Preparation and characterization of thiocyanate-selective electrodes based on new complexes of copper(II) as neutral carriers

The complexes of hydroxycitronellal (o-aminobenzoic acid) copper(II) (Cu(II)-HXAB) and salicylaldehyde (o-aminobenzoic acid) copper(II) (Cu(II)-SHAB) were used as neutral carriers in PVC-based membrane ion-selective electrodes. The electrode based on Cu(II)-HXAB exhibited near-Nernstian potential response to thiocyanate (SCN⁻) in a linear range of 1.0 × 10^− 6 to 1.0 × 10^− 1 M with a detection limit of 8.5 × 10^− 7 M and a slope of − 57.3 mV/decade in 0.01 M phosphate buffer solution (pH 5.0). The electrode exhibited high selectivity to SCN⁻ over other tested anions with an anti-Hofmeister selectivity sequence. The selectivity behavior might be discussed in terms of UV-Vis spectrum and infrared spectrum. The transfer process of thiocyanate across the membrane interface was investigated by making use of the AC impedance technique. The electrode containing Cu(II)-HXAB could be applied to thiocyanate analysis in waste water with satisfactory results.     

Chloride selective potentiometric sensor based on a newly synthesized hydrogen bonding anion receptor

A new potentiometric chloride sensor based on the use of anion receptor 2-(1-H-imidazo [4,5-f][1,10]phenanthroline-2-yl)-6methoxyphenol (HIPM) in poly(vinyl chloride) (PVC) matrix is reported. Effect of various plasticizers: 2-nitrophenyloctylether (o-NPOE), di-n-butylphthalate (DBP), diethyl phthalate (DEP), dioctylpthalate (DOP), tri-n-butyl phosphates (TBP), chloronapthalene (CN) and cation excluder, cetryltrimethylammoniumbromide (CTAB) was studied. The best performance was obtained with a membrane composition of PVC: o-NPOE:HIPM: CTAB ratio (w/w, %) of 31:60:7:3. The sensor exhibits significantly enhanced selectivity towards chloride over the concentration range 5.0 × 10⁻⁸ to 1.0 × 10⁻¹ M with a lower detection limit of 1.0 × 10⁻⁸ M and a Nernstian slope of 59.8 mV decade⁻¹ of activity. Influence of the membrane composition and possible interfering ions was investigated on the response properties of the electrode. Fast and stable response, good reproducibility and long-term stability are demonstrated. The sensor shows response time of <10 s and can be used for about 3 months without any considerable divergence in their potential response. Selectivity coefficients determined with matched potential method (MPM) indicate high selectivity for chloride. The proposed electrode shows fairly good discrimination of chloride from anions. It was successfully applied to determination of chloride ion in packed drinking water. The electrode can be used in the pH range of 6.5-8.0 and mixtures containing up to 20% (v/v) non-aqueous content. It was used as an indicator electrode in potentiometric titration of chloride ion against silver nitrate.     

The ultrasonic electropolymerization of an 5-[o-(4-bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) film electrode and its electrocatalytic properties to dopamine oxidation in aqueous solution

5-[o-(4-Bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) was electropolymerized on a glassy carbon electrode (GCE), and the electrocatalytic properties of the prepared film electrode response to dopamine (DA) oxidation were investigated. A stable o-BrPETPP film was formed on the GCE under ultrasonic irradiation through a potentiodynamic process in 0.1 M H₂SO₄ between −1.1 V and 2.2 V versus a saturated calomel electrode (SCE) at a scan rate of 0.1 V s⁻¹. The film electrode showed high selectivity for DA in the presence of ascorbic acid (AA) and uric acid (UA), and a 6-fold greater sensitivity to DA than that of the bare GCE. In the 0.05 mol L⁻¹ phosphate buffer (pH 6.0), there was a linear relationship between the oxidation current and the concentration of DA solution in the range of 5 × 10⁻⁷ mol L⁻¹ to 3 × 10⁻⁵ mol L⁻¹. The electrode had a detection limit of 6.0 × 10⁻⁸ mol L⁻¹(S/N = 3) when the differential pulse voltammetric (DPV) method was used. In addition, the charge transfer rate constant k = 0.0703 cm s⁻¹, the transfer coefficient α = 0.709, the electron number involved in the rate determining step n_α = 0.952, and the diffusion coefficient D_o = 3.54  10⁻⁵ cm² s⁻¹ were determined. The o-BrPETPP film electrode provides high stability, sensitivity, and selectivity for DA oxidation.     

