Application of a new potentiometric method for determination of phosphate based on a surfactant-modified zeolite carbon-paste electrode (SMZ-CPE)

A phosphate-selective electrode based on surfactant-modified zeolite (SMZ) particles into carbon-paste has been proposed (SMZ-CPE). The electrode was fully characterized in terms of composition, response time, ionic strength, thermal stability and usable pH range. The electrode containing 20% SMZ exhibited linear response range to phosphate species in the range of 1.58 × 10⁻⁵ to 1.00 × 10⁻² M with a detection limit of 1.28 × 10⁻⁵ M and a Nernstian slope of 29.9 ± 0.9 mV per decade of phosphate concentration. The electrode response to phosphate remains constant in the pH range of 4-12 and in the presence of 1 × 10⁻⁴ to 4 × 10⁻³ M NaNO₃. The response of the electrode reaches equilibrium within several seconds after immersing the electrode in phosphate solution. Common anions such as Cl⁻, Br⁻, I⁻, NO₃⁻, SO₄²⁻ and Cr₂O₇²⁻ have little effect on the determination of phosphate but AsO₄³⁻ shows some interference. A successful application of the electrode for determination of phosphate in a fertilizer, using direct potentiometry, is presented. The electrode was also used for the potentiometric titration of phosphate. The validation of the obtained results in each case was proved by statistical methods.     

Determination of Inorganic and Organic Thiophosphates with Ion-Selective Membrane Electrodes

Abstract

The conditions for the determination of sodium thiophosphate and sodium S-(2-amino-ethyl)-thiophos-phate in the presence of phosphate ion using ion-selective membrane electrodes are described. Both thiophosphates (inorganic and organic) are hydrolyzed in acidic medium. The phosphate ion is determined with Pb(II) using a Pb²⁺ – selective membrane electrode.

Cysteamine is determined by potentiometric titration with Hg(I1) using a Ag⁺/S^2? – crystal membrane electrode or by direct potentiometry with a Cu²⁺ selective membrane electrode. The results were verified by the iodination method.     

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 phosphate-selective electrode based on immobilized alkaline phosphatase and glucose oxidase.

An enzyme electrode which senses oxygen consumption for the assay of phosphate ion (10^-3-10^-4M), was constructed by using two enzymes together:

The competitive inhibition by phosphate ion added caused a smaller and slower oxygen consumption which could be detected by a platinum disc electrode at -0.6 V vs. SCE amperometrically. This dual enzyme electrode was also found useful for the assay of oxyacids other than phosphate, such as arsenate, tungstate, molybdate and borate.     

银-磷酸钛纳米复合膜的构建及其电化学生物传感

利用ITO基底上层层组装构建的多层内嵌银纳米粒子的磷酸钛薄膜固定了血红蛋白并且用于生物传感研究。由于银纳米粒子与磷酸钛膜的协同作用,实验中可以观察到Hb的直接电子传递。研究表明所制备的Hb-Ag-TiP/PDDA/ITO电极对H₂O₂响应迅速、稳定,检测限达3.3×10^-6 mol·L^-1。     

Determination of phosphate based on inhibition of crystal growth of calcite

The growth of calcium carbonate seed crystals (calcite) is strongly inhibited by the presence of phosphate ions. The kinetics of crystal growth were followed potentiometrically using a calcium ion-selective electrode or through the use of a pH electrode. The study of different variables on such a process was carried out with the aim of developing kinetic methods to determine phosphate ions (50–400 ng ml^?1). The selectivity and sensitivity of these processes allow the application of crystallization reactions to the determination of phosphate in human urine.     

Tetracyanoquinodimethane (TCNQ) modified electrode for NADH oxidation

A TCNQ-modified edge-plane pyrolytic graphite electrode prepared by a dip-coating procedure shows electrocatalytic activity for NADH oxidation in phosphate buffer solutions (pH 7.0). The modified electrode is stable and shows a linear relation for NADH in the concentration range 1–10 mM. The rate constant between adsorbed TCNQ and NADH in solution has been estimated to be 1.46 × 10⁶ M⁻¹s⁻¹ at 25°C. The modified electrode has the potential use as a sensor for dehydrogenase-enzyme-based substrates.     

