A New Highly Selective Neodymium(III) Polyvinylchloride Membrane Electrode Based on 4-Hydroxypyrrolidine-2-Carboxylic Acid as an Active Material

The present paper describes the fabrication of a new polyvinylchloride (PVC) membrane electrode for the determination of Nd(III) ion based on 4-hydroxypyrrolidine-2-carboxylic acid (LHP) as an active material along with sodium tetraphenyl borate (NaTPB) as an anionic additive and acetophenone (AP) as solvent mediator. The optimum composition (%, w/w) of the best performing membrane contained 66 AP, 30 PVC, 2 NaTPB and 3 LHP. The basic analytical parameters of this electrode such as slope characteristics, detection limit, response time, selectivity and pH effect were assessed. The electrode displayed a Nernstian response in terms of slope (20.5 ± 0.4 mV per decade) and response time (~8 ± 1 s) over a wide neodymium ion concentration range of 10^–6 to 10^–2 M with a detection limit of 7.5 × 10^?7 M. The potentiometric response of the electrode was constant in the pH range of 3.2?8.9. According to the selectivity coefficients determined by the matched potential method, the interference of many common alkaline, alkaline earth, transition, heavy metals and specially lanthanide ions in determination of Nd(III) ion was very low. The proposed electrode has been successfully used as an indicator electrode in the potentiometric titration of Nd(III) ion with EDTA and applied for determination of Nd(III) ion in mixtures of different ions.     

A Calcium Ion-Selective Electrode Based on a Neutral Carrier

Abstract

The performance of a liquid-membrane electrode using a synthesized ligand in p-nitroethylbenzene as membrane component is described. Selectivities, working range, lifetime, pH-dependence, and dependence on the dielectric constant of the membrane solvent are discussed. The electrode makes possible the measurement of calcium ion activities in the range of 10^?1 M to 10^?5 M In unbuffered systems with a selectivity of calcium over sodium and magnesium of 175 and 33,000 respectively. The selectivity constants of the cell discussed are heavily influenced by the dielectric constant of the membrane solvent.     

Organoselenide as a Novel Ionophore for a Silver-Selective Membrane Electrode

Abstract

Three selenium-containing compounds, diphenyl selenide, benzyl phenyl selenide and dibenzyl selenide were used as neutral carriers to make a Ag⁺-selective membrane electrode. All three compounds exhibited higher sensitivity to Ag⁺ than the corresponding sulfides and ethers. The highest sensitivity was obtained by dibenzyl selenide and an electrode constructed using this carrier and bis(l-butylpentyl) adipate as a membrane solvent in a poly(vinyl chloride) membrane matrix exhibited a near-Nernstian response to Ag⁺ in the concentration range from 1 x 10^?5 to 1 x 10^?2 M with a slope of 52 mV per concentration decade. The lower limit of detection was around 1 μM. The ion selectivity of this electrode for Ag⁺ was over 10⁴ times that for other metal cations. Dibenzyl selenide and Ag⁺ interactions were examined by ¹H-NMR spectroscopy.     

Determination of l-phenylalanine in serum by flow-injection analysis using immobilized phenylalanine dehydrogenase and fluorimetric detection

A flow-injection system with an immobilized enzyme reactor is proposed for the determination of l-phenylalanine. Phenylalanine dehydrogenase from Rhodoccus sp. M4 was immobilized on tresylated poly (vinyl alcohol) beads (13 mum) and packed into a stainless-steel column (5 cm x 4 mm i.d.). Serum sample was deproteinized with tungstic acid and filtered through an ultrafiltration membrane. The sample solution (30 mul) was injected into the carrier stream (water). The NADH formed was detected at 465 nm (excitation at 340 nm). The calibration graph was linear for 0.9-600 mum l-phenylalanine; the detection limit was 0.3 mum. The sample throughout was 25 h(-1) without carryover. The half-life period of the immobilized enzyme was 23 days.     