Novel all-solid-state copper(II) microelectrode based on a dithiomacrocycle as a neutral carrier

A novel dithiomacrocycle (4-phenyl-11-decanoyl-1,7-dithia-11-azacyclotetradecane-4-sulfide) has been synthesized and used as a new ionophore in order to develop a plasticized poly(vinyl chloride) membrane for copper ion detection. The performance of these novel planar copper(II)-selective potentiometric microelectrodes was investigated using potentiometric measurements. The developed microelectrodes exhibits a good linear response of 29.5 ± 1 mV per decade within the concentration range of 1.0 × 10⁻⁶ to 1.0 × 10⁻² M (r = 0.9995) of Cu²⁺. The detection limit was determined as 5.62 × 10⁻⁷ M and the selectivity coefficients for possible interfering cations were evaluated. The microelectrodes are suitable for use with aqueous solutions of pH 3.5-6.0 and were found to be insensitive to the nature of the anions used in the sample.     

Insights into the simultaneous chronopotentiometric stripping measurement of indium(III), thallium(I) and zinc(II) in acidic medium at the in situ prepared antimony film carbon paste electrode

The simultaneous measurement of microgram per liter concentration levels of indium(III), thallium(I) and zinc(II) at the antimony film carbon paste electrode (SbF-CPE) is demonstrated. The antimony film was deposited in situ on a carbon paste substrate electrode and employed in chronopotentiometric stripping mode in deoxygenated solutions of 0.01 M hydrochloric acid (pH 2). The chronopotentiometric stripping performance of the SbF-CPE was studied and compared with constant current chronopotentiometric stripping and anodic stripping voltammetric operation. In comparison with its bismuth and mercury counterparts, the SbF-CPE exhibited advantageous electroanalytical performance; namely, at the bismuth film electrode, the measurement of zinc(II) was practically impossible due to hydrogen evolution, whereas the mercury film electrode exhibited a poorly developed signal for thallium(I). The SbF-CPE revealed favorable calculated LoDs (3σ) of 1.4 μg L⁻¹ for thallium(I) and 2.4 μg L⁻¹ for indium(III) along with good linear response in the examined concentration range from 10 to 100 μg L⁻¹ with correlations coefficients (R²) of 0.992 for thallium(I) and 0.994 for indium(III) associated with a 120 s deposition time. The chronopotentiometric stripping performance of the SbF-CPE was characterized also by satisfactory reproducibility of 1.62% for indium(III), 3.96% for thallium(I) and 2.11% for zinc(II) (c = 40 μg L⁻¹, n = 11).     

Aluminium (III)-selective PVC membrane sensor based on a Schiff base complex of N,N′-bis (salicylidene)-1, 2-cyclohexanediamine

