New insight into the mechanism of catalytic hydrogenation allows the structure of the key intermediate in asymmetric hydrogenation to be predicted

An approach has been developed to determine the regioselectivity of hydrometalation in homogeneous and heterogeneous hydrogenation of alkenes. By studying the electronic effects on the orientation of hydrometalation it is found with palladium and rhodium that this key step is a two electron process that can occur by two modes (a Pd^δ+-H^δ− or b Pd^δ− - H^δ+). This provides valuable information about the structure of the metal-alkyl intermediate and helps rationalise how chiral induction occurs.     

A novel thin-layer amperometric detector based on chemically modified ring-disc electrode and its application for simultaneous measurements of nitric oxide and nitrite in rat brain combined with in vivo microdialysis

A novel thin-layer amperometric detector (TLAD) based on chemically modified ring-disc electrode and its application for simultaneous measurements of nitric oxide (NO) and nitrite (NO₂⁻) in rat brain were demonstrated in this work. The ring-disc electrode was simultaneously sensitive to nitric oxide (NO) and nitrite (NO₂⁻) by modifying its inner disc with electropolymerized film of cobalt(II) tetraaminophthalocyanine (polyCoTAPc)/Nafion and its outer ring with poly(vinylpyridine) (PVP), respectively. The ring-disc electrode was used to constitute a novel TLAD in radial flow cell for simultaneous measurements of NO and NO₂⁻ in rat brain combined with techniques of high performance liquid chromatography (HPLC) and in vivo microdialysis. It was found that the basal concentration of NO in the caudate nucleus of rat brain is lower than 1.0×10⁻⁷ mol l⁻¹, NO₂⁻ concentration is 5.0×10⁻⁷ mol l⁻¹ and NO exists in brain maybe mainly in the form of its decomposed product.     

Simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional ion-selective electrodes in series

An on-line automated system for the simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional calcium-selective and fluoride-selective membrane electrodes as sensors in series is described. Samples (30 μl) are injected into a TISAB II (pH=5.50) carrier solution as an ionic strength adjustment buffer. The sample-buffer zone formed is first channeled to a fluoride-selective membrane electrode and then via the calcium-selective membrane electrode to the reference electrodes. The system is suitable for the simultaneous on-site monitoring of calcium (linear range 10⁻⁵–10⁻² mol l⁻¹ detection limit 1.94×10⁻⁶ mol l⁻¹ recovery 99.22%, RSD<0.5%) and fluoride (linear range 10⁻⁵–10⁻² mol l⁻¹ detection limit 4.83×10⁻⁶ mol l⁻¹ recovery 98.63%, RSD=0.3%) at a sampling rate of 60 samples h⁻¹.     

Persulphate quenching of the excited state of ruthenium(II) tris-bipyridyl dication: thermal reactions

The rate constant for the reaction between the sulphate radical (SO₄^√−) and the ruthenium (II) tris-bipyridyl dication (Ru(bipy)₃²⁺) is (3.3±0.2)×10⁹ mol⁻¹ dm³ s⁻¹ in 1 mol dm⁻³ H₂SO₄ and (4.9±0.5)×10⁹ mol⁻¹ dm³ s⁻¹ in 0.1 mol dm⁻³, pH 4.7 acetate buffer. The SO₄^√−radical produced by the electron transfer quenching of Ru(bipy)₃^2+* by S₂O₈²⁻ reacts rapidly with both acetate buffer and chloride ions. These side reactions result in a reduction in the overall quantum yield of Ru(bipy)₃³⁺ production and reduced reaction selectivity when Ru(bipy)₃^2+* is quenched by persulphate.     