Silver nanoparticle assemblies supported on glassy-carbon electrodes for the electro-analytical detection of hydrogen peroxide

Electrochemical detection of hydrogen peroxide using an edge-plane pyrolytic-graphite electrode (EPPG), a glassy carbon (GC) electrode, and a silver nanoparticle-modified GC electrode is reported. It is shown, in phosphate buffer (0.05 mol L^–1, pH 7.4), that hydrogen peroxide cannot be detected directly on either the EPPG or GC electrodes. However, reduction can be facilitated by modification of the glassy-carbon surface with nanosized silver assemblies. The optimum conditions for modification of the GC electrode with silver nanoparticles were found to be deposition for 1 min at –0.5 V vs. Ag from 5 mmol L^–1 AgNO₃/0.1 mol L^–1 TBAP/MeCN, followed by stripping for 2 min at +0.5 V vs. Ag in the same solution. A wave, due to the reduction of hydrogen peroxide on the silver nanoparticles is observed at –0.68 V vs. SCE. The limit of detection for this modified nanosilver electrode was 2.0×10^–6 mol L^–1 for hydrogen peroxide in phosphate buffer (0.05 mol L^–1, pH 7.4) with a sensitivity which is five times higher than that observed at a silver macro-electrode. Also observed is a shoulder on the voltammetric wave corresponding to the reduction of oxygen, which is produced by silver-catalysed chemical decomposition of hydrogen peroxide to water and oxygen then oxygen reduction at the surface of the glassy-carbon electrode.     

A disposable cobalt-based phosphate sensor based on screen printing technology

Screen printing is a promising technology because of its simplicity, low-cost, high reproducibility, and efficiency in large-scale production. In this work, a cobalt-based phosphate sensor was successfully fabricated using the screen printing technology for the determination of phosphate concentration in the aqueous solution. The disposable sensor consists of a fully integrated cobalt(Co) electrode, which is a layer of carbon conductive ink(C) physically doped with Co powder, and Ag/AgCl reference electrode. The SEM images show that the morphology of the Co electrode changes after exposure to the phosphate solution, indicating that the expendable reaction exists during the measurement. At the Co/C ratio of 1:99, the cobalt-based phosphate sensor shows phosphate-selective potential response in the range of 10-4 to 10-1 mol/L, yielding a detection limit of 1×10-5 mol/L and a slope of over 30 mV/decade in acidic solution(pH 4.5) for H2PO4-. The proposed screen-printed sensor also exhibited significant reproducibility with a small repeated sensing deviation(i.e., relative standard deviation(R.S.D.) of 0.5%) on a single sensor and a small electrode-to-electrode deviation(i.e., R.S.D. 3.2%). The recovery study of H2PO4- in real wastewater samples gave values from 95.4% to 101.8%, confirming its application potential in the measurement of phosphate in real samples. Apart from its high selectivity, sensitivity, and stability comparable with a conventional bulk Co-wire electrode, the proposed phosphate sensor still yields many other advantages, such as low price, compactness, ease of use, and the possibility of integration with other analytical devices such as flow injection analysis.     

Modified glassy carbon electrode by electropolymerization of tetrakis-(2-aminopheny)porphyrin for the determination of norepinephrine in the presence of ascorbic acid

A glassy carbon electrode (GCE) was modified with electropolymerization of meso-tetrakis(2-aminophenyl)porphyrin (TAPP) in acetonitrile by cyclic voltammetry (CV). The voltammetric behavior of norepinephrine (NE) in the presence of excess ascorbic acid (AA) was investigated at the modified electrode by cyclic and square wave voltammetry (SWV) in phosphate buffer solution. The modified electrode gave higher selectivity and highly effective electroactivity to NE oxidation in voltammetric measurements of NE in the presence of AA and epinephrine. In pH 7.4 phosphate buffer solution, the peak current increased linearly with the concentration of NE in two concentration ranges of 1.0×10⁻⁶ to 5.0×10⁻⁵ mol dm⁻³.     