A potentiometric aluminium sensor, based on the use N,N′-bis(salicylidene)-1,2-cyclohexanediamine (NBSC) as a neutral carrier, in poly(vinyl chloride) (PVC) matrix, is reported. Effect of various plasticizers; 2-nitrophenyloctylether (o-NPOE), tri-n-butyl phosphates (TBP), dioctylpththalate (DOP) & chloronapthalen (CN), and anion excluder, sodium tetraphenylborate (NaTPB) was studied. The best performance was obtained with a membrane composition of PVC: o-NPOE: NBSC: NaTPB ratio (w/w; mg) of 150:150:5:5. The sensor exhibits significantly enhanced selectivity toward Al³⁺ ions over the concentration range 1.0 × 10⁻⁸-1.0 × 10⁻¹ M with a lower detection limit of 5.0 × 10⁻⁹ M and a Nernstian slope of 20.3 ± 0.1 mV decade⁻¹ of activity. Influence of the membrane composition and possible interfering ions was investigated on the response properties of the electrode. Fast and stable response, good reproducibility and long-term stability are demonstrated. The sensor shows response time of <5 s and can be used for about 3 months without any considerable divergence in their potential response. Selectivity coefficients determined by matched potential method (MPM) indicate high selectivity for aluminium (III) ion. The proposed electrode shows fairly good discrimination of aluminium (III) from many metal ions. It was successfully applied for direct determination of aluminium (III) in biological, industrial and environmental samples. The electrode can be used in the pH range of 2.0-9.0 and mixtures containing up to 20% (v/v) non-aqueous content. It was used as an indicator electrode in potentiometric titration of aluminium ion vs. EDTA.     

On the p-doping of PEDOT layers in various ionic liquids studied by EQCM and acoustic impedance

Poly(3,4-ethylenedioxythiophene) (PEDOT) films have been polymerized in several ionic liquids on the polycrystalline gold electrode of an electrochemical quartz crystal microbalance (EQCM) at 25 °C and 85 °C. Under cyclic potential variation, the EQCM resonance frequency decreased in the anodic potential region, indicating that the p-doping process is accompanied by the incorporation of anions. Elastic shear moduli G′, G″ - calculated from acoustic impedance measurements - were about 10⁷ Pa, values that are 2 orders of magnitude higher than for PEDOT polymerized in acetonitrile solutions. This difference is explained by the stiffening of the film by incorporated charged species because of the absence of a neutral molecular solvent plasticizing the film.     

Electrochemical sensor for sulfite determination based on a nanostructured copper-salen film modified electrode

The electrochemical preparation described herein involved the electrocatalytic oxidation of sulfite on a platinum electrode modified with nanostructured copper salen (salen = N,N′-ethylenebis(salicylideneiminato)) polymer films. The complex was prepared and electropolymerized at a platinum electrode in a 0.1 mol L⁻¹ solution of tetrabutylammonium perchlorate in acetonitrile by cyclic voltammetry between 0 and 1.4 V vs. SCE. After cycling the modified electrode in a 0.50 mol L⁻¹ KCl solution, the estimated surface concentration was found to be equal to 2.2 × 10⁻⁹ mol cm⁻². This is a typical behavior of an electrode surface immobilized with a redox couple that can usually be considered as a reversible single-electron reduction/oxidation of the copper(II)/copper(III) couple. The potential peaks of the modified electrode in the electrolyte solution (aqueous) containing the different anions increase with the decrease of the ionic radius, demonstrating that the counter-ions influence the voltammetric behavior of the sensor. The potential peak was found to be linearly dependent upon the ratio [ionic charge]/[ionic radius]. The oxidation of the sulfite anion was performed at the platinum electrode at +0.9 V vs. SCE. However, a significant decrease in the overpotential (+0.45 V) was obtained while using the sensor, which minimized the effect of oxidizable interferences. A plot of the anodic current vs. the sulfite concentration for chronoamperometry (potential fixed = +0.45 V) at the sensor was linear in the 4.0 × 10⁻⁶ to 6.9 × 10⁻⁵ mol L⁻¹ concentration range and the concentration limit was 1.2 × 10⁻⁶ mol L⁻¹. The reaction order with respect to sulfite was determined by the slope of the logarithm of the current vs. the logarithm of the sulfite concentration.     