Sulfate-selective PVC membrane electrode based on a strontium Schiff's base complex

A strontium Schiff's base complex (SS) can be used as a suitable ionophore to prepare a sulfate-selective PVC-based membrane electrode. The use of oleic acid (OA) and hexadecyltrimethylammonium bromide (HTAB), as additives, and nitrobenzen (NB), dibutyl phthalate (DBP) and benzyl acetate (BA) as solvent mediators, were investigated. The best performance was observed with a membrane composition PVC: NB: SS: HTAB of 30%: 62%: 5%: 3% ratio. The resulting sensor works well over a wide concentration range (1.0 x 10(-2)-1.0 x 10(-6) M) with a Nernstian slope of -29.2 mV per decade of sulfate activity over a pH range 4.0-7.0. The limit of detection of the electrode is 5 x 10(-7) M. The proposed sensor shows excellent discriminating ability toward SO4(2-) ions with regard to many anions. It has a fast response time of about 15 s. The membrane electrode was used to the determination of zinc in zinc sulfate tablets. The sensor was also used as an indicator electrode in the potentiometric titration of SO4(2-) against barium ion.     

Enhanced Performance of CNT‐doped Imine Based Receptors as Fe(III) Sensor Using Potentiometry and Voltammetry

A highly sensitive and selective potentiometric and voltammteric assay for the detection of Fe³⁺ using (E)‐3‐((2‐(2‐(2‐aminoethylamino) ethylamino) ethylimino)methyl)‐4H‐chromen‐4‐one (IFE(III)) ionophore was developed. To demonstrate the ion‐to‐electron transfer ability of MWCNT, these were incorporated in the ion‐selective membrane and response characteristics of Fe³⁺ electrode was compared with those of the traditional ion selective electrode. The electrode showed an improved Nernstian slope, lower detection limit, response time of less than 5 s and working in a pH range of 3.0 to 8.0. Differential pulse voltammetric studies were performed for IFE(III)‐Fe³⁺ complex in DMSO solvent medium at glassy carbon (GC) electrode. A linear relationship between the cathodic peak current and concentration of Fe³⁺ was observed in the range of 1.6×10^?5 to 4.4×10^?5 mol/L with a detection limit of 5.2×10^?8 mol/L. The electrode shows remarkable selectivity for Fe³⁺ ions over alkali, alkaline earth, transition and heavy metal ions. The optimized electrode was successfully applied for the determination of Fe³⁺ ion in different real‐life samples using potentiometric technique. Theoretical calculations were used to support the complexation behavior of Fe³⁺ with IFE(III).     

Determination of SCN- in urine and saliva of smokers and non-smokers by SCN(-)-selective polymeric membrane containing a nickel(II)-azamacrocycle complex coated on a graphite electrode.

The construction, performance characteristics, and application of a novel polymeric membrane coated on a graphite electrode with unique selectivity towards SCN- are reported. The electrode was prepared by incorporating Ni(II)-2,2,4,9,9,11-hexamethyltetraazacyclotetradecanediene perchlorate into a plasticized poly(vinyl chloride) membrane. The influences of membrane composition, pH and foreign ions were investigated. The electrode displays a near Nernstian slope (-57.8 mV decade-1) over a wide concentration range of 1 x 10(-7)-1 x 10(-1) M of SCN- ion. The electrode has a detection limit of 4.8 x 10(-8) M (2.8 ng/cm3) SCN- and shows response times of about 15 s and 120 s for low to high and high to low concentration sequences, respectively. The proposed sensor shows high selectivity towards SCN- over several common organic and inorganic anions. The electrode revealed a great enhancement in selectivity coefficients and detection limit for SCN-, in comparison with the previously reported electrodes. It was successfully applied to the direct determination of SCN- in milk and biological samples, and as an indicator electrode in titration of Ag+ ions with thiocyanate.     

Highly Selective and Sensitive Triiodide PVC‐Membrane Electrode Based on a New Charge‐Transfer Complex of Bis(2,4‐Dimethoxybenzaldehyde)butane‐2,3‐Dihydrazone with Iodine