Electrode Hybride Pour la Determination de Phosphates et de Polyphosphates. Application aux Problemes Lies a l'Environnement

Abstract

The determination of phosphates and polyphosphates has been effected using an hybrid electrode, which consists of a glucose oxydase enzymic membrane and a Solanum tuberosum tissue slice. This membrane, rich in acid phosphatase, catalyses the glucose-6-phosphate hydrolysis. This reaction is quantitatively inhibited by phosphate. Several factors involved in the electrode response, like substrate concentration, pH, ionic strength and type of buffer are discussed in detail. At a 4.10^?4 M glucose-6-phosphate concentration, the linear ranges of phosphates and polyphosphates are, respectively, 6.10^?5 M to 1.6.10^?3 M and 3.10^?5 M to 1.10^?3 M. The urinary phosphate contents determinated by this biosensor are in good agrement with those obtained by usual spectrophotometric techniques.     

Anion-selective properties of alkali metal-free lead phosphate glasses containing silver chloride and their application in an ion-selective electrode

Alkali-free lead phosphate glasses containing silver chloride have been developed for anion responsive sensors. From measurements of the final glass compositions by electron probe microanalysis, it became clear that some of chloride ions in the glass bulk were not volatilized during the glass melting process. Compared with phosphate glasses containing silver oxide, the new glass electrodes containing silver chloride could respond more rapidly, although the response behaviour for anionic species were similar. From the electrode potential vs. time curve for the anionic species, the potential rapidly reached equilibrium when these concentrations varied from 10^?5 to 10^?2 M. The response times, t₉₅, to thiocyanate of the new glass electrode and the phosphate glass electrode containing silver oxide were 30 and 110 s, respectively. Moreover, the response time required for an initial potential change with a concentration jump of thiocyanate with the new glass electrode was found to be independent of the membrane thickness within about 2 mm and of the measuring temperature between 15 and 40°C. It is concluded that the diffusion process of species such as silver ion in the glass bulk does not take part in the initial part of the response behaviour.     

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.     

Selective determination of cysteine in the presence of tryptophan by carbon paste electrode modified with quinizarine

A sensitive and selective electrochemical method for the determination of L-cysteine was developed using a modified carbon paste electrode (MCPE) with quinizarine. Cyclic voltammetry was used to investigate the redox properties of this modified electrode at various solution pH values and at various scan rates. The apparent charge transfer rate constant, ks and transfer coefficient for electron transfer between quinizarine and carbon paste electrode (CPE) were calculated as 2.76 s^?1 and 0.6, respectively. This modified carbon paste electrode shows excellent electrocatalytic activity toward the oxidation of L-cysteine in a phosphate buffer solution (pH 7.0). The linear range of 1.0 × 10^?6 to 1.0 × 10^?3 M and a detection limit (3s) of 2.2 × 10^?7 M were observed in pH 7.0 phosphate buffer solutions. In differential pulse voltammetry, the quinizarine modified carbon paste electrode (QMCPE) could separate the oxidation peak potentials of L-cysteine and tryptophan present in the same solution, though at the unmodified CPE the peak potentials were indistinguishable. This work introduces a simple and easy approach to selective detection of L-cysteine in the presence of tryptophan. Also, the modified electrode was employed for the determination of L-cysteine in the real samples such as serum of blood and acetylcysteine tablet.     

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.     

