Oxidation of substituted pyridines by dimethyldioxirane: kinetics and solvent effects

The oxidation of a series of substituted pyridines by dimethyldioxirane (1) produced the expected N-oxides in quantitative yields. The second order rate constants (k₂) for the oxidation of a series of substituted pyridines (2a-g) by dimethyldioxirane were determined in dried acetone at 23 °C. An excellent correlation with Hammett sigma values was found (ρ = −2.91, r = 0.995). Kinetic studies for the oxidation of 4-trifluoromethylpyridine by 1 were carried out in the following dried solvent systems: acetone (k₂ = 0.017 M⁻¹ s⁻¹), carbon tetrachloride/acetone (7:3; k₂ = 0.014 M⁻¹ s⁻¹), acetonitrile/acetone (7:3; k₂ = 0.047 M⁻¹ s⁻¹), and methanol/acetone (7:3; k₂ = 0.68 M⁻¹ s⁻¹). Kinetic studies of the oxidation of pyridine by 1 versus mole fraction of water in acetone [k₂ = 0.78 M⁻¹ s⁻¹ (χ = 0) to k₂ = 11.1 M⁻¹ s⁻¹ (χ = 0.52)] were carried out. The results showed the reaction to be very sensitive to protic, polar solvents.     

Kinetic study of the oxidative addition reaction between methyl iodide and [Rh(FcCOCHCOCF3)(CO)(PPh3)]: Structure of [Rh(FcCOCHCOCF3)(CO)(PPh3)(CH3)(I)]

The kinetics of oxidative addition of CH₃I to [Rh(FcCOCHCOCF₃)(CO)(PPh₃)], where Fc = ferrocenyl and (FcCOCHCOCF₃)⁻ = fctfa = ferrocenoylacetonato, have been studied utilizing UV/Vis, IR, ¹H and ³¹P NMR techniques. Three definite sets of reactions involving isomers of at least two distinctly different classes of Rh^III-alkyl and two different classes of Rh^III-acyl species were observed. Rate constants for this reaction in CHCl₃ at 25 °C, applicable to the reaction sequence below, were determined as k₁ = 0.00611(1) dm³ mol⁻¹ s⁻¹, k₋₁ = 0.0005(1) s⁻¹, k₃ = 0.00017(2) s⁻¹ and k₄ = 0.0000044(1) s⁻¹ while k₋₃ ? k₃ and k₋₄ ? k₄ but both ≠0. The indeterminable equilibrium K₂ was fast enough to be maintained during Rh^I depletion in the first set of reactions and during the Rh^IIIalkyl2 formation in the second set of reactions. From a ¹H and ³¹P NMR study in CDCl₃, K_c1 was found to be 0.68, K_c2 = 2.57, K_c3 = 1.00, K_c4 = 4.56 and K_c5 = 1.65.     

Development of a novel flow injection liquid-liquid microextraction method for the on-line separation and preconcentration for determination of zinc(II) using 5-(8-hydroxy-2-quinolinylmethyl)-2,8-dithia-5-aza-2,6-pyridinophane as a sensitive and selective fluorescent chemosensor

A novel flow injection analysis (FIA) system based on liquid-liquid microextraction and fluorimetric determination was developed for the determination of traces of the Zn²⁺ ion using 5-(8-hydroxy-2-quinolinylmethyl)-2,8-dithia-5-aza-2,6-pyridinophane (L) as a sensitive and selective fluorimetric sensor, with λ_ex = 373 nm and λ_em = 530 nm, and hexanol as the extracting organic solvent. In the designed FIA system, the phase separation takes place via gravitation forces in the absence of any segmenter. The influence of pH and ionic strength of the solution, amount of ligand, nature of counter ion, volume of organic solvent, extraction time and coil length was investigated. Under optimized experimental conditions, the calibration curve found to be liner over a concentration range of 0.025-4.53 μg mL⁻¹ (R² = 0.9951) with a limit of detection of 2.3 ng mL⁻¹. The enrichment factor was 45 and relative standard deviation for 7 replicate determinations was 2.43%. The method is very fast and uses low levels of organic solvents. The proposed method was applied successfully to the determination of zinc(II) in human hair, human serum and two inorganic sludge samples.     