In this work, a highly selective membrane triiodide sensor based on a new charge‐transfer complex of bis(2,4‐dimethoxybenzaldehyde)butane‐2,3‐dihydrazone with iodine (Iodide Charge Transfer complex: ICT) as membrane carrier is introduced. The influences of five different solvent mediators on sensitivity and selectivity of the proposed sensor were considered. The best performance was obtained with the membrane composition containing 30% poly (vinyl chloride), 63% DBP, 5% ICT and 2% HTAB. The electrode shows a Nernstian behavior over a very wide triiodide ion concentration range (1.0 × 10^?7‐1.0 × 10^?2 M), and a detection limit value of 8.0 × 10^?8 M. The effect of pH on the potentiometric response of the sensor was also studied, and it was found that the response of the electrode is independent of the pH of the solution in the pH range of 4.0–10. The proposed sensor has a very fast response time (< 12 s), and good selectivities relative to a wide variety of common inorganic and organic anions, including iodide, acetate, bromide, chloride, fluoride, nitrite, nitrate, sulfite, sulfate, cyanide and thiocyanate. In fact the selectivity behavior of the proposed triiodide ion‐selective electrode shows great improvements compared to the previously reported electrodes for triiodide ion. The proposed membrane sensor can be used for at least 6 months without any divergence in the potentials. The electrode was successfully applied as an indicator electrode in the titration of triiodide with thiosulfate ion.     

A selective membrane electrode for lanthanum(III) ion based on a hexaaza macrocycle derivative as ionophore.

We have developed a highly La(III)-selective PVC membrane electrode based on a hexaaza macrocycle, 8,16-dimethyl-6,14-diphenyl-2,3,4:10,11,12-dipyridine-1,3,5,9,11,13-hexaazacyclohexadeca-3,5,8,11,13,16-hexaene [Bzo2Me2Pyo2(16)-hexaeneN6] (I) as membrane carrier, dibutylbutyl phosphonate (DBBP) as solvent mediator and sodium tetraphenylborate (NaTPB) as lipophilic additive. The best performance was given by the membrane of macrocycle I having a composition 10:260:5:120 (I:DBBP:NaTPB:PVC). The electrode exhibits a Nernstian response to La(III) ion in the concentration range 1.0x10(-1)-7.94x10(-7) M with a slope of 19.8+/-0.2 mV/decade of concentration and a detection limit of 5.62x10(-7) M. The response time of the sensor is 12 s and it can be used over a period of 4 months with good reproducibility. The electrode works well over a pH range of 2.5-10.0 and in partially non-aqueous medium with up to 30% organic content. The sensor was also used as an indicator electrode in potentiometric titration of La(III) ions with EDTA and for determining La(III) concentration in real samples.     

Determination of uranyl ion by potentiometric titration using an uranyl-selective electrode

Summary A potentiometric titration of uranyl ion is described using an uranyl selective electrode based on a membrane containing a complex of UO₂-bis[di-4-(1,1,3,3-tetra-methylbutyl)phenyl phosphate] as an ion-exchanger and tritolyl phosphate as a solvent mediator. The titrations were carried out with various titrants: sodium hydroxide, potassium fluoride and sodium salts of acetate, oxalate and citrate. The equivalence points were determined by Gran's method. Good results were obtained by using sodium oxalate as a titrant for the determination of uranium in several samples of ammonium diuranate. The results were quite comparable with those obtained by X-ray fluorescence spectrometry.     

Characteristics of Lariat Crown Ether‐Copper (II) Ion‐Selective Electrodes

An ion‐selective electrode using ionophore 2′‐picolyl sym‐dibenzo‐16‐crown‐5 ether as membrane carrier, sodium tetraphenylborate (NaTPB) as an anion excluder, and 2‐nitrophenyl‐octyl ether (NOPE) as the plasticing solvent mediator has been successfully developed. This electrode exhibits al in ear response with a slope of 42 mV/decade in concentration ranging from 10^?5 molL^?1 to 10^?1 molL^?1, slightly larger than the 30 mV expected from the one‐to‐one complex. The reason for the super‐Nernstain slope is the partial dimmer formation in side the membrane of the electrode, because this dimmer [Cu(C₂₅H₂₇NO₆)₂(H₂O)₂] 2ClO₄, has been isolated and confirmed by single crystal X‐ray crystallography. The detection limit for the cop per (II) ion was estimated to be 1 × 10^?6 molL^?1. Electrades composed of other plasticing solvent mediators such as tris(2‐ethylhexyl) phosphate (TOP), bis (2‐ethylhexyl) sebacate (DOS) and dibutyl phthalate (DBP) were also investigated. Stability constants (logKs) of the two to one and the one to one 2‐picolylsym‐dibenzo‐16‐crown‐5 ether‐Cu (II) complexes have been determined by potentiometric titration in methanol.     