Electrochemical Study of Moroxydine and its Voltammetric Determination with a Silver Amalgam Film Electrode

Moroxydine (Mor.) is an antiviral agent of biguanide structure. The paper presents a new silver amalgam film electrode (Hg(Ag)FE) for determination of Mor. in phosphate buffer, pH 6.2 (LOD=4×10^?9 mol L^?1, LOQ= 1×10^?8 mol L^?1) and in spiked urine using square wave adsorptive stripping voltammetry. It was found that the compound can act as an electrocatalyst not only at hanging mercury drop electrode but also at the Hg(Ag)FE. The electrode mechanism is connected with the hydrogen evolution reaction catalyzed by moroxydine. Adsorption of moroxydine at the mercury electrode was studied and special arrangements of molecules enabling electron transfer of the protonated form of moroxydine is suggested.     

Voltammetric Determination of Phosphate in Brazilian Biodiesel Using Two Different Electrodes

The presence of contaminants, such as phosphate, in biodiesel, has several drawbacks for instance: current engines perform poorly, fuel tanks deteriorate, catalytic conversion is damaged, and particles emission is increased. Therefore, biodiesel quality control is extremely important for biodiesel acceptance and commercialization worldwide. In this context, a bare glassy carbon electrode (GCE) and another chemically modified electrode with iron hexacyanoferrate (Prussian Blue – PB) were developed for determination of phosphate in biodiesel. The LODs of 6.44 and 1.19 mg kg^?1, and LOQs of 21.43 and 3.97 mg kg^?1 were obtained for the bare GCE and the PB‐modified GCE, respectively. The methodology was employed for analysis of Brazilian biodiesel samples, and it led to satisfactory results, demonstrating its potential application for biodiesel quality control. Additionally, recovery and interference tests were conducted, which revealed that the developed methods are suitable for analysis of phosphate in biodiesel samples.     

Electrocatalytic Oxidation of Hydrazine at the Poly(Glutamic Acid) Chemically Modified Electrode and Its Amperometric Determination

ABSTRACT

The electrochemical polymerization of glutamic acid at the glassy carbon electrode was investigated in phosphate buffer solution by cyclic voltammetry. The applied voltage range, pH of electrolyte, cyclic number were explored for optimal polymerization conditions. The resulting film exhibits an electrocatalytic ability to hydrazine, reducing the overpotential by 500mV. The electrocatalytic response of hydrazine is evaluated with regard to pH, scan rate, applied voltage, hydrazine concentration and other variables. The rate constant of the catalytic reaction was 1.2 × 10⁴ M^?1.s^?1. When used as amperometric detector, the modified electrode yields a detection limit of 1 × 10^?8 M hydrazine. The electrode is rather stable even after use for a month and a reproducible response was obtained.     

A bio-selective membrane electrode prepared with living bacterial cells.

A novel potentiometric sensor has been devised by coupling intact microorganisms (Streptococcus faecium) with an ammonia gas-sensing membrane electrode. The resulting electrode provides a linear response to arginine over the concentration range 5.0 × lO^-5–1. 0 × 10^-3 M in phosphate buffer pH 7.4, with selectivity over other amino acids. The slope of the calibration graph is -40 to -45 mV/decade during the period 2–20 days after preparation. This membrane electrode with living bacterial cells may serve as a model for the development of other new sensing systems.     

Potentiometric detection of monovalent anions separated by ion chromatography using all solid-state contact PVC matrix membrane electrode

Summary An all solid-state contact tubular PVC-matrix membrane electrode has been applied for potentiometric detection of inorganic and organic monovalent anions using phosphate and hydrogen phosphate eluents at low concentrations. This is a “monovalent detection method” as the selectivity of the electrode towards monovalent anions results in some other anions being undetected unless the concentration of those other anions is higher than 10⁻³ mol dm⁻³ in the sample solution injected. It takes only eight minutes to complete the separation with a good resolution. Theoretical and practical considerations are discussed, and in particular, sensitivity, linearity, detection limit and dynamic behaviour are presented. The use of an all solid-state contact bromide-selective electrode as a detector offers so far the best simultaneous sensitivity toward all anions when compared with other detection methods. Determination of Cl⁻ and NO₃ ⁻ ions in river, rain and drinking water samples without any further sample preconcentration has been successfully achieved. The detection limit is sub-ppb for most of anions in a 20 μL injection volume.