A microfluidic chip based liquid-liquid extraction system with microporous membrane

A robust and simple approach for microfabricated chip based liquid-liquid extraction was developed for on-chip sample pretreatment. The chip based extraction system was composed of two microfabricated glass plates with a microporous membrane sandwiched in between. A simple bonding approach using epoxy was used to achieve bonding and sealing of the L-L extraction chip. Gravity was employed to drive the aqueous and organic flows through separate channels in the extraction system, separated by the membrane. During extraction, the analyte in an aqueous sample stream was transferred through the membrane into the organic stream. The fluorescence intensity of the analyte extracted into the organic stream was monitored in situ by a laser induced fluorescence detection system. The performance of the system was demonstrated using an aqueous solution of butyl rhodamine B (BRB) and isobutanol as sample and extractant, respectively. The system proved to be an efficient means for achieving chip based microporous membrane liquid-liquid extraction. The precision of fluorescence measurements was 1.5% R.S.D. (n = 4). A linear response range of 1 × 10⁻⁷ to 1 × 10⁻⁴ M BRB was obtained with a regression equation: I = 8.00 × 10⁶ C + 4.91. An enrichment factor of ca. 3 was obtained with an extraction efficiency of 69%.     

Adsorption of arsenic(III) into modified lamellar Na-magadiite in aqueous medium—Thermodynamic of adsorption process

Synthetic Na-magadiite sample was used for organofunctionalization process with N-propyldiethylenetrimethoxysilane and bis[3-(triethoxysilyl)propyl]tetrasulfide, after expanding the interlayer distance with polar organic solvents such as dimethylsulfoxide (DMSO). The resulted materials were submitted to process of adsorption with arsenic solution at pH 2.0 and 298±1 K. The adsorption isotherms were adjusted using a modified Langmuir equation with regression nonlinear; the net thermal effects obtained from calorimetric titration measurements were adjusted to a modified Langmuir equation. The adsorption process was exothermic (Δ_intH=−4.15-5.98 kJ mol⁻¹) accompanied by increase in entropy (Δ_intS=41.32-62.20 J k⁻¹ mol⁻¹) and Gibbs energy (Δ_intG=−22.44−24.56 kJ mol⁻¹). The favorable values corroborate with the arsenic (III)/basic reactive centers interaction at the solid-liquid interface in the spontaneous process.     

Comparison of pyrimethanil-imprinted beads and bulk polymer as stationary phase by non-linear chromatography

A pyrimethanil-imprinted polymer (P1) was prepared by iniferter-mediated photografting a mixture of methacrylic acid and ethylene dimethacrylate onto homemade near-monodispersed chloromethylated polydivinylbenzene beads. The chromatographic behaviour of a column packed with these imprinted beads was compared with another column packed with irregular particles obtained by grinding a bulk pyrimethanil-imprinted polymer (P2). The comparison was made using the kinetic model of non-linear chromatography, studying the elution of the template and of two related substances, cyprodinil and mepanipyrim. Extension of the region of linearity, capacity factors for the template and the related substances, column selectivity, binding site heterogeneity, apparent affinity constant (K) and lumped kinetic association (k_a) and dissociation rate constant (k_d) were studied during a large interval of solute concentration, ranging between 1 and 2000 μg/ml. From the experimental results obtained, in the linearity region of solute concentration column selectivity and binding site heterogeneity remained essentially the same for the two columns, while column capacity (at 20 μg/ml, P1 = 23.1, P2 = 11.5), K (at 20 μg/ml, P1 = 8.3 × 10⁶ M⁻¹, P2 = 2.5 × 10⁶ M⁻¹) and k_a (at 20 μg/ml, P1 = 3.5 μM⁻¹ s⁻¹, P2 = 0.47 μM⁻¹ s⁻¹) significantly increased and k_d (at 20 μg/ml, P1 = 0.42 s⁻¹, P2 = 0.67 s⁻¹) decreased for the column packed with the imprinted beads. These results are consistent with an influence of the polymerisation method on the morphology of the resulting polymer and not on the molecular recognition properties due to the molecular imprinting process.     

