Voltammetric studies on the α-tocopherol anion and α-tocopheroxyl (Vitamin E) radical in acetonitrile

The α-tocopheroxyl radical was generated voltammetrically by one-electron oxidation of the α-tocopherol anion (E ^r_1/2=−0.73 V versus Ag|Ag⁺) that was prepared by reacting α-tocopherol with Et₄NOH in acetonitrile (with Bu₄NPF₆ as the supporting electrolyte). Cyclic voltammograms recorded at variable scan rates (0.05–10 V s⁻¹), temperatures (−20 to 20°C) and concentrations (0.5–10 mM) were modelled using digital simulation techniques to determine the rate of bimolecular self-reaction of α-tocopheroxyl radicals. The k values were calculated to be 3×10³ l mol⁻¹ s⁻¹ at 20°C, 2×10³ l mol⁻¹ s⁻¹ at 0°C and 1.2×10³ l mol⁻¹ s⁻¹ at −20°C. In situ electrochemical-EPR experiments performed at a channel electrode confirmed the existence of the α-tocopheroxyl radical.     

Copper(II)-cetyltrimethylammonium chloride-chromal blue G ternary complex and its application to the spectrophotometric determination of copper(II)

A sensitive spectrophotometric method for the determination of copper(II) based on a ternary complex with chromal blue G, a triphenylmethane reagent in the presence of cetyltrimethylammonium chloride, is described. The sensitivity of color reaction between copper and chromal blue G has been greatly increased by the sensitizing action of cetyltrimethylammonium chloride, a cationic surfactant. The color development of the ternary complex can be utilized in the highly sensitive spectrophotometric determination of copper. The molar absorptivity of the binary complex between copper and chromal blue G ε_630nm = 9.56 × 10³liters · mol⁻¹ · cm⁻¹ is enchanced on ternary complex formation to ε_{542 nm} = 4.78 × 10⁴liters · mol⁻¹ · cm⁻¹. The ternary complex gave a maximal absorbance at 542 nm in the pH range 9.8–11. Beer's law is obeyed up to at least 1.2 ppm of copper. The maximal absorbance of the ternary complex was found to develop within 5 min and then it remains constant for several hours. The formation constant of the ternary complex is calculated to be 8.6 × 10¹⁰ under these conditions.     

UV–visible spectrum of the phenyl radical and kinetics of its reaction with NO in the gas phase

Pulse radiolysis transient UV–visible absorption spectroscopy was used to study the UV–visible absorption spectrum (225–575 nm) of the phenyl radical, C₆H₅(), and kinetics of its reaction with NO. Phenyl radicals have a strong broad featureless absorption in the region of 225–340 nm. In the presence of NO phenyl radicals are converted into nitrosobenzene. The phenyl radical spectrum was measured relative to that of nitrosobenzene. Based upon σ(C₆H₅NO)_{270 nm}=3.82×10⁻¹⁷ cm² molecule⁻¹ we derive an absorption cross-section for phenyl radicals at 250 nm, σ(C₆H₅())_{250 nm}=(2.75±0.58)×10⁻¹⁷ cm² molecule⁻¹. At 295 K in 200–1000 mbar of Ar diluent k(C₆H₅()+NO)=(2.09±0.15)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹.     

A study on terbium sensitized chemiluminescence of ciprofloxacin and its application

A novel rapid flow injection method with chemiluminescence (CL) detection was established for the determination of ciprofloxacin (CPLX), which is an antibiotic commonly used. The method is based on CL of Ce(IV)–SO₃²⁻ sensitized by Tb³⁺–CPLX, and showed the intensive bands characteristic of Tb³⁺ (⁵D₄→⁷F₅). The optimum conditions for CL emission were investigated. The linear relationship between the relative CL intensity and the concentration of CPLX is in the range of 9.0×10⁻⁹–1.0×10⁻⁶ mol/l with a detection limit of 3.1×10⁻¹⁰ mol/l. The relative standard deviation is 2.8% (n=11) for a level of 5.0×10⁻⁸ mol/l. The method was applied to the analysis of CPLX in human serum and urine samples with satisfactory results. The possible mechanism for this sensitized CL reaction is also discussed.     