Complex impedance spectra of porous electrode with fractal structure

Transmission line model (TLM) is often used for the analysis of porous electrode since the current distribution in the pore can be calculated. In the present paper, the TLM is applied to the impedance analysis of complicated porous electrode with fractal structure. Activated carbon is used as electrode material for electric double layer capacitor and its pore has branch structure. Considering the fractal structure of the activated carbon, we proposed new equivalent circuit with TLM for three sizes of pores: macropore, mesopore and micropore. In this model, we defined the ratio ξ of electric double layer parts in whole inner wall of the pore because the inner wall is separated to electrolyte/electrode interface and branch base parts. Calculated electrochemical impedance of blocking electrode shows the lumped constant region in low frequency range and the distributed constant region in low frequency range. General TLM yields the straight line with slope of 45° in the distributed constant region on the Nyquist plots. In the present analysis, it was found that the slope was 22.5° at ξ = 0 in the case of double-pores (macropore and mesopore) and 11.25° at ξ = 0 in the case of triple-pores (macropore, mesopore and micropore).     

Preparation and electrochemistry of cadmium pentacyanonitrosylferrate film modified glassy carbon electrode

A glassy carbon (GC) electrode surface was modified with a cadmium pentacyanonitrosylferrate (CdPCNF) film as a novel electrode material. The modification procedure of the GC surface includes two consecutive procedures: (i) the electrodeposition of metallic cadmium on the GC electrode surface from a CdCl₂ solution and (ii) the chemical transformation of the deposited cadmium to the CdPCNF films in 0.05 M Na₂[Fe(CN)₅NO] + 0.5 M KNO₃ solution. The modified GC electrode showed a well-defined redox couple due to [Cd^IIFe^III/II(CN)₅NO]^0/−1 system. The effects of supporting electrolytes and solution pH were studied on the electrochemical behavior of the modified electrode. The diffusion coefficients of alkali-metal cations in the film (D), the transfer coefficient (α) and the charge transfer rate constant at the modifying film | electrode interface (k_s), were calculated in the presence of various alkali-metal cations. The stability of the modified electrode was investigated under various experimental conditions.     

Mass transport studies at rotating cylinder electrode (RCE): Influence of using plates and concentric cylinder as counter electrodes

This study shows the effect of using four-plate, six-plate and concentric cylinder as counter electrodes on the RCE mass transport characterization. The Cu(II)/Cu(0) process in sulfuric acid is employed as the model reaction. Based on the analysis of Sh = aRe^bSc^0.356 correlation, the values of the constant a, associated with shape and cell dimensions, were 0.012, 0.014, and 0.022 for the four-plate, six-plate and concentric cylinder, respectively. This behavior suggests that both, the shape and area of the counter electrode, influence the values of a. The values of b, associated with the hydrodynamic regime, were similar for the six-plate and the concentric counter electrodes, b = 0.91; whereas for the four-plate counter electrode, b = 0.95. The fact that the four-plate device gives greater values of b suggests that this device has a higher turbulence-promoting action than the others. The mass transport at the RCE with four plates has a performance similar to the concentric counter electrode, which is important from the technical and economics standpoints.     

Thermal conductivities from temperature profiles in the polymer electrolyte fuel cell

We report measured temperatures inside the single polymer fuel cell, and thermal conductivities and heat transfer coefficients calculated from these. Temperatures were measured next to the membrane on its two sides, and in the gas channels. Higher temperatures (5 °C or more at 1 A/cm²) were found at the membrane electrode surface than in the gas channels. The thermal conductivity of the membrane (λ^m) was small, as expected from the properties of water and polymer, while the heat transfer coefficient of the electrode surfaces (λ^s) was smaller, 1000±300 W/m² K for a layer thickness of 10 μm. The real coefficient is smaller, since the measured temperatures are systematically smaller than the real ones. The electrode surface heat transfer coefficient is not previously reported. The average value for the catalyst surface plus gas diffusion layer was 0.2 W/m K.     