A novel application of immobilization on membranes for the separation and spectrofluorimetric quantification of amiloride and furosemide in pharmaceutical samples

A new, simple and highly sensitive method for spectrofluorimetric determination of amiloride (AMI) and furosemide (FUR) in pharmaceuticals is presented. The proposed method is based on the separation of AMI from FUR by solid-phase extraction using a nylon membrane, followed by spectrofluorimetric determination of both drugs, on the solid surface and the filtered aqueous solution, respectively. AMI shows low native fluorescence, but its separation-preconcentration by immobilization (solid-phase extraction) on nylon membrane surface provides a considerable enhancement in fluorescence intensity. The fluorescence determination is carried out at λ_ex = 237, λ_em = 415 nm for FUR; and λ_ex = 365, λ_em = 406 nm for AMI. The calibration graphs are linear in the range 3.20 × 10⁻⁴ to 0.8 μg mL⁻¹and 1.33 × 10⁻³ to 4.0 μg mL⁻¹, for AMI and FUR, respectively, with a detection limit of 9.62 × 10⁻⁵ and 4.01 × 10⁻⁴ μg mL⁻¹ (S/N = 3). The commonly found excipients in commercial pharmaceutical formulations do not interfere. The developed method is successfully applied to the determination of both drugs in pharmaceutical formulations.     

Magnetic properties of linear trimers in fluoride analogs of tetragonal tungsten bronze

The compounds KZnTiF₆, KZnVF₆, KVScF₆, KCrScF₆, and KMnScF₆ are fluoride analogs of Tetragonal Tungsten Bronze. M²⁺-M³⁺ ionic ordering in these fluorides provided systems which contained linear trinuclear complexes of their respective paramagnetic ions. Magnetic coupling within these linear trimers occurred below 100 K in each of the five systems. Derived magnetic susceptibility equations were fitted to observed magnetic susceptibilities for each of the possible spin systems: KZnTiF₆ (S=1/2), J/k=−114 K; KZnVF₆ (S=1), J/k=−39 K; KVScF₆ (S=3/2), J/k=−16 K; KCrScF₆ (S=2), J/k=−4 K; and KMnScF₆ (S=5/2), J/k=−7.5 K.     

Kinetics of the CH3 + HCl/DCl → CH4/CH3D + Cl and CD3 + HCl/DCl → CD3H/CD4 + Cl reactions: An experimental H atom tunneling investigation

The kinetics of the radical reactions of CH₃ with HCl or DCl and CD₃ with HCl or DCl have been investigated in a temperature controlled tubular reactor coupled to a photoionization mass spectrometer. The CH₃ (or CD₃) radical, R, was produced homogeneously in the reactor by a pulsed 193 nm exciplex laser photolysis of CH₃COCH₃ (or CD₃COCD₃). The decay of CH₃/CD₃ was monitored as a function of HCl/DCl concentration under pseudo-first-order conditions to determine the rate constants as a function of temperature, typically from 188 to 500 K. The rate constants of the CH₃ and CD₃ reactions with HCl had strong non-Arrhenius behavior at low temperatures. The rate constants were fitted to a modified Arrhenius expression k = QA exp (−E_a/RT) (error limits stated are 1σ + Students t values, units in cm³ molecule⁻¹ s⁻¹): k(CH₃ + HCl) = [1.004 + 85.64 exp (−0.02438 × T/K)] × (3.3 ± 1.3) × 10⁻¹³ exp [−(4.8 ± 0.6) kJ mol⁻¹/RT] and k(CD₃ + HCl) = [1.002 + 73.31 exp (−0.02505 × T/K)] × (2.7 ± 1.2) × 10⁻¹³ exp [−(3.5 ± 0.5) kJ mol⁻¹/RT]. The radical reactions with DCl were studied separately over a wide ranges of temperatures and in these temperature ranges the rate constants determined were fitted to a conventional Arrhenius expression k = A exp (−E_a/RT) (error limits stated are 1σ + Students t values, units in cm³ molecule⁻¹ s⁻¹): k(CH₃ + DCl) = (2.4 ± 1.6) × 10⁻¹³ exp [−(7.8 ± 1.4) kJ mol⁻¹/RT] and k(CD₃ + DCl) = (1.2 ± 0.4) × 10⁻¹³ exp [−(5.2 ± 0.2) kJ mol⁻¹/RT] cm³ molecule⁻¹ s⁻¹.     

