Calcium‐Selective Electrodes for Measurements in the Presence of Anionic Surfactants

Calcium selective electrodes with various membrane formulations were studied in solutions containing CaCl₂ and sodium dodecylsulfate (NaDS). It is shown that electrodes based on neutral ionophores ETH 1001 and ETH 129 cannot be used as Ca²⁺ ion sensors in these solutions because of strong anion interference from DS^? anion. Among other formulations, that based on calcium bis(tetramethylbutylphenyl)phosphate in tri(2‐ethylhexyl)phosphate appear the most promising. The interpretation of the ISE response in solutions under study relied on a novel approach which considers three forms of calcium: Ca²⁺ free ions, Ca in Ca(DS)₂ precipitate, and Ca²⁺ bound by the DS^? micelles. Data needed for the respective calculations were obtained by DS^? selective electrode based on tetradecylammonium, and Na⁺ selective glass electrode.     

Determination of betamethasone sodium phosphate in pharmaceuticals by potentiometric membrane sensor based on its lidocaine ion pair

Betamethasone sodium phosphate (BMNaP) has been employed as an electroactive material in the design of an ion-selective electrode (ISE). The electrode incorporates PVC membrane with betamethasone sodium phosphate-lidocaine ion pair complex. The influences of membrane composition, temperature, pH of the test solution, and the interfering ions on the electrode performance were investigated. The sensor exhibits a Nernstian response for betamethasone sodium phosphate ions over a relatively wide concentration range (1.0 × 10^?1 to 1.0 × 10^?5 M) with a slope of 28.4 ± 0.9 mV per decade at 25°C. It can be used in the pH range 4.0–10.0. The isothermal temperature coefficient of this electrode amounted to ?0.0008 V/°C. The membrane sensor was successfully applied to the determination of betamethasone sodium phosphate in pharmaceutical products.     

A Calcium Solid State Ion Selective Minisensor Based on Lipid Films on ZnO Nanorods

This article describes the development of a calcium solid minisensor based on lipid films on ZnO nanorods. The lipid film was composed from dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylcic acid (DPPA). The calibration graph was logarithmically related to the concentration of calcium ions in the range from 10^?6 to 10^?3 M. The response times of the electrode were less than 5 s. No memory effects were observed for this electrode. The reproducibility of the electrode was less than 5 %. The electrode has shown good stability for a period of 1 month. The influence of a wide range of metals and anions as potential interferences was checked. The method was applied to the rapid detection of calcium ions in mineral waters.     

Determination of citric acid based on inhibition of the crystal growth of calcium fluoride

Inhibition of the growth of calcium fluoride crystals in the presence of citrate was followed using a kinetic-potentiometric technique and a calcium ion-selective electrode, and as a consequence, a method for the determination of citrate in the range 0.5-2.4 micrograms ml-1 has been developed. The method was successfully applied to the determination of citrate contained in pharmaceutical products and urine. Urine analysis requires prior separation of phosphate, sulphate and magnesium(II). Elimination of these interferences was studied and accomplished using precipitation processes. Magnesium and phosphate were jointly eliminated in basic media by the addition of ammonium ions. Phosphate and sulphate were eliminated with barium(II). Phosphate was also eliminated as a lithium salt.     

Calcium ion selective membrane electrode

The calcium salts of the mono- and diesters of [4-(1,1,3,3-tetramethylbutyl)phenyl phosphoric acid] have been prepared, and the individual esters as well as mixtures of the esters have been used with several varieties of polyvinyl chloride to construct macro membrane electrodes selective to calcium ions. These electrodes have been calibrated by using solutions of CaCl₂ and Ca ion buffers. The mixed ester electrodes showed Nernstian response in the concentration range 10^-1 to 10^-7M; the diester electrodes showed Nernstian response down to 7.9 x 10^-8M. The detection limit of the mixed ester electrode was 10^-8M, whereas that of the diester electrode was 7.9 x 10^-9M. Contrary to these results, the monoester electrodes showed unsatisfactory behavior. The responses of both the mixed ester and diester electrodes to calcium ions were not affected by the presence of sodium, potassium, or other divalent ions. Only ferric and lanthanum ions showed interferences with the electrode response to calcium ions. p]The electrode response was independent of pH in the approximate range 5–8 at a CaCl₂ concentration of 10^-4M. As the Ca ion concentration was increased, the range of pH independence widened to approximately 4–8. The dynamic response time constant of the mixed ester electrode was in the range 0.7–1.5 sec, whereas that of the diester electrode was in the range 0.5–0.75 sec.     