A selective sensing membrane for the determination of tetracycline with heptakis (2,6-di-O-isobutyl)-β-cyclodextrin as the substrate

When heptakis (2,6-di-O-isobutyl)-β-cyclodextrin(DOB-β-CD) is immobilized in a plasticized poly vinyl chloride (PVC) membrane, it extracts tetracycline (TC) from the sample solution into the organic membrane phase to form a complex of DOB-β-CD and TC. Since the complex formation results in an enhancement of fluorescence intensity of TC at 506 nm, the chemical recognition process can be directly translated into an optical signal. The maximum response of the sensitive membrane for TC was obtained in 0.2 mol/l KH₂PO₄–KOH buffer solution (pH 8.01). In the optimum conditions described, the proposed sensor responds linearly in the measuring range of 2.00×10⁻⁶ mol/l to 4.00×10⁻⁴ mol/l, and has a detection limit of 8.00×10⁻⁷ mol/l. The response time of the sensor is within 2.0 min. In addition to high reproducibility and reversibility, the sensor also exhibits good selectivity over some common pharmaceutical species and some common organic and inorganic compounds.     

Pulsed microwave irradiation effects on the dynamic behavior of a photochemically generated radical pair in a micellar solution

The radical pair dynamics in a photochemical hydrogen abstraction reaction of 2-methyl-1,4-naphthoquinone in a sodium dodecylsulfate micelle was modulated by a microwave pulse. After a short resonant 180° microwave pulse, the recombination of the radical pair was enhanced, its rate constant being determined to be (8.3±0.8)×10⁶ s⁻¹. Other kinetic parameters were determined by the scanning of the microwave pulse position as follows: the formation of the radical pair (3.3±0.3)×10⁷ s⁻¹, the relaxation rate from the triplet (T_±1) levels to the singlet–triplet (T₀) mixed one (3.3±0.3)×10⁵ s⁻¹ at 331 mT, and the radical escape rate (5.8±0.6)×10⁵ s⁻¹.     

Studies of the hydrogen evolution reaction on smooth Co and electrodeposited Ni–Co ultramicroelectrodes

The hydrogen evolution reaction (HER) was studied on smooth Co and on electrodeposited Ni–Co ultramicroelectrodes (UMEs) in alkaline solutions at several temperatures by steady-state polarisation curves. The real electrochemical area was previously estimated by cyclic voltammetry to account for the large difference in roughness factor of the two surfaces. The values obtained for the Tafel slopes were very close to 2.303RT/βnF while the ‘apparent’ energies of activation were 59 and 41 kJ mol⁻¹ for Co and Ni–Co, respectively. A common Volmer–Heyrovsky mechanism with Heyrovsky as the rate-determining step (RDS) was initially proposed. This was confirmed when the experimental results were mathematically treated by a non-linear fitting procedure using the kinetic equations derived for that mechanism. The calculations revealed that Ni–Co is a more efficient catalyst for the HER then pure Co, with a rate constant value of 0.16×10⁻¹⁰ mol s⁻¹ cm⁻² at 25°C for the slow step. Although this value is more than one order of magnitude smaller than that already reported for deposited Ni, it is considerably larger than the one measured here (0.02×10⁻¹⁰ mol s⁻¹ cm⁻²) for pure Co at 25°C.     

Spectrophotometric determination of titanium (IV) with chlorpromazine hydrochloride

Spectrophotometric Determination of Titanium(IV) with Chlorpromazine HydrochlorideThe extraction of titanium(IV)-thiocyanate complex from hydrochloric acid with chlorpromazine (CPA) dissolved in chloroform has been studied. The composition of the extracted species was found to be (CPAH⁺)₂[Ti(CNS)₆²⁻]. This complex, dissolved in chloroform, has maximum absorbance at 417 nm and a molar absorptivity of 2.6 × 10⁴ mol⁻¹ · cm⁻¹ · liter. Titanium(IV) was determined spectrophotometrically in the organic phase. Beer's law is obeyed in the titanium concentration range 0.2-2.2 μg/ml.     