Development of a voltammetric sensor based on a molecularly imprinted polymer (MIP) for caffeine measurement

A new voltammetric sensor for caffeine measurement is introduced. A caffeine-selective molecularly imprinted polymer (MIP) and a non-imprinted polymer (NIP) were synthesized and then used for carbon paste (CP) electrode preparation. The MIP, embedded in the carbon paste electrode, functioned as a selective recognition element and a pre-concentrator agent for caffeine determination. The prepared electrode was used for caffeine measurement via a three-step procedure including analyte extraction in the electrode, electrode washing and electrochemical measurement of caffeine. The MIP-CP electrode showed very high recognition ability in comparison to NIP-CP. It was shown that electrode washing after caffeine extraction led to enhanced selectivity. Differential pulse voltammetry for caffeine determination was more effective than square wave voltammetry. Some parameters affecting sensor response were optimized, and a calibration curve was then plotted. A linear range of 6 × 10⁻⁸ to 2.5 × 10⁻⁵ mol L⁻¹ was obtained. The detection limit of the sensor was calculated to be equal to 1.5 × 10⁻⁸ mol L⁻¹. This sensor was used successfully for caffeine determination in spiked beverage and tea samples.     

Glassy carbon electrode modified with Nafion-Au colloids for clenbuterol electroanalysis

Stable Nafion-Au colloids were immobilized on a glassy carbon electrode (GCE) for detection of β-agonist clenbuterol by electroanalysis. The Au colloids were prepared by a one-step electrodeposition onto GCE, with obvious electrocatalytic activity present. The negatively charged Nafion film was an efficient barrier to negatively charged interfering compounds, resulting in accumulation of positively charged clenbuterol at the Nafion film. The electrochemical characters of the electrode during various modified steps in a redox probe system of K₄[Fe(CN)₆]/K₃[Fe(CN)₆] were confirmed by cyclic voltammetry (CV) and AC-impedance. In Britton-Robinson (B-R) buffer solution (pH = 2.0) and the potential range of −0.2 to 1.2 V, the Nafion-Au colloid modified electrode, compared to a bare GCE, exhibits obvious electrocatalytic activity towards the redox of clenbuterol by greatly enhancing the peak current with a linear calibration curve from 8.0 × 10⁻⁷ to 1.0 × 10⁻⁵ mol/L and a detection limit of (1.0 × 10⁻⁷ mol/L) (R = 0.996). The modified electrode shows high sensitivity, selectivity and reproducibility. The recovery for detecting clenbuterol (∼10⁻⁶ mol/L) in human serum is up to 98.19%.     

Prediction of coal grindability based on petrography,proximate and ultimate analysis using multiple regression and artificial neural network models

The effects of proximate and ultimate analysis, maceral content, and coal rank (R_max) for a wide range of Kentucky coal samples from calorific value of 4320 to 14960 (BTU/lb) (10.05 to 34.80 MJ/kg) on Hardgrove Grindability Index (HGI) have been investigated by multivariable regression and artificial neural network methods (ANN). The stepwise least square mathematical method shows that the relationship between (a) Moisture, ash, volatile matter, and total sulfur; (b) ln (total sulfur), hydrogen, ash, ln ((oxygen + nitrogen)/carbon) and moisture; (c) ln (exinite), semifusinite, micrinite, macrinite, resinite, and R_max input sets with HGI in linear condition can achieve the correlation coefficients (R²) of 0.77, 0.75, and 0.81, respectively. The ANN, which adequately recognized the characteristics of the coal samples, can predict HGI with correlation coefficients of 0.89, 0.89 and 0.95 respectively in testing process. It was determined that ln (exinite), semifusinite, micrinite, macrinite, resinite, and R_max can be used as the best predictor for the estimation of HGI on multivariable regression (R² = 0.81) and also artificial neural network methods (R² = 0.95). The ANN based prediction method, as used in this paper, can be further employed as a reliable and accurate method, in the hardgrove grindability index prediction.     