Flow-injection determination of phosphate species in detergents with a calciumion-selective electrode

A calcium ion-selective electrode is applied for the determination of ligands which complex calcium ions. The response of the electrode is measured when ligand solutions are injected into a buffered carrier stream containing calcium. Injections of EDTA, citrate, tripolyphosphate and pyrophosphate provide peaks with heights dependent on both the ligand and calcium concentrations. Ligand concentrations down to. 2 × 10^?5 M can be detected. Tripolyphosphate in detergents is easily determined. Use of the electrode has advantages over spectrophotometric methods for phosphates in flow systems.     

Use of ethylene-vinyl-acetate as a new membrane matrix for calcium ion-selective electrode preparation

A new polymer matrix based membrane electrode with an ion-exchanger responding to calcium was constructed by dissolving the copolymer ethylene-vinyl-acetate together with the ion-exchanger in chloroform in the presence of a mixture of dioctylphthalate-nitrobenzene as plasticizer. The ion-exchanger used as the electroactive component was calcium didecyl phosphate in di-(n-octylphenyl) phosphonate (Orion). This electrode exhibited near-Nernstian response over the concentration range 10(-1)-4 x 10(-6)M calcium. The pH did not affect the electrode performance within the range 8-11. Response time varied from 15 to 120 sec and the lifetime exceeded six months. The membrane is subject to static charge buildup, but this is avoided by controlling the level of dryness of the membrane. Selectivity coefficients determined for both monovalent and divalent cations showed negligible interference by most of these ions. The electrode was applied successfully to the determination of calcium in commercial mineral waters.     

Processing of signals from an ion-elective electrode array by a neural network

Neural network software is described for processing the signals of arrays of ion-selective electrodes. The performance of the software was tested in the simultaneous determination of calcium and copper(II) ions in binary mixtures of copper(II) nitrate and calcium chloride and the simultaneous determination of potassium, calcium, nitrate and chloride in mixtures of potassium and calcium chlorides and ammonium nitrate. The measurements for the Ca²⁺/Cu²⁺ determinations were done with a pH-glass electrode and calcium and copper ion-selective electrodes; results were accurate to ±8%. For the K⁺/Ca²⁺NO^?₃/Cl^? determinations, the measurements were made with the relevant ion-selective electrodes and a glass electrode; the mean relative error was ±6%, and for the worst cases the error did not exceed 20%.     

Radiotracer studies on calcium ion-selective electrode membranes based on poly(vinyl chloride) matrices

Radiotracer studies with ⁴⁵Ca and ³⁶Cl demonstrate that PVC matrix membranes containing Orion 92-20-02 liquid calcium ion-exchanger are permselective to counter-cations. Diffusion coefficients are quoted for the migration of ⁴⁵Ca between pairs of calcium solutions and are discussed in terms of solution concentration, membrane thickness and concentration level of sensor in the membrane. Migration of calcium ions from calcium chloride solution to a Group (II) metal chloride solution through a PVC membrane containing calcium liquid ion-exchanger is discussed in terms of solvent extraction and electrode selectivity coefficient parameters. Thus, magnesium, strontium and barium ions appear to inhibit migration through the membrane by their low affinity for the membrane liquid ion-exchanger sites, while the inhibition by beryllium ions is attributed to site blockage by the strong affinity of dialkylphosphate sites for beryllium.     

Application of Tetra Cyclohexyl Tin(IV) as an Anionic Carrier for the Construction of a New Salicylate Membrane Sensor

A new tin complex namely tetracyclohexyl tin(IV) (TCHT) was synthesized and used as the ion carrier for the construction of a highly selective salicylate sensor. This sensor shows a Nernstian response to salicylate ions over a very wide concentration (1.0 × 10^?7–1.0 × 10^?1 M) in a pH range of 5.5–10.5. The optimum selectivity and response could be obtained for a membrane incorporating 30% PVC, 61% BA, 3% of cationic additive (HTAB) and 6% of TCHT. The response time of the electrode is very short in the whole concentration range (15 s). The electrode also shows an excellent discriminating ability for salicylate ions with respect to the most common organic and inorganic anions including chloride, sulfate, nitrate, nitrite, cyanide, sulfite, iodide, thiocyanate, phosphate, acetate, oxalate, citrate, and tartarate ions. The detection limit of the proposed sensor is 8.0 × 10^?8 M. The electrode was successfully used for determining the concentration of salicylate ion in synthetic serums.     