Reactivity studies of trans-[PtClMe(SMe2)2] towards anionic and neutral ligand substitution processes

Reaction of trans-[PtClMe(SMe₂)₂] with the mono anionic ligands azide, bromide, cyanide, iodide and thiocyanate result in substitution of the chloro ligand as the first step. In contrast the neutral ligands pyridine, 4-Me-pyridine and thiourea substitute a SMe₂ ligand in the first step as confirmed by ¹H NMR spectroscopy and the kinetic data. Detailed kinetic studies were performed in methanol as solvent by use of conventional stopped-flow spectrophotometry. All processes follow the usual two-term rate law for square-planar substitutions, k_obs = k₁ + k₂[Y] (where k₁ = k_MeOH[MeOH]), with k₁ = 0.088 ± 0.004 s⁻¹ and k₂ = 1.18 ± 0.13, 3.8 ± 0.3, 17.8 ± 1.3, 34.9 ± 1.4, 75.3 ± 1.1 mol⁻¹ dm³ s⁻¹ for Y⁻ = N₃, Br, CN, I and SCN respectively at 298 K. The reactions with the neutral ligands proceed without an appreciable intercept with k₂ = 5.1 ± 0.3, 15.3 ± 1.8 and 195 ± 3 mol⁻¹ dm³ s⁻¹ for Y = pyridine, 4-Me-pyridine and thiourea, respectively, at 298 K. Activation parameters for MeOH, , Br⁻, CN⁻, I⁻, SCN⁻, and Tu are ΔH^≠ = 47.1 ± 1.6, 49.8 ± 0.6, 39 ± 3, 32 ± 8, 39 ± 5, 34 ± 4 and 31 ± 3 kJ mol⁻¹ and ΔS^≠ = −107 ± 5, −77 ± 2, −104 ± 9,−113 ± 28, −85 ± 18, −94 ± 14 and −97 ± 10 J K⁻¹ mol⁻¹, respectively. Recalculation of k₁ to second-order units gives the following sequence of nucleophilicity: (1:13:42:57:170:200:390:840:2170) at 298 K. Variation of the leaving group in the reaction between trans-[PtXMe(SMe₂)₂] and SCN⁻ follows the same rate law as stated above with k₂ = 75.3 ± 1.1, 236 ± 4 and 442 ± 5 mol⁻¹ dm³ s⁻¹ for X⁻ = Cl, I and N₃, respectively, at 298 K. The corresponding activation parameters were determined as ΔH^≠ = 34 ± 4, 32 ± 2 and 39.3 ± 1.7 kJ mol⁻¹ and ΔS^≠ = −94 ± 14, −86 ± 8 and −68 ± 6 J K⁻¹ mol⁻¹. All the kinetic measurements indicate the usual associate mode of activation for square planar substitution reactions as supported by large negative entropies of activation, a significant dependence of the reaction rate on different entering nucleophiles and a linear free energy relationship.     

Multivariate optimization of molecularly imprinted polymer solid-phase extraction applied to parathion determination in different water samples

In this work a parathion selective molecularly imprinted polymer was synthesized and applied as a high selective adsorber material for parathion extraction and determination in aqueous samples. The method was based on the sorption of parathion in the MIP according to simple batch procedure, followed by desorption by using methanol and measurement with square wave voltammetry. Plackett-Burman and Box-Behnken designs were used for optimizing the solid-phase extraction, in order to enhance the recovery percent and improve the pre-concentration factor. By using the screening design, the effect of six various factors on the extraction recovery was investigated. These factors were: pH, stirring rate (rpm), sample volume (V₁), eluent volume (V₂), organic solvent content of the sample (org%) and extraction time (t). The response surface design was carried out considering three main factors of (V₂), (V₁) and (org%) which were found to be main effects. The mathematical model for the recovery percent was obtained as a function of the mentioned main effects. Finally the main effects were adjusted according to the defined desirability function. It was found that the recovery percents more than 95% could be easily obtained by using the optimized method. By using the experimental conditions, obtained in the optimization step, the method allowed parathion selective determination in the linear dynamic range of 0.20-467.4 μg L⁻¹, with detection limit of 49.0 ng L⁻¹ and R.S.D. of 5.7% (n = 5). Parathion content of water samples were successfully analyzed when evaluating potentialities of the developed procedure.     