Individual and simultaneous fluorimetric determination of glycine and cysteine by flow injection analysis

A dual-channel flow injection configuration is proposed for individual and simultaneous determinations of glycine and cysteine (based on their reaction with phthaldialdehyde) in the range 1 × 10⁻⁷ – 1.5 × 10⁻⁵ M, with relative standard deviations of ± 1.7 and ± 1.3%, respectively, and a sampling frequency of 75–80 hr⁻¹ for both analytes. The inclusion of a selecting valve in the configuration allows the simultaneous determination of these analytes in the concentration range achieved in their individual determinations. The resolution of mixtures is feasible with average errors of ± 2.2%.     

Evaluation of a chloramine-T-selective electrode for catalytic titration of silver and mercury(II) based on iodide-catalyzed chloramine-T reactions

Semiautomatic methods are described for the catalytic titrimetric determination of microamounts of silver and mercury(II) using a chloramine-T-selective electrode as monitor. The methods are based on the inhibitory effect of Ag(I) and Hg(II) on the iodide-catalyzed chloramine-T-arsenite and chloramine-T-H₂O₂ reactions. Microamounts of silver in the range 0.2–200 μg (1 × 10⁻⁷−1 × 10⁻⁴ M) and of mercury(II) in the range 0.1–200 μg (2.5 × 10⁻⁸−5 × 10⁻⁵ M) were determined using the chloramine-T-As(III) indicator reaction. Mercury(II) in the range 4–2000 μg (1 × 10⁻⁶−5 × 10⁻⁴ M) was also determined using the chloramine-T-H₂O₂ indicator reaction. The accuracy and precision were in the range 0.1–1%.     

Hollow fiber membrane of yttrium-stabilized zirconia and strontium-doped lanthanum manganite dual-phase composite for oxygen separation

The hollow fiber composite membrane involving Zr_0.84Y_0.16O_1.92 (YSZ) as an oxygen ionic conductor and La_0.8Sr_0.2MnO_3−δ (LSM) as an electronic conductor was explored for oxygen separation application. The hollow fiber precursor was prepared by the phase-inversion process, and transformed to a gas-tight ceramic by sintering at 1350 °C. The as-prepared fiber exhibited a thermal expansion coefficient of 11.1 × 10⁻⁶ K⁻¹ and a three-point bending strength of 152 ± 12 MPa. An oxygen permeation flux of 2.1 × 10⁻⁷ mol cm⁻² s⁻¹ was obtained under air/He gradient at 950 °C for a hollow fiber of length 57.00 mm and wall thickness 0.16 mm. The oxygen permeation flux remained unchanged when the sweeping gas was changed from helium to high concentration of CO₂. Considering the satisfactory trade-off between the permeability and stability, the YSZ–LSM hollow fiber is promising for oxygen production applications.     

Determination of C4–C14 carboxylic acids by capillary zone electrophoresis: Application to the identification of diamide degradation products and partitioning studies

Capillary zone electrophoresis (CZE) was investigated for the determination of linear saturated carboxylic acid homologues ranging from C₄ to C₁₄. Separation conditions were optimised to overcome the problems of decreasing solubility and decreasing selectivity between successive homologues with increasing chain length. Separations were performed at 20°C, using a 20 kV separation voltage and a pH 8 electrolyte containing 30% methanol. A suitable chromophore (4-aminobenzoate) was added to ensure indirect UV detection of the analytes. Calibration curves and repeatability were established. Minimum detectable concentrations of 3·10⁻⁶ mol l⁻¹ were achieved. Resolution between successive homologues was better than 2. The electrophoretic mobility of each homologue (n=7–14) was assessed and a quasi-linear relationship between the mobility value and 1/n was observed. The quantitative analysis of a diamide degradation solution was performed and compared to potentiometric results. The CZE method was also applied to the determination of C₇–C₁₄ partitioning between an organic medium containing tributylphosphate in n-dodecane and different basic solutions. Their behaviour was established according to the chain length and the pH of the aqueous phase. For C₁₀–C₁₄ compounds, results were validated by comparison with gas chromatographic analysis of the organic phases.     