Preparation and examination of calcium ion-selective electrodes for flow analysis

The preparation of calcium ion-selective electrodes based on known alkylphenylphosphate exchangers or on the ETH 1001 ionophore, and their use in a flow-through cell in a flow-injection system for the determination of calcium are described. The response and lifetime of the electrodes and the effects of magnesium and sodium ions on the determination of 10^?3?10^?5 M calcium are examined in detail. The ionophore electrode is shown to be most satisfactory.     

Determination of calcium and magnesium in magnesite by a complexometric method employing co-precipitation of calcium with strontium sulphate

In the classical gravimetric method calcium is precipitated as phosphate along with the magnesium and recovered by adding sulphuric acid and alcohol, then Ca²⁺ and Mg²⁺ are determined separately by the usual method [1]. In the proposed method calcium is co-precipitated with strontium sulphate and in the filtrate magnesium ions are determined by EDTA. Then calcium is calculated by difference after estimating combined Ca²⁺ and Mg²⁺ ions [2].     

A study of the mechanism of response of liquid ion exchanger calcium selective electrodes: Part III. Membrane behaviour under non-zero current conditions

The I-E response of the liquid membrane of the calcium selective electrode is studied under constant or linearly varying current and voltage. An increase in the membrane resistance, recorded when an electrical current crosses the membrane, is due to the outflow of Cl^? ions initially present in the membrane. When calcium ions are replaced by alkaline ions inside the membrane at constant current, the decrease of the membrane resistance due to an ion exchange is in agreement with the conductivity measurements (Part II). When the applied voltage is imposed besides the ion exchange one must take into account the interfacial overpotential to explain the important rectification effect observed. The interfacial transfer constant rate of alkaline ions seems greater than that of Ca²⁺ ion.     

Incongruent dissolution of hydroxyapatite in the presence of phosphoserine

Concentrations of phosphate and calcium ions, liberated from the surface of hydroxyapatite (HAP) during the adsorption of phosphoserine (PSer), were determined at 30°C. HAP showed a marked incongruent dissolution behavior in the presence of PSer. That is, the concentration of phosphate ion in solution increases with the addition of PSer due to the ion-exchange between PSer and phosphate ion on HAP (molar ratio of the former to the latter=32), whereas the concentration of calcium ion decreases with this release of phosphate ion, because the solubility product of HAP restricts the concentrations of both ions in solution (calculated values of — log (Ca²⁺)¹⁰ (PO ₄ ^3– )⁶ (OH^–)² were 115.8±1.0). The affinity of PSer to HAP was highest at pH 5.8 where the PSer and the HAP surface had the opposite charges. This electrostatic attraction force between PSer and HAP was shielded to some extent by the addition of KCl.     

Flow-through calcium-selective electrode: application in flow-injection analysis and ion chromatography

The use of solid contact flow-through calcium-selective electrodes as potentiometric detectors in flow-injection analysis and non-suppressed ion chromatography is discussed. Owing to the high selectivity of the membrane electrode based on tetratolyl-m-xylylenediphosphine dioxide, it can be used to monitor trace amounts of calcium ions in the presence of a 100-fold excess of alkali metal, ammonium and magnesium ions. The detection limit is about 1 × 10^?6 M. The composition and thickncss of the calcium selective membrane influence both the detection limit and selectivity of the electrode. The sensitivity of this potentiometric detector in ion chromatography relative to alkaline earth and heavy metals is significantly higher than that of a commercial conductivity detector.     