Preparation of multiwall carbon nanotubes film modified electrode and its application to simultaneous determination of oxidizable amino acids in ion chromatography

A multiwall carbon nanotubes (MWNTs) film modified electrode was prepared and used as an amperometric sensor for the simultaneous determination of oxidizable amino acids including cysteine, tryptophane and tyrosine. The electrochemical behaviors of these amino acids at this modified electrode were studied by cyclic voltammetry (CV). The results indicated that the MWNTs chemically modified electrode (CME) exhibited efficient electrocatalytic activity towards the oxidation of these amino acids with relatively high sensitivity, stability and long-life. Following separation by ion chromatography (IC) with 2.0×10⁻³ mol l⁻¹ citric acid buffer solution (pH 6.5) as eluent, cysteine, tryptophane and tyrosine could be determined by the MWNTs CME successfully. Under the optimal conditions, the detection limits were 7.0×10⁻⁷ mol l⁻¹ for cysteine, 2.0×10⁻⁷ mol l⁻¹ for tryptophane and 3.5×10⁻⁷ mol l⁻¹ for tyrosine at the signal-to-noise of 3, respectively. The method was applied successfully to the determination of these substances in plasma.     

Electrocatalytic response of norepinephrine at a thiolactic acid self-assembled gold electrode

The thiolactic acid (TLA) self-assembled monolayer modified gold electrode (TLA/Au) is demonstrated to catalyze the electrochemical response of norepinephrine (NE) by cyclic voltammetry. A pair of well-defined redox waves were obtained and the calculated standard rate constant (k_s) is 5.11×10⁻³ cm s⁻¹ at the self-assembled electrode. The electrode reaction is a pseudo-reversible process. The peak current and the concentration of NE are a linear relationship in the range of 4.0×10⁻⁵–2.0×10⁻³ mol l⁻¹. The detection limit is 2.0×10⁻⁶ mol l⁻¹. By ac impedance spectroscopy the apparent electron transfer rate constant (k_app) of Fe(CN)³⁻/Fe(CN)⁴⁻ at the TLA/Au electrode was obtained as 2.5×10⁻⁵ cm s⁻¹.     

Cathodic adsorptive stripping voltammetric behaviour of guanine in the presence of copper at the static mercury drop electrode

The cathodic adsorptive electrochemical behavior of guanine in the presence of some metal ions at the static mercury drop electrode was investigated. A 1.0×10⁻³ mol l⁻¹ NaOH or a 2.0×10⁻² mol l⁻¹ Hepes buffer at pH 8.0 solutions were used as supporting electrolytes. The reduction peak potential for guanine was found to be around −0.15 V, which is very close to the mercury reduction wave. A new peak appears at −0.60 V in the presence of copper or at −1.05 V in the presence of zinc. A square wave voltammetric procedure for electroanalytical determination of guanine in 2.0×10⁻² mol l⁻¹ Hepes buffer at pH 8.0 containing 1.6×10⁻⁵ mol l⁻¹of copper ions, was developed. An accumulation potential of −0.15 V during 270 s for the prior adsorption of guanine at the electrode surface was used. The response of the system was found to be linear in the range of guanine concentration from 6.62×10⁻⁸ to 1.32×10⁻⁷ mol l⁻¹ and the detection limit was 7.0×10⁻⁹ mol l⁻¹. The influence of DNA bases such as adenine, cytosine and thymine was also examined. Cyclic voltammetry was used to characterize the interfacial and redox mechanism.     

A new flow-injection determination of glucose based on the redox reaction of hydroquinone with iron(III) in the presence of 1,10-phenanthroline

A new spectrophotometric flow-injection (FI) method is proposed for the determination of glucose based on the redox reaction of hydroquinone with iron(III). When a glucose solution containing quinone is passed through the immobilized glucose oxidase column introduced in FI system, quinone is reduced to hydroquinone by glucose. In the presence of 1,10-phenanthroline (phen), iron(III) is then quantitatively reduced by hydroquinone to iron(II) followed by the formation of iron(II)-phen complex (λ_max=510 nm). An FI peak observed at 510 nm corresponds to the concentration of glucose. The wide dynamic range for glucose was obtained in the range of 1×10⁻⁶–1×10⁻³ mol l⁻¹ at a sampling rate of 24 h⁻¹ and the detection limit (S/N=3) was 5×10⁻⁷ mol l⁻¹. Relative standard deviations were 0.78, 0.44 and 0.23% (n=5) for 5×10⁻⁶, 5×10⁻⁵ and 5×10⁻⁴ mol l⁻¹ of glucose, respectively. The proposed method was successfully applied to the determination of glucose in control blood sera, human blood plasma and wine.     