Kinetics of the R + HBr RH + Br (CH3CHBr, CHBr2 or CDBr2) equilibrium. Thermochemistry of the CH3CHBr and CHBr2 radicals

The kinetics of the reaction of the CH₃CHBr, CHBr₂ or CDBr₂ radicals, R, with HBr have been investigated in a temperature-controlled tubular reactor coupled to a photoionization mass spectrometer. The CH₃CHBr (or CHBr₂ or CDBr₂) radical was produced homogeneously in the reactor by a pulsed 248 nm exciplex laser photolysis of CH₃CHBr₂ (or CHBr₃ or CDBr₃). The decay of R was monitored as a function of HBr concentration under pseudo-first-order conditions to determine the rate constants as a function of temperature. The reactions were studied separately from 253 to 344 K (CH₃CHBr + HBr) and from 288 to 477 K (CHBr₂ + HBr) and in these temperature ranges the rate constants determined were fitted to an Arrhenius expression (error limits stated are 1σ + Student’s t values, units in cm³ molecule⁻¹ s⁻¹, no error limits for the third reaction): k(CH₃CHBr + HBr) = (1.7 ± 1.2) × 10⁻¹³ exp[+ (5.1 ± 1.9) kJ mol⁻¹/RT], k(CHBr₂ + HBr) = (2.5 ± 1.2) × 10⁻¹³ exp[−(4.04 ± 1.14) kJ mol⁻¹/RT] and k(CDBr₂ + HBr) = 1.6 × 10⁻¹³ exp(−2.1 kJ mol⁻¹/RT). The energy barriers of the reverse reactions were taken from the literature. The enthalpy of formation values of the CH₃CHBr and CHBr₂ radicals and an experimental entropy value at 298 K for the CH₃CHBr radical were obtained using a second-law method. The result for the entropy value for the CH₃CHBr radical is 305 ± 9 J K⁻¹ mol⁻¹. The results for the enthalpy of formation values at 298 K are (in kJ mol⁻¹): 133.4 ± 3.4 (CH₃CHBr) and 199.1 ± 2.7 (CHBr₂), and for α-C–H bond dissociation energies of analogous compounds are (in kJ mol⁻¹): 415.0 ± 2.7 (CH₃CH₂Br) and 412.6 ± 2.7 (CH₂Br₂), respectively.     

Fluorescence enhancement effect for the determination of DNA with calcein-cetyl trimethyl ammonium bromide system

A new spectrofluorimetric method was developed for the determination of trace amounts of DNA using the calcein as a fluorescent probe. In the presence of appropriate amounts of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB), the anionic dye calcein dimerizes. The weak fluorescence intensity of the dimer was enhanced by adding DNA at pH 6–7. The interaction between calcein–CTAB and DNA was studied on the basis of this behavior and a new method was developed for determining DNA. Under the optimal conditions, the enhanced fluorescence intensity was in proportion to the concentration of DNA in the range of 4.0 × 10⁻⁶ to 8.0 × 10⁻⁵ g L⁻¹ for fsDNA and thermally denatured ctDNA (4.5 × 10⁻⁶ to 9.0 × 10⁻⁵ g L⁻¹). The detection limits (S/N = 3) were 2.0 × 10⁻⁶ and 2.2 × 10⁻⁶ g L⁻¹, respectively. This method was used for determining the concentration of DNA in synthetic samples with satisfactory results.     

Spectrophotometric determination of vanadium in steel and alloys with 2-(5-chloro-2-pyridylazo)-5-dimethylaminophenol

Using spectrophotometric methods, the protopysis constant of the 5.ClDMPAP reagent (pK_a1 = −0.19; pK_a2 = 1.97; pK_a3 = 11.98) and the stability constant of its vanadic complex (6.0 ± 0.11) × 10¹⁴ were determined. A high-sensitivity spectrophotometric method was developed to determine V(V) using 0.1–1.2 ppm and pH = 3.8. ε₅₈₆ = 55,300 ± 400 liters · mol⁻¹ · cm⁻¹. A study on the most important interferences and the way to eliminate them was carried out. The method can be applied to the determination of the element in steels and ferrovanadiums.     