Flow injection potentiometry by a new coated graphite ion-selective electrode for the determination of Pb(2+)

A new graphite coated electrode for the determination of Pb²⁺ based on a recently synthesized ionophore 1-hydroxy-2-{2-[2-(2-hydroxy-ethoxy)-ethoxy]-ethoxy methyl}-anthracene-9,10-dione (L) has been developed. The electrode was used in flow injection potentiometry by a home-made flow cell. Under both the batch and flow conditions, the electrode revealed a near Nernstian response over a wide lead ion concentration range (10⁻⁶ to 10⁻¹ M) and very low limit of detection. In flow injection potentiometry, excellent reproducibility (RSD%=0.49%), very high sampling rate (170 injections h⁻¹) and stable baseline was observed in the presence of 10⁻³ M KCl as ionic strength adjuster. The electrode showed high sensitivity and good selectivity for Pb²⁺ over a wide variety of alkali, alkali earth and transition metal ions and the electrode can be used for at least 3 months without any considerable change in potential response. The proposed sensor was successfully applied to the direct determination of lead in real samples and also used for the titrimetric determination of phosphate ions by both batch and flow injection potentiometry.     

Phosphate-sensitive enzyme electrode: a potential sensor for environment control

A highly sensitive enzyme electrode was designed for the assay of phosphate ions. For this purpose, a bienzyme membrane with co-immobilized nucleoside phosphorylase and xanthine oxidase was used with a platinum amperometric electrode for the detection of enzymatically generated hydrogen peroxide. A detection limit of 10^?7 M was obtained and phosphate assays could be easily performed in the range 0.1–10 μM, which is of interest in the control of water pollution.     

Ion-selective electrode for measuring low Ca2+ concentrations in the presence of high K+, Na+ and Mg2+ background

In this work, ion-selective electrodes for calcium ion were investigated. Two ionophores were used in the membranes: ETH 1001 and ETH 129. An internal filling solution buffered for primary ion was used that allowed the lower detection limit to be decreased down to 10^−8.8 M. Theoretical and experimental electrode characteristics pertaining to both primary and interfering ions are discussed. Better behavior was obtained with the electrode prepared with ETH 129 in the membrane. This electrode would be the most likely candidate for obtaining a low Ca²⁺ detection limit in measurements performed with high K⁺, Na⁺, Mg²⁺ background, which is found inside the cells of living organisms, for example. The potentiometric response of the electrode in solutions containing main and interfering ions is in good agreement with simulated curves obtained using the Nernst–Planck–Poisson (NPP) model.     

Preparation and characterization of a new organic-inorganic nano-composite poly-o-toluidine Th(IV) phosphate: Its analytical applications as cation-exchanger and in making ion-selective electrode

An organic-inorganic hybrid poly-o-toluidine Th(IV) phosphate was chemically synthesized by mixing ortho-tolidine into the gel of Th(IV) phosphate in different mixing volume ratios, concentration of inorganic reactant with a fixed mixing volume ratios of organic polymer. The physico-chemical characterization was carried out by elemental analysis, TEM, SEM, XRD, FTIR and simultaneous TGA-DTA studies. The ion-exchange capacity, chemical stability, effect of eluant concentration, elution behavior and pH titration studies were also carried out to understand the ion-exchange capabilities. The distribution studies revealed that the cation-exchange material is highly selective for Hg²⁺, which is an important environmental pollutant. Due to selective nature of the cation-exchanger ion-selective membrane electrode was fabricated for the determination of Hg(II) ions in solutions. The analytical utility of this electrode was established by employing it as an indicator electrode in electrometric titrations.     

Reversible Electrochemical Modulation of a Catalytic Nanosystem

A catalytic system based on monolayer‐functionalized gold nanoparticles (Au NPs) that can be electrochemically modulated and reversibly activated is reported. The catalytic activity relies on the presence of metal ions (Cd²⁺ and Cu²⁺), which can be complexed by the nanoparticle‐bound monolayer. This activates the system towards the catalytic cleavage of 2‐hydroxypropyl‐p‐nitrophenyl phosphate (HPNPP), which can be monitored by UV/Vis spectroscopy. It is shown that Cu²⁺ metal ions can be delivered to the system by applying an oxidative potential to an electrode on which Cu⁰ was deposited. By exploiting the different affinity of Cd²⁺ and Cu²⁺ ions for the monolayer, it was also possible to upregulate the catalytic activity after releasing Cu²⁺ from an electrode into a solution containing Cd²⁺. Finally, it is shown that the activity of this supramolecular nanosystem can be reversibly switched on or off by oxidizing/reducing Cu/Cu²⁺ ions under controlled conditions.