Amperometric detection of amino acids in a flow-injection system with a nickel(II)-modified electrode with an Eastman-AQ polymer film

Amperometic flow measurements were made at +0.55 V (vs. Ag/AgCl) in 0.1 mol l⁻¹ KOH electrolyte with an Ni(II) chemically modified electrode (CME) with an Eastman-AQ polymer film. The use and characteristics of a Ni(II)-containing crystalline and polymer-modified electrode obtained by a double coating step as a detector for amino acids in a flow-injection system using reversed-phase liquid chromatography are described. The detection of these analytes is based on the higher oxidation state of nickel (NiOOH) controlled by the applied potential. The electroanalytical parameters and the detection current for a series of amines and amino acids were investigated. The use of such a CME in the flow-injection technique was found to be suitable in a solution at low pH. The linear range for glycine is 5 × 10⁻⁶-0.1 mol l⁻¹ with a detection limit of 1.0×10⁻⁶ mol l⁻¹. A 1 × 10⁻⁴ mol l⁻¹ mixture of serine and tyrosine was also detected after separation on an Nucleosil C₁₈ column.     

Interactions of mercury(II), lead(II), calcium(II), aluminium(III) or ferric(III) nitrate with single and double chain linear alkylbenzenesulfonates in aqueous and sea-water media

The solubility products of mercury(II) and lead(II) dodecylbenzenesulfonates were calculated on the basis of light scattering measurements at 20°C to be (9.33 ± 0.90) × 10⁻¹³ and (1.03 ± 0.10) × 10⁻¹² respectively. The investigations of precipitation phenomena performed in diluted natural sea-water ([Cl⁻] = 10⁻¹² mol dm ⁻³), including tenside and added heavy metal ions, showed similar behaviour for all metal dodecylbenzenesulfonates investigated in this work, i.e. the synergistic effect of tenside, of added electrolyte, and of cations and anions from sea-water. A comparison made by a statistical test of significance, chosen to measure agreement between the estimates of the solubility constants obtained in aqueous solutions and in the above mentioned sea-water solution, showed a noticeable effect, evident particularly in the case of mercury(II). The precipitation of tenside and metal nitrates in natural sea-water ([Cl⁻] = 5 × 10⁻¹ mol dm⁻³) covers a wide concentration region of dodecylbenzenesulfonic acid (from high concentrations to 5 × 10⁻⁶ mol dm⁻³) and from high metal nitrate concentrations to very low. The microscopic textures of phases precipitated in the systems with sea-water obviously confirmed favouring formation of the liquid crystalline phase.     

Amberlite XAD-2 functionalized with o-aminophenol: synthesis and applications as extractant for copper(II), cobalt(II), cadmium(II), nickel(II), zinc(II) and lead(II)

