyl]porphyrin I and similar urea-functionalized porphyrins Ⅱ-Ⅳ as neutral ionophores were prepared. The membrane based on porphyrin I exhibits the best potentiometric properties in pH 6.0 rather than pH 7.0: linear stable response over a wide concentration range (6.0 <TEX>${\times}$</TEX><TEX>$10^{-5}$</TEX>-1.0 <TEX>${\times}$</TEX><TEX>$10^{-2}$</TEX>) with a slope of -59.6 mV/decade and a detection limit of log[CH3CO<TEX>$O^-$</TEX>] = -5.32. Selectivity coefficients obtained from the matched potential method (MPM) in pH 6.0 indicate that interferences of hydrophobic anions are very small for the membranes of porphyrins I and II having the strong withdrawing group. The electronic effect of urea-functionalized porphyrins and pH effect of buffer solutions are discussed on the potentiometric response.

A selective membrane electrode for iodide ion based on a thiopyrilium ion derivative as a new ionophore

A highly selective electrode for iodide ion based on a thiopyrilium derivative as an excellent ionophore is described. At pH 5.5–8.0, the electrode responds to iodide ion in a linear range from 1.0×10 −1 to 8.0×10 −7 M with a slope of 60.2 mV per decade, and a detection limit of 2.0×10 −7 M. Selectivity coefficients determined with the match potential method (MPM) indicate that the interference from inorganic and organic anions is very small. The proposed sensor shows a fast response time of approximately 15 s. It was applied as an indicator electrode in titration of iodide with Ag +.

A SELECTIVE MEMBRANE ELECTRODE FOR THIOCYANATE ION BASED ON A COPPER-1,8-DIMETHYL-1,3,6,8,10,13-AZACYCLOTETRADECANE COMPLEX AS IONOPHORE

An ion selective electrode for thiocyanate based on Cu(II)-1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane complex as an ionophore is described. At pH 4.5–10.5, the electrode based on this ionophore and cationic sites (40 percent of the ionophore) responds to thiocyanate in linear (E vs log [SCN−]) range from 1.0 × 10−1 to 7.0 × 10−6 M. Selectivity coefficients determined with the match potential method (MPM) indicate that interference from inorganic and organic anions is very small. The proposed sensor shows a fast response time of about 15 s. It was applied as an indicator electrode in titration of thiocyanate with Ag+ and in potentiometric determination of thiocyanate in saliva and urine samples.

Nickel hexacyanoferrate membrane as a coated wire cation-selective electrode.

Nickel hexacyanoferrates containing alkali metal cations as counter ions were used to prepare ion-selective electrodes for potentiometric sensing of intercalated species in the coated wire electrode (CWE) configuration. All the electrodes developed display a quasi-Nernstian response towards potassium ion, whereas the highest sensitivity is generally achieved when Cs+ is the counter cation in the sensing material. The selectivity constants of the electrodes were calculated by the matched potential method considering K+ as the primary ion. The selectivity order is Cs+ > K+ > Na+ > Li+ and reflects the effective dimension of the hydrated cations.

Lead-selective poly(vinyl cholride) membrane electrode based on piroxicam as a neutral carrier

An ion-selective electrode (ISE) for lead based on piroxicam as a suitable ionophor is described. The electrode responds to Pb 2+ in a linear range from 1.0×10 −5 to 1.0×10 −1 mol l −1 with a slope of 30±0.2 mV per decade and a detection limit of (4.0±1.0)×10 −6 mol l −1. The proposed electrode could be used in a pH range of 4.0–8.0. Selectivity coefficients were determined with the conventional fixed interference method (FIM), modified FIM combined with non-linear curve fitting of the Nickolskii–Eisenman equation and matched potential method (MPM). The electrode was successfully used for the direct determination of Pb 2+ in solution and in potentiometric titration of Pb 2+ ions.

Sulfate-selective electrodes based on hydrotalcites

An ion-selective electrode (ISE) for sulfate based on a dispersion of hydrotalcite particles into a polymeric membrane using coated-wire (CW) configuration is described. Membranes, based both on poly(vinyl chloride) (PVC) containing different plasticizers and poly(dimethylsiloxane) (PDMS) were studied. The best performance in terms of slope (−29.6±0.8 mV per decade) and response time (<1 min) was displayed by the PDMS based membrane whereas the linearity range (4.0×10 −5 to 4.0×10 −2 M) was practically the same for all the developed ISEs. The interference of hydrophobic inorganic anions was very low in comparison with the conventional ion-exchanger based electrodes, as shown by the selectivity coefficients determined by the matched potential method (MPM). The selectivity was also good towards the most common univalent and divalent anions and improved in the presence of 2-[4-(2-Hydroxyethyl)-1-piperarazinyl]ethanesulfonic acid (HEPES). The potentiometric response was practically constant in the pH range 4–7. A successful application of sulfate determination in commercial mineral waters, using direct potentiometry, is presented.

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.