Nafion membrane electrophoresis with direct and simplified end-column pulse electrochemical detection of amino acids

A novel electrophoresis technique, in which the separation column was replaced by a strip of Nafion membrane (5.0 cm x 0.20 mm x 0.25 mm), was developed for the separation of an amino acid mixture (glycine, asparic acid and lysine), followed by quadruple-pulse electrochemical detection. Nafion membrane contains hydrophilic pores (10-20 A and 50-60 A in size) acting as very narrow electrophoresis channels. The fixed-charge sites (-SO(3) (-)) on the hydrophilic pore surface provide a strong charged background. A platinum disk electrode (0.90 mm inner diameter) was employed as the detection electrode and the electrophoresis cathode was used as the quasi-reference and counter electrode for the end-column electrochemical detector, without decoupler. Under optimized conditions the mixture of amino acids could be separated at a voltage of only 90 V with a detection limit of 10(-7) M, indicating that Nafion membrane electrophoresis is a potentially attractive technique for the separation of small organic molecules or ions.     

On-line flow-injection monitoring of the enzyme inductor l-phenylalanine in the continuous cultivation of rhodococcus sp. M4

The o-phthaldialdehyde (OPA) assay for amino acids was adapted for flow injection analysis. With a dual-channel manifold and 2-μl injections, injection rates of 90–100 h^?1 were possible. Depending on the selected fluorimeter sensitivity, linear response ranges of 0.1–1.2 mM or 1–30 mM were obtained for l-phenylalanine with a relative standard deviation (RSD) of 0.9% for the slope of the calibration line. For ten injections of a 0.1 mM standard, the RSD was 1.6%. The detection limit was 0.01 mM. When the flow-injection method was combined with a fermentation sampling device and controlled by a timer, it was possible to monitor the decrease of l-phenylalanine on-line over a period of 87 h in the initial phase of a continuous cultivation of Rhodococcus sp. M4 producing the intracellular enzyme l-phenylalanine dehydrogenase. Correlation of the results with those obtained with an amino acid analyzer was good. Because OPA reacts with all primary amino groups, the applicability of the proposed method is restricted to cases in which only one amino acid and only small amounts of other amino group donators are present.     

Phase inversion cellulose acetate membranes for suppression of protein interferences in anodic stripping voltammetry 2(1). Improvement of the membrane preparation procedure

Phase inversion (PI) cellulose acetate membranes were cast on glassy carbon electrodes from a solution containing acetone as solvent and aqueous magnesium perchlorate as pore former. It is shown that a significant improvement of the reproducibility and permselective properties of the membrane is obtained by allowing complete evaporation of the solvent in a controlled humidity environment before the membrane is gelated. By using cadmium and lead as test analytes and differential pulse anodic stripping voltammetry as the detection method, it was found that the modification of the electrode greatly reduces the interference from albumin, lysozyme, gelatin and polyethylene glycol (MW 6000). The permselectivity of the PI membrane can be controlled by varying the amount of magnesium perchlorate in the casting solution and the relative humidity during the pre-gelation conditioning of the membrane.     

Preparation of a novel iodide-selective electrode based on iodide-miconazole ion-pair and its application to pharmaceutical analysis.

An iodide-miconazole ion-paired complex was used as a suitable ion-exchanger for the preparation of a plasticized-PVC membrane electrode. Among different solvent mediators tested, dioctylsebacate exhibited the proper response characteristics, including Nernstian slope of the calibration curve, fast response time and good reproducibility of the emf values. The electrode exhibits a Nernstian slope of -59.8 +/- 0.5 mV decade(-1) for I- ion over a concentration range of 1.0 x 10(-5) - 1.0 x 10(-2) M with a limit of detection of 7.0 x 10(-6) M. The electrode displays a good selectivity for I- with respect to a number of inorganic and organic species. It canbe used over a pH range of 2.5 - 8.5. The membrane sensor was successfully applied to the determination of iodide in water samples and blood serum, as well as in pharmaceutical products such as iodoquinol and thyroxin.     

Monitoring metal ion contamination onset in hydrofluoric acid using silicon-diamond and dual silicon sensing electrode assembly.