A stable chelating resin matrix was synthesized by covalently linking o-aminophenol (o-AP) with the benzene ring of the polystyrene–divinylbenzene resin, Amberlite XAD-2, through a –N=N– group. Elemental analyses, thermogravimetric analysis (TGA) and infrared spectra have characterized the resulting chelating resin. It has been used to preconcentrate Cu²⁺, Cd²⁺, Co²⁺, Ni²⁺, Zn²⁺ and Pb²⁺, prior to their determination by flame atomic absorption spectrometry. The optimum pH values for quantitative sorption of Cu, Cd, Co, Ni, Zn and Pb are 6.2–7.4, 5.6–7.2, 5.6–9.0, 6.0–9.0, 5.7–7.0 and 5.0–6.0, respectively. These metals are desorbed (recovery 91–98%) with 4 mol dm⁻³ HNO₃. The sorption capacity of the resin is 3.37, 3.42, 3.29, 3.24, 2.94 and 3.32 mg of metal g⁻¹ of resin, respectively, for Cu, Cd, Co, Ni, Zn and Pb. The effect of NaF, NaCl, NaNO₃, Na₂SO₄, and Na₃PO₄ on the sorption of these metal ions has been investigated. These electrolytes are tolerable up to 0.01 mol dm⁻³ in case of all the metal ions, except Cl⁻ which is tolerable even up to 0.1 mol dm⁻³ for Zn and 1.0 mol dm⁻³ for Pb. The preconcentration factor for Cu, Cd, Co, Ni, Zn and Pb are 50, 50, 100, 65, 40 and 40 (concentration level 10–25 μg dm⁻³) respectively. Simultaneous enrichment of the six metals is possible. The method has been applied to determine Cu, Cd, Co, Ni, Zn and Pb content in well water samples (RSD≤8%).     

On-line extraction-spectrophotometric determination of neostigmine in pharmaceuticals using double membrane phase separator and monovalent dyestuff

A simple, sensitive and rapid spectrophotometric method for the determination of neostigmine by flow injection analysis (FIA) coupled with an ion associate extraction has been developed. The three-line manifold was assembled. Neostigmine(200 μl) was injected into a distilled water stream and the pH was adjusted to 10 with a borate–phosphate buffer solution. Then, the stream was mixed with the ion-pairing tetrabromophenolphthalein ethylester (TBPEH)-1,2-dichloroethane solution. After phase separation with a double membrane phase separator, absorbance was measured at 610 nm. A linear calibration graph was obtained between 1×10⁻⁷ mol l⁻¹ and 5×10⁻⁷ mol l⁻¹ of neostigmine. Up to 48 samples h⁻¹ could be processed with a relative standard deviation (R.S.D.) of 0.5% (n=5) for 4×10⁻⁷ mol l⁻¹ neostigmine. The proposed system was applied to the simple, reproducible and rapid determination of neostigmine in commercial pharmaceuticals.     

Nitrate ion-sensitive field effect transistor based on bis(bathocuproin)-copper(I) nitrate dissolved in solid solvents

Nitrate-selective ion-sensitive field effect transistors (ISFETs) were prepared by dip-coating the silica gate insulator surface with an ion exchanger based on bis(bathocuproin)-copper(I) nitrate in a molten mixture of docosan 1-ol and Triphenylphosphate. This sensor showed a linear response from 10⁻¹ mol dm⁻³ to at least 10⁻⁵ mol dm⁻³ and maintained its response characteristics for as long as 5 weeks. The selectivity sequence for common anions such as Cl0−₄ and Cl− followed the conventional Hofmeister series. This sensor was applied to rain water analysis.     

Catalytic flow injection determination of vanadium by oxidation of N-(3-sulfopropyl)-3,3',5,5'-tetramethylbenzidine using bromate

A catalytic flow-injection (FI) method was developed for the determination of 10⁻⁹ mol l⁻¹ levels of vanadium(IV, V). The method is based on the catalytic effect of vanadium(V) on oxidation of N-(3-sulfopropyl)-3,3′,5,5′-tetramethylbenzidine (TMBZ·PS) using bromate as oxidant to form a yellow dye (λ_max=460 nm). The use of 5-sulfosalicylic acid (SSA) as an activator enhanced the sensitivity of the method. The calibration graphs with a working range 0.05–8.0 ng ml⁻¹ were obtained for vanadium(V). Vanadium(IV) was also determined, being oxidized by bromate. The detection limit (signal/noise, S/N=3) was 0.01 ng ml⁻¹ (ca. 2×10⁻¹⁰ mol l⁻¹) vanadium. The relative standard deviations (R.S.D.) for 15 determinations of 0.5 ng ml⁻¹ vanadium, and for ten determinations of 0.1 and 1.0 ng ml⁻¹ vanadium were 0.41, 2.6 and 0.25%, respectively, with a sampling rate of 15 samples h⁻¹. The proposed method was successfully applied to the determination of vanadium in natural waters.     