Enthalpy change and mechanism of oxidation of o-phenylenediamine by hydrogen peroxide catalyzed by horseradish peroxidase

The hydrogen peroxide-oxidation of o-phenylenediamine (OPD) catalyzed by horseradish peroxidase (HRP) at 37 °C in 50 mM phosphate buffer (pH 7.0) was studied by calorimetry. The apparent molar reaction enthalpy with respect to OPD and hydrogen peroxide were −447 ± 8 kJ mol⁻¹ and −298 ± 9 kJ mol⁻¹, respectively. Oxidation of OPD by H₂O₂ catalyzed by HRP (1.25 nM) at pH 7.0 and 37 °C follows a ping-pong mechanism. The maximum rate V_max (0.91 ± 0.05 μM s⁻¹), Michaelis constant for OPD K_m,S (51 ± 3 μM), Michaelis constant for hydrogen peroxide Km,H₂O₂ (136 ± 8 μM), the catalytic constant k_cat (364 ± 18 s⁻¹) and the second-order rate constants k₊₁ = (2.7 ± 0.3) × 10⁶ M⁻¹ s⁻¹ and k₊₅ = (7.1 ± 0.8) × 10⁶ M⁻¹ s⁻¹ were obtained by the initial rate method.     

A novel magnetic poly(aniline-naphthylamine)-based nanocomposite for micro solid phase extraction of rhodamine B

A novel Fe₃O₄–poly(aniline-naphthylamine)-based nanocomposite was synthesized by chemical oxidative polymerization process as a magnetic sorbent for micro solid phase extraction. The scanning electron microscopy images of the synthesized nanocomposite revealed that the copolymer posses a porous structure with diameters less than 50 nm. The extraction efficiency of this sorbent was examined by isolation of rhodamine B, a mutagenic and carcinogenic dye, from aquatic media in dispersion mode. Among different synthesized polymers, Fe₃O₄/poly(aniline-naphthylamine) nanocomposite showed a prominent efficiency. Parameters including the desorption solvent, amount of sorbent, desorption time, sample pH, ionic strength, extraction time and stirring rate were optimized. Under the optimum condition, a linear spiked calibration curve in the range of 0.35–5.00 μg L⁻¹ with R² = 0.9991 was obtained. The limits of detection (3S_b) and limits of quantification (10S_b) of the method were 0.10 μg L⁻¹ and 0.35 μg L⁻¹ (n = 3), respectively. The relative standard deviation for water sample with 0.5 μg L⁻¹ of RhB was 4.2% (n = 5) and the absolute recovery was 92%. The method was applied for the determination of rhodamine B in dishwashing foam, dishwashing liquid, shampoo, pencil, matches tips and eye shadows samples and the relative recovery percentage were in the range of 94–99%.     

A new anionic exchange stir bar sorptive extraction coating based on monolithic material for the extraction of inorganic anion

A novel anionic exchange stir bar sorptive extraction (SBSE) coating based on poly(2-(methacryloyloxy)ethyltrimethylammonium chloride-co-divinylbenzene) monolithic material for the extraction of inorganic anion was prepared. The effect of preparation conditions such as ratio of functional monomer to cross-linker, content of porogenic solvent on the extraction efficiencies were investigated in detailed. The monolithic material was characterized by elemental analysis, scanning electron microscopy and infrared spectroscopy. In order to investigate the extraction capacity of the new coating for inorganic anion, the new SBSE was combined with ionic chromatography with conductivity detection, Br⁻, NO₃⁻, PO₄³⁻ and SO₄²⁻ were selected as detected solutes. Several extractive parameters, including pH value and ionic strength in sample matrix, desorption solvent, extraction and desorption time were optimized. The results showed that strongly ionic strength did not favor the extraction of anlaytes. Under the optimum experimental conditions, low detection limits (S/N = 3) and quantification limits (S/N = 10) of the proposed method for the target anions were achieved within the range of 0.92–2.62 and 3.03–9.25 μg/L, respectively. The method also showed good linearity, simplicity, practicality and low cost for the extraction inorganic anions. Finally, the proposed method was successfully used to detect the two different trademarks of commercial purified water with satisfactory recovery in the range of 70.0–92.6%. To the best of our knowledge, this is the first to use SBSE to enrich inorganic anions.     