Piezoelectric Sensor in Situ Monitoring Adsorption of Fibrinogen and Surfactant onto a Self-Assembled Monolayer of Alkanethiols

This report describes the use of a piezoelectric quartz crystal (PQC) sensor to investigate the nonspecific adsorption of fibrinogen (FN) and sodium dodecyl sulfate (SDS) onto a self-assembled monolayer (SAM) of alkanethiols on gold. The change in adsorption mass was monitored in situ by the PQC sensor. A kinetics model was proposed to describe the adsorption of the FN and SDS on the hydrophobic SAM surface. The adsorption kinetics parameters were determined from the responses of the PQC. The adsorption and desorption rate constants of the FN on the SAM surface were estimated to be (6.18 ± 0.53) × 10³ M⁻¹ s⁻¹ and (6.74 ± 0.72) × 10⁻³ s⁻¹, respectively. The rate constants for the adsorption and desorption of SDS on the SAM are (24.3 ± 1.4 M⁻¹ s⁻¹) and (1.52 ± 0.11) × 10⁻² s⁻¹, respectively. The adsorption of SDS on the SAM was reversible. The fractional coverage of the FN on the SAM surface was estimated from kinetics analyses to be 42–86% for the FN concentration range 25–400 μg/ml. Over 80% of the FN is irreversibly adsorbed on the SAM surface with respect to dilution of the bulk phase. The fraction of FN reversibly adsorbed increases with the bulk concentration of FN.     

Comparative studies on hydrolysis of bis(p-nitrophenyl) phosphate catalyzed by short- and long-alkyl-multiamine metallomicelles

Four short- and long-alkyl-multiamine ligands L¹–L⁴ have been synthesized and characterized. The catalytic efficiency of complex CuL¹ and functional metallomicelles CuL²–CuL⁴ were comparatively investigated for the hydrolysis of bis(p-nitrophenyl) phosphate (BNPP) in buffered solution at 30 °C. The ternary kinetic model for metallomicellar catalysis was suggested to analyze the experimental data. The kinetic and thermodynamic parameters k^′_N, K_T and pK_a were obtained. The results indicated that the complexes with 1:1 ratio of ligands L²–L⁴ to copper(II) ion were the kinetic active catalysts, and the deprotonized Cu(II) complex formed by activated water molecule was the real active species for BNPP catalytic hydrolysis. The real rate constant of the reaction catalyzed by CuL¹–CuL⁴ was 4.00 × 10⁻⁶, 7.44 × 10⁻⁵, 1.42 × 10⁻⁴ and 4.10 × 10⁻⁴ s⁻¹, respectively. The effects of ligand and microenvironment on the hydrolytic reaction of BNPP have been discussed in detail.     

Palladized aluminum electrode covered by Prussian blue film as an effective transducer for electrocatalytic oxidation and hydrodynamic amperometry of N-acetyl-cysteine and glutathione

The mediated oxidation of N-acetyl cysteine (NAC) and glutathione (GL) at the palladized aluminum electrode modified by Prussian blue film (PB/Pd–Al) is described. The catalytic activity of PB/Pd–Al was explored in terms of Fe^III[Fe^III(CN)₆]/Fe^III[Fe^II(CN)₆]¹⁻ system by taking advantage of the metallic palladium layer inserted between PB film and Al, as an electron-transfer bridge. The best mediated oxidation of NAC and GL on the PB/Pd–Al electrode was achieved in 0.5 M KNO₃ + 0.2 M potassium acetate of pH 2. The mechanism and kinetics of the catalytic oxidation reactions of the both compounds were monitored by cyclic voltammetry and chronoamperometry. The charge transfer-rate limiting step as well as overall oxidation reaction of NAC or GL is found to be a one-electron abstraction. The values of transfer coefficients α, catalytic rate constant k and diffusion coefficient D are 0.5, 3.2 × 10² M⁻¹ s⁻¹ and 2.45 × 10⁻⁵ cm² s⁻¹ for NAC and 0.5, 2.1 × 10² M⁻¹ s⁻¹ and 3.7 × 10⁻⁵ cm² s⁻¹ for GL, respectively. The modifying layers on the Pd–Al substrate have reproducible behavior and a high level of stability in the electrolyte solutions. The modified electrode is exploited for hydrodynamic amperometry of NAC and GL. The amperometric calibration graph is linear in concentration ranges 2 × 10⁻⁶–40 × 10⁻⁶ for NAC and 5 × 10⁻⁷–18 × 10⁻⁶ M for GL and the detection limits are 5.4 × 10⁻⁷ and 4.6 × 10⁻⁷ M, respectively.     