Solvent effect on K to Na ion mutation: a Monte Carlo simulation study

Monte Carlo simulation studies of statistical perturbation theory (SPT) have been carried out to investigate the solvent effects on the relative free energies of solvation and the difference in partition coefficients (log P) for K⁺ to Na⁺ ion mutation in the several solvents. We compared the relative free energies for interconversion of K⁺ to Na⁺, in H₂O (TIP4P) in this study with those published works, that in H₂O (TIP4P) is −16.55 kcal/mol in this study, those of the published works are −17.6, −17.3 and −17.31 kcal/mol and that of the experiment is −17.6 kcal/mol, respectively. Comparing the relative free energies for interconversion of K⁺ to Na⁺, in CH₃OH in this study with those published works, that in CH₃OH is −18.08±0.28 kcal/mol in this study, that of molecular dynamic simulation is −19.6±0.4 kcal/mol and that of the experimental work is −17.3 kcal/mol, respectively. There is good agreement among the several studies if we consider both methods of obtaining the solvation (or hydration) free energies and the standard deviations. For the present K⁺ and Na⁺ ions, the relative free energies of solvation vs Born's function of solvents are decreased with increasing Born's function of solvent except for CH₃OH, THF and MEOME. There is also good agreement between the calculated structural properties in this study and the computer simulation, ab initio and experimental works.     

Influence of ionic strength and pH on the first 60 min of Pseudomonas aeruginosa attachment to ZnSe and to TiO2 monitored by ATR-IR spectroscopy

Establishing the factors which influence the attachment of bacteria to surfaces is important in both preventing and enhancing biofilm formation. The initial hour of attachment of Pseudomonas aeruginosa to ZnSe and to TiO₂ from solutions of different ionic strength and pH was studied using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. The TiO₂ surface was prepared by dip-coating a ZnSe internal reflection element, which produced a 50 nm thick, continuous flat film. At pH 6.3 attachment was found to increase with ionic strength up to 0.03 mol l⁻¹ but to decrease at 0.15 mol l⁻¹. At an ionic strength of 0.003 mol l⁻¹ attachment increased with pH from 4 to 6.3 to 10, but at ionic strength of 0.03 mol l⁻¹ attachment was greater at pH 6.3 than at pH 10. The influence of ionic strength appears to be due to charge factors and/or related changes in the degree of extension of bacterial surface polymers. The complex trends in the influence of pH on attachment can not be explained solely in terms of bacterial and substrate charge, bacterial surface polymer extension or bacterial metabolic activity.     

Spectrophotometric determination of nitrite based on its catalytic effect on the oxidation of carminic acid by bromate

A highly sensitive and selective method is described for the determination of trace amounts of nitrite based on its effect on the oxidation of carminic acid with bromate. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of carminic acid at 490 nm after 3 min of mixing the reagents. The optimum reaction conditions were 1.8×10⁻¹ mol l⁻¹ H₂SO₄, 3.8×10⁻³ mol l⁻¹ KBrO₃, and 1.2×10⁻⁴ mol l⁻¹ carminic acid at 30°C. By using the recommended procedure, the calibration graph was linear from 0.2 to 14 ng ml⁻¹ of nitrite; the detection limit was 0.04 ng ml⁻¹; the R.S.D. for six replicate determinations of 6 ng ml⁻¹ was 1.7%. The method is mostly free from interference, especially from large amounts of nitrate and ammonium ions. The proposed method was applied to the determination of nitrite in rain and river water.     