Hollow fiber-based liquid phase microextraction combined with high-performance liquid chromatography for extraction and determination of some antidepressant drugs in biological fluids

The applicability of hollow fiber-based liquid phase microextraction (HF-LPME) was evaluated for the extraction and preconcentration of three antidepressant drugs (amitriptyline, imipramine and sertraline) prior to their determination by HPLC-UV. The target drugs were extracted from 11.0 mL of aqueous solution with pH 12.0 (source phase) into an organic extracting solvent (n-dodecane) impregnated in the pores of a hollow fiber and finally back extracted into 24 μL of aqueous solution located inside the lumen of the hollow fiber and adjusted to pH 2.1 using 0.1 M of H₃PO₄ (receiving phase). The extraction was performed due to pH gradient between the inside and outside of the hollow fiber membrane. In order to obtain high extraction efficiency, the parameters affecting the HF-LPME including pH of the source and receiving phases, the type of organic phase, ionic strength and volume of the source phase, stirring rate and extraction time were studied and optimized. Under the optimized conditions, enrichment factors up to 300 were achieved and the relative standard deviation (R.S.D.%) of the method was in the range of 2-12%. The calibration curves were obtained in the range of 5-500 μg L⁻¹ with reasonable linearity (R² > 0.998) and the limits of detection (LODs) ranged between 0.5 and 0.7 μg L⁻¹ (based on S/N = 3). Finally, the applicability of the proposed method was evaluated by extraction and determination of the drugs in urine, plasma and tap water samples. The results indicated that hollow fiber microextraction method has excellent clean-up and high-preconcentration factor and can be served as a simple and sensitive method for monitoring of antidepressant drugs in the biological samples.     

High-throughput quantification of palladium in water samples by ion pair based-surfactant assisted microextraction

A novel method for the determination of palladium as a metal ion model was developed by ion pair based surfactant-assisted microextraction (IP-SAME) and inductively coupled plasma-optical detection (ICP-OES). In this methodology, a cationic surfactant was used in extraction process. It has two fundamental functions: (1) the formation of an emulsified phase and (2) the ion pair formation with Pd(II) in the presence of iodide ions and making PdI₄²⁻$\text{Pd}{{\text{I}}_4}^{2-}$ extractable into organic phase (active microextraction). The effective parameters on the extraction recovery such as the types of extraction solvent and the surfactant, surfactant concentration, KI amount and HCl concentration of the sample were investigated and optimized. In the proposed approach, tetradecyl trimethyl ammonium bromide (TTAB) was used as emulsifier and ion pairing agent, and 1-octanol was selected as extraction solvent. Under the optimum conditions, the enhancement factor as large as 146 was obtained. The detection limit for palladium was 0.2 μg L⁻¹, and the relative standard deviation (RSD) was 4.1% (n = 5, C = 10.0 μg L⁻¹). The proposed method was applied for extraction and determination of palladium in different water samples.     

On-line preconcentration and separation of Co, Ni and Cd via capillary microextraction on ordered mesoporous alumina coating and determination by inductively plasma mass spectrometry (ICP-MS)

In this paper, an ordered mesoporous alumina coating was prepared and applied to capillary microextraction (CME) of trace Co, Ni and Cd for the first time. The coated capillary was used for on-line coupling CME with inductively plasma mass spectrometry (ICP-MS) for the determination of trace metals of Co, Ni and Cd. The porous structure of Al₂O₃ coating was examined by SEM and TEM. The effects of the extraction parameters including pH, sample flow rate and volume, elution solution and interfering ions on the recovery of analytes have been investigated and optimized. Under the optimized conditions, the limits of detection were 0.33, 1.5 and 1.4 ng L⁻¹ for Co, Ni and Cd, respectively, with a preconcentration factor of 10 times. The precisions for all investigated elements were 2.7, 4.1 and 2.5% (c = 0.05 ng L⁻¹, n = 7), for Co, Ni and Cd, respectively, and the sample frequency was 8 h⁻¹.The proposed method was successfully applied for the analysis of trace metals in water, rice and urine samples with the recovery of 94-105%. In order to validate the proposed method, two certified reference materials of GBW 0913 human urine and NIES No.10-b rice flour were analyzed, and the determination values are in good agreement with the certified values. The ordered mesoporous Al₂O₃ coated capillary can be used more than 20 times without decreasing the extraction efficiency.     