Hydrogenated hydroxy-functionalized polyisoprene (H-HTPI) and isocyanurate of isophorone diisocyanates (I-IPDI): reaction kinetics study using FTIR spectroscopy

The reaction between a hydrogenated hydroxyl-functionalized polyisoprene (H-HTPI) and isophorone diisocyanate isocyanurate (I-IPDI) is followed by using direct FTIR spectroscopy. The reaction kinetics is studied using a simple model taking into consideration the I-IPDI structure. The rates of individual isocyanate groups are described by a second order equation. Influence of dibutyltin dilaurate (DBTL) concentration and temperature on selectivity, defined as the ratio between the rate constant of secondary isocyanate group and the rate constant of the primary isocyanate group, is investigated. It is observed that selectivity decreases when temperature or DBTL concentration increases. Eyring parameters are determined for the catalyzed [ΔH^*=77/35 (kJ mol⁻¹), ΔS^*=12/−100 (J mol⁻¹ K⁻¹)] and uncatalyzed reactions [ΔH^*=48/43 (kJ mol⁻¹), ΔS^*=−179/−167 (J mol⁻¹ K⁻¹)] primary and secondary isocyanate groups being differentiated.     

An electrochemical methanol sensor based on a Pd-Ni/SiNWs catalytic electrode

A novel electrochemical methanol sensor based on a catalytic electrode of palladium-nickel/silicon nanowires (Pd-Ni/SiNWs) is presented in this paper. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electrochemical methods are employed to investigate the Pd-Ni/SiNWs electrode materials. These nanocomposite materials exhibit a highly ordered, wire-like structure with a wire length of ∼50 μm and a wire diameter ranging from 100 to 300 nm. The substrate has good electrocatalytic activity towards the oxidation of methanol in alkaline solutions. The performances of the prototype sensor are characterized by cyclic voltammetry and fixed potential amperometry techniques. In a 1 mol L⁻¹ KOH solution containing different methanol concentrations, the sensor exhibits a good sensitivity of 1.96 mA mmol⁻¹ L cm⁻² with R² = 0.99 and the corresponding detection limit of 18 μmol L⁻¹ (signal-to-noise ratio = 3, S/N = 3) for cyclic voltammetry. Meanwhile, the electrode also displays a sensitivity of 0.48 mA mmol⁻¹ L cm⁻² with R² = 0.98 and the corresponding detection limit of 25 μmol L⁻¹ (S/N = 3) for a fixed potential amperometry at −0.3 V versus an Ag/AgCl reference electrode. The results demonstrate that the Pd-Ni/SiNWs catalytic electrode has potential as an efficient and integrated sensor for methanol detection.     

Construction of barium (II) PVC membrane electrochemical sensor based on 3-deoxy-d-erythro-hexos-2-ulose bis (thiosemicarbazone) as a novel ionophore

In this study, a new ion-selective electrode for Ba²⁺ is described, illustrating 3-deoxy-d-erythro-hexos-2-ulose bis (thiosemicarbazone) (DHUT) in a poly(vinylchloride) (PVC) membrane with benzyl acetate (BA) as a plasticizer and sodium tetraphenyl borate (NaTPB) as an anionic additive. This sensor presented very good selectivity and sensitivity towards the Ba²⁺ ions over a wide variety of cations, including alkali, alkaline earth, transition and heavy metal ions. The electrode revealed a great improvement in the selectivity coefficients for the Ba²⁺ ions in comparison with the formerly reported Ba²⁺ sensors. The proposed electrode exhibited a significantly enhanced response towards the Ba²⁺ ions across the concentration range of 1.0 × 10^− 6-1.0 × 10^− 2 M for the pH variation from 2.6 to 11 with a lower detection limit of 5.6 × 10^− 7 M. Moreover, the sensors displayed the Nernstian slope of 29.6 ± 0.5 mV per decade, having a fast response time within 15 s over the entire concentration range. It could also be used for at least 2 months with no potential divergence. As a result, the developed sensor was successfully applied to the direct barium ion determination in solutions, rock samples and as an indicator electrode in the Ba²⁺ ion potentiometric titration with EDTA.