A polynuclear mixed-valent ruthenium oxide/cyanoruthenate composite that yields thin coatings on a glassy carbon electrode with high catalytic activity toward methanol oxidation

Polynuclear ruthenium oxide/cyanoruthenate films on carbon substrates were grown by cycling the potential between 0.5 and 1.0 V (vs SCE) for 5–90 min in fresh 2 mM RuCl₃·3 H₂O, 2 mM K₄Ru(CN)₆·3 H₂O, 0.5 M KCl solution at pH 2. During the positive scans, the cationic Ru(III,IV)-oxo polynuclear species interacted with the simultaneously formed anionic CN-bridged Ru dimers to yield sparingly soluble deposits on the electrode surfaces. Different thicknesses, typically corresponding to 2–50×10⁻¹⁰ mol cm⁻², could be obtained by varying the cycling times. The absorption spectra of the coatings obtained at SnO₂-covered glass electrodes were different from those known for RuO₂ films and exhibited two absorption bands at about 400 and 700 nm. IR spectroscopic measurements confirmed the presence of a CN group in the deposit, as well as showing significant aquation of the entire coating. Ru, O, N and K (from KCl) were detected by means of Auger electron spectroscopy. The surface electrochemistry and stability of the film were significantly enhanced in K⁺-containing supporting electrolytes. The system provides a durable catalytic surface which allows the voltammetric oxidation of methanol; in 0.5 M H₂SO₄+0.5 M K₂SO₄ electrolyte the resulting anodic peak is at 1.01 V (vs. SCE). Electrooxidation was not possible at bare carbon electrodes, at least before the onset of the electrolyte decomposition.     

Solid substrate-room temperature phosphorimetry for the determination of trace protein using ion association complex [Cu(BPY)2]·[(Fin)2] as a phosphorescent probe

Fluorescein (HFin) emitted strong and stable room temperature phosphorescence (RTP) on filter paper after set at 50 °C for 10 min using Li⁺ as the ion perturber. HFin existed as Fin⁻ when the pH value was in the range of 5.45–7.36. Fin⁻ could react with [Cu(BPY)₂]²⁺ (BPY: α,α-bipyridyl) to produce ion association complex [Cu(BPY)₂]²⁺·[(Fin)₂]²⁻, which could enhance the RTP signal of Hfin. In the presence of bovine serum albumin (BSA), the –COOH group of Fin⁻ in the [Cu(BPY)₂]²⁺·[(Fin)₂]²⁻ could react with the –NH₂ group of BSA to form the ion association complex [Cu(BPY)₂]²⁺·[(Fin-BSA)₂]²⁻, which contained –CO–NH– bond. This complex could sharply enhance the RTP signal of Hfin and the ΔI_p was directly proportional to the content of BSA. According to the facts above, a new solid substrate-room temperature phosphorimetry (SS-RTP) for the determination of trace protein had been established using the ion association complex [Cu(BPY)₂]²⁺·[(Fin)₂]²⁻as a phosphorescent probe. This method had wide linear range (0.40 × 10⁻⁹–280 × 10⁻⁹ mg l⁻¹), high sensitivity (the detection limit (LD) was 1.4 × 10⁻¹⁰ mg l⁻¹), good precision (RSD: 3.4–4.9%) and high selectivity (the allowed concentration of coexistent ions or coexistent materials was high). It had been applied to the determination of the content of protein in 10 kinds of real samples, and the result agreed well with pyrocatechol violet-Mo (VI) method (P.V.M.M.), which indicated it had high accuracy. Meanwhile, reaction mechanism for the determination of trace protein with [Cu(BPY)₂]²⁺·[(Fin)₂]²⁻ phosphorescent probe was also discussed. The academic thought of this research could not only be used to develop many kinds of ion association complex phosphorescent probes, but also provided a new way to promote the sensitivity of SS-RTP.