Study of diffusion-controlled and activation-diffusion-controlled electron-transfer processes from quenching measurements

The diffusion-controlled rate constants, k_d, of various quenching reactions, [Ru(L)₃]^2+∗ (L = bpy, phen and 4,7-(CH₃)₂phen) + [Fe(CN)₆]³⁻, were measured through fluorescence measurements. From them, the effective values of viscosity coefficients for several methanol + water mixtures were calculated. These coefficients were checked through calculations of the rate constants of the reaction [IrCl₆]²⁻ + [Ru(bpy)₃]^2+∗, which were also obtained by fluorescence quenching measurements. The agreement between the two sets of data (experimental and predicted) is excellent. Besides, the trends of association, k_d, and dissociation, k_−d, rate constants for 2+/3−, 2+/2− and 2+/2+ reactions in methanol-water mixtures are discussed. The use of effective diffusion coefficients for estimating k_d and k_−d allowed us to obtain the intrinsic electron transfer rate constant, k_et, for the activation-diffusion-controlled process between [Ru(bpy)₃]^2+∗ and [Co(NH₃)₅Cl]²⁺ complexes from the observed (quenching) rate constant. The influence of methanol-water mixtures on k_et was rationalized by using the Marcus electron-transfer treatment.     

Thin Pd–23%Ag/stainless steel composite membranes: Long-term stability,life-time estimation and post-process characterisation

The long-term stability of Pd–23%Ag/stainless steel composite membranes has been examined in H₂/N₂ mixtures as a function of both temperature and feed pressure. During continuous operation, the membrane shows a good stability at 400 °C while the N₂ leakage increases very slowly at a temperature of 450 °C (P_feed = 10 bar). After 100 days of operation (P_feed = 5–20 bar, T = 350–450 °C), the N₂ permeance equals 7.0 × 10⁻⁹ mol m⁻² s⁻¹ Pa⁻¹, which indicates that the H₂/N₂ permselectivity still lies around 500, based on a H₂ permeance equal to 3.0 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹. Despite the generation of small pinholes, a membrane life-time of several (2–3) years (T ≤ 425 °C) is estimated for the experimental conditions employed based on long-term stability tests over 100 days. Post-process characterisation shows a considerable grain growth and micro-strain relaxation in the Pd–23%Ag membrane after the prolonged permeation experiment. Changes in surface area are relatively small. In addition, segregation of Ag to the membrane surfaces is observed. The formation of pinholes is identified as the main source for the increased N₂ leakage during testing at higher temperature.     

Simple hollow fiber renewal liquid membrane extraction method for pre-concentration of Cd(II) in environmental samples and detection by Flame Atomic Absorption Spectrometry

A hollow fiber renewal liquid membrane (HFRLM) extraction method to determine cadmium (II) in water samples using Flame Atomic Absorption Spectrometry (FAAS) was developed. Ammonium O,O-diethyl dithiophosphate (DDTP) was used to complex cadmium (II) in an acid medium to obtain a neutral hydrophobic complex (ML₂). The organic solvent introduced to the sample extracts this complex from the aqueous solution and carries it over the poly(dimethylsiloxane) (PDMS) membrane, that had their walls previously filled with the same organic solvent. The organic solvent is solubilized inside the PDMS membrane, leading to a homogeneous phase. The complex strips the lumen of the membrane where, at higher pH, the complex Cd-DDTP is broken down and cadmium (II) is released into the stripping phase. EDTA was used to complex the cadmium (II), helping to trap the analyte in the stripping phase. A multivariate procedure was used to optimize the studied variables. The optimized variables were: sample (donor phase) pH 3.25, DDTP concentration 0.05% (m/v), stripping (acceptor phase) pH 8.75, EDTA concentration 1.5 × 10⁻² mol L⁻¹, extraction temperature 40 °C, extraction time 40 min, a solvent mixture N-butyl acetate and hexane (60/40%, v/v) with a volume of 100 μL, and addition of ammonium sulfate to saturate the sample. The sample volume used was 20 mL and the stripping volume was 165 μL. The analyte enrichment factor was 120, limit of detection (LOD) 1.3 μg L⁻¹, relative standard deviation (RSD) 5.5% and the working linear range 2-30 μg L⁻¹.