Cellulose based macromolecular chelator having pyrocatechol as an anchored ligand: synthesis and applications as metal extractant prior to their determination by flame atomic absorption spectrometry

Pyrocatechol is immobilized on cellulose via ---NH---CH₂---CH₂---NH---SO₂---C₆H₄---N=N--- linker and the resulting macromolecular chelator characterized by IR, TGA, CPMAS ¹³C NMR and elemental analyses. It has been used for enrichment of Cu(II), Zn(II), Fe(III), Ni(II), Co(II), Cd(II) and Pb(II) prior to their determination by flame atomic absorption spectrometry (FAAS). The pH ranges for quantitative sorption (98.0–99.4%) are 4.0–7.0, 5.0–6.0, 3.0–4.0, 5.0–7.0, 5.0–8.0, 7.0–8.0 and 4.0–5.0, respectively. The desorption was found quantitative with 0.5 mol dm⁻³ HCl/HNO₃ (for Pb). The sorption capacity of the matrix for the seven metal ions has been found in the range 85.3–186.2 μmol g⁻¹. The optimum flow rate of metal ion solution for quantitative sorption of metal onto pyrocatechol functionalized cellulose as determined by column method, is 2–6 cm³ min⁻¹, whereas for desorption it is 2–4 cm³ min⁻¹. The tolerance limits for NaCl, NaBr, NaI, NaNO₃, Na₂SO₄, Na₃PO_4, humic acid, EDTA, ascorbic acid, citric acid, sodium tartrate, Ca(II) and Mg(II) in the sorption of all the seven metal ions are reported. Ascorbic acid is tolerable up to 0.8 mmol dm⁻³ with Cu and Pb where as sodium tartrate does not interfere up to 0.6 mmol dm⁻³ with Pb. There is no interference of NaBr, NaCl and NaNO₃ up to a concentration of 0.5 mol dm⁻³, in the sorption of Cu(II), Cd(II) and Fe(III) on to the chelating cellulose matrix The preconcentration factors are between 75 and 300 and t_1/2 values ≤5 min for all the metal ions. Simultaneous sorption of Cu, Zn, Ni and Co is possible at pH 5.0 if their total concentration does not exceed lowest sorption capacity. The present matrix coupled with FAAS has been used to enrich and determine the seven metal ions in river and tap water samples (relative standard deviation (R.S.D.) 1.05–7.20%) and synthetic certified water sample SLRS-4 (NRC, Canada) with R.S.D. 2.03%. The cobalt present in pharmaceutical vitamin tablets was also preconcentrated on the modified cellulose and determined by FAAS (R.S.D. 1.87%).     

Radiolysis of aqueous solutions of thiamine

The results of the radiolysis of aqueous solutions of thiamine (vitamin B₁) are presented. The yields for decomposition of thiamine and the product of radiolytic products were determined. The G values decrease as the dose increases. Some radiolytic products were identified. Decomposition of thiamine was slightly dependent on the presence of oxygen and on the pH of the solution. At pH 4.4 with a concentration of 2.5 × 10⁻⁴ mol L⁻¹ of thiamine in an oxygen free aqueous solution, the G⁰ value for decomposition is 5.0.     

Chemiluminescence assay of promethazine hydrochloride using acidic permanganate employing flow injection mode operated with syringe and peristaltic pumps

For the first time, promethazine hydrochloride chemiluminescence emission was monitored. The paper describes a new, specific and highly sensitive flow injection (FI) method for the determination of promethazine hydrochloride using both a peristaltic and a syringe pump. The method was based on the chemiluminescence emission intensity produced as a result of its oxidation reaction with permanganate in sulfuric acid medium. Reaction variables were thoroughly investigated employing chemometrical methods with few number of experiments. The optimum system and chemical conditions were 2.1519×10⁻⁴ mol l⁻¹ permanganate in 0.01 mol l⁻¹ sulfuric acid when operating the peristaltic pump at a flow rate of 45 μl s⁻¹ and injecting the drug by a syringe pump operated at a speed of 40 μl s⁻¹. The method was found to be applicable in the concentration range of promethazine hydrochloride between 1.558×10⁻⁵ and 1.8697×10⁻³ mol l⁻¹ with a linear calibration plot of 0.992 correlation coefficient and the following equation: I=92.74+0.08048C. The method adopted proved to be highly suitable for the assay of promethazine hydrochloride in drug formulations without fear of interferences in dosage form.