Potentiometric detection of trace levels of metallic contamination onset in hydrofluoric acid using a silicon-based sensor in conjunction with two non-contaminating reference electrode systems is presented in this paper. In the first case, conductive diamond was used as a non-contaminating reference electrode. Cyclic voltammetry and open-circuit potential experiments demonstrated the feasibility of using a conductive diamond film electrode as a quasi-reference electrode in the HF solution. In the second case, a dual silicon electrode system was used with one of the silicon-based electrodes protected with an anion permeable membrane behaving as the quasi-reference electrode. The dual silicon sensing electrode system possessed an additional operational advantage of being unaffected by the solution acidity. Though both sensing configurations were able to detect the metal ion contamination onsets at the parts-per-trillion to parts-per-billion levels, the dual silicon electrode design showed a greater compatibility for the on-line detection of metallic impurities in HF etching baths commonly used in semiconductor processing.     

Carbonate ion electrode based on HTPP sites

Several quaternary phosphonium salts have been used as the site materials for construction of carbonate ion electrodes. Among them the electrode based on hexadecyltriphenylphosphonium salt showed best performance characteristics. The Nerstian response range of the electrode is from 1 × 10^?2 down to 6.3 × 10^?7 mol · dm^?3 with a detection limit of 1.8 × 10^?7 mol · dm³. The selectivity order of ions can be altered by the introduction of trifluoroacetyl-tert-butylbenzene as a solvent mediator. The strong solvatoin of the primary ion of interest in the membrane phase by the solvent mediator favors the improvement of the selectivity of the proposed electrode.     

Polycation-sensitive membrane electrode for determination of heparin based on controlled release of protamine

A polycation-selective polymeric membrane electrode using dinonylnaphthalene sulfonate as an ion exchanger has been developed as a protamine controlled-release system for potentiometric detection of heparin. The incorporation of tetradodecylammonium tetrakis(4-chlorophenyl)borate as a lipophilic salt in the membrane dramatically improves the sensor's selectivity towards protamine over sodium ions via influencing the activity coefficient of protamine in the membrane, and a stable potential baseline can be obtained in the presence of an electrolyte background. The electrostatic binding interaction between heparin and protamine decreases the concentration of free protamine released at the sample-membrane interface and facilitates the stripping of protamine out of the membrane surface via the ion-exchange process with sodium ions, thus decreasing the membrane potential. Under optimal conditions, the proposed polymeric membrane electrode exhibits a linear relationship between the initial slope of the potential change and the heparin concentration in the range of 0.025-1.25 U mL(-1) with an improved detection limit of 0.01 U mL(-1).     

Membrane pH-Sensitive Electrodes Based on New Nitrogen-containing Neutral Carriers

A series of new symmetrical and asymmetrical aliphatic tertiary amines and nicotinic acid derivatives synthesized as hydrogen ion carriers have been used for preparing solvent polymeric membrane (SPM) pH electrodes. The electrodes prepared have wide linear mV-pH response range, and can be used for determination of pH of human blood serum, preparation of gas-sensing probes and end-point indication of acid-base titrations.The relationships between the structure of ionophores and electrode potential response characteristics were studied using quantum chemistry calculations.Super-Nernstian response was explained by establishment of a general membrane potential equation.     

Development of a dielectric barrier discharge ion source for ambient mass spectrometry

A new ion source based on dielectric barrier discharge was developed as an alternative ionization source for ambient mass spectrometry. The dielectric barrier discharge ionization source, termed as DBDI herein, was composed of a copper sheet electrode, a discharge electrode, and a piece of glass slide in between as dielectric barrier as well as sample plate. Stable low-temperature plasma was formed between the tip of the discharge electrode and the surface of glass slide when an alternating voltage was applied between the electrodes. Analytes deposited on the surface of the glass slide were desorbed and ionized by the plasma and the ions were introduced to the mass spectrometer for mass analysis. The capability of this new ambient ion source was demonstrated with the analysis of 20 amino acids, which were deposited on the glass slide separately. Protonated molecular ions of [M + H](+) were observed for all the amino acids except for L-arginine. This ion source was also used for a rapid discrimination of L-valine, L-proline, L-serine and L-alanine from their mixture. The limit of detection was 3.5 pmol for L-alanine using single-ion-monitoring (SIM). Relative standard deviation (RSD) was 5.78% for 17.5 nmol of L-alanine (n = 5). With the advantages of small size, simple configuration and ease operation at ambient conditions, the dielectric barrier discharge ion source would potentially be coupled to portable mass spectrometers.