Rate coefficient for the reaction OH + HO2 = H2O + O2 at 1 atmosphere pressure and 308 K

The rate coefficient for the reaction OH + HO₂ =H₂O + O₂ has been determined from measurements of the steady-state absorption of HO₂ at 210 nm, in low-frequency square-wave modulated photolysis of O₃ + H₂O mixtures. The value obtained was (9.9 ± 2.5) × 10^?11 cm³ molecule^?1 s^?1 at 308 K and 1 atm pressure.     

Electrochemical investigation of redox reactions of herbal drug-shikonin and its determination in pharmaceutical preparations

The electrochemical behaviour of the anticancer herbal drug shikonin was investigated at glassy carbon electrode in 0.16 M HAc-NaAc (20% ethanol, pH 3.98) buffer solution using cyclic voltammetry, square-wave voltammetry and chronocoulometry. Shikonin gives a pair of quasi-reversible redox peaks at potentials of E _pc = 0.698 V and E _pa =0.632 V by absorption-controlled process at a scan rate of 100 mV/s. The electrode process dynamics parameters (saturated adsorptive amount Γ, charge transfer coefficient α, and apparent rate constant K _s) and reaction mechanism were also investigated with result of two electrons and two hydrogen ions participating in electrode reaction. The experimental conditions were optimized for the determination of shikonin and the square-wave anodic peak currents were linearly related to the shikonin concentrations in the range from 2.08 × 10⁻⁸ to 1.82 × 10⁻⁶ M with correlation coefficient of 0.998 and detection limit of 7.8 × 10⁻⁹ M. Using the established method without pretreatment and pre-separation, shikonin in herbal drug Gromwell Root was determined with satisfactory result.     

Square-wave voltammetry and square-wave voltacoulommetry applied to the study of the electrocatalytic behaviour of surface confined myoglobin

Analytical previous results corresponding to square-wave voltammetry (SWV) and square-wave voltacoulommetry (SWVC) have been applied to the electrochemical characterization of myoglobin immobilized at a self-assembled monolayer of l-cystine on a gold electrode and its electrocatalytic activity towards the oxidation of sodium ascorbate. The obtained results have been compared with those previously reported in Langmuir (2011) 27:2052–2057 by using cyclic voltammetry. It has been shown that double layer effects can be easily removed in SWV and SWVC techniques. Accurate values for the thermodynamic and electrochemical kinetic parameters have been obtained by assuming dispersion in the formal potential of the redox center, and a value k _c ′?=?1.37?×?10⁵?M^?1?s^?1 has been found for the catalytic constant.     

Increased rates of initiation of polymerization by 4-4′-dicyano-4-4′-azopentanoic acid in the presence of ferric salts

The initiation of the polymerization of acrylamide by 4-4′-dicyano-4-4′-azopentanoic acid in aqueous solution has been studied kinetically at 25°C. Ferric chloride and ferric sulfate were used to terminate polymerization so that rates of initiation could be calculated from the rates of production of ferrous iron. Velocity coefficients at 25°C. for the initiation reaction were found to be (25.7 ± 2.8) × 10^?7 sec.^?1 for the ferric chloride terminated reaction and (73.6 ± 0.6) × 10^?7 sec.^?1 for the ferric sulfate-terminated polymerization. The value reported for the initiation reaction when acrylamide is polymerized in the absence of metal salts is 1.29 × 10^?7 sec.^?1. Velocity coefficients for the termination reaction have been calculated from the overall rates of polymerization obtained with ferric salts present. In the case of the ferric chloride-terminated reaction, it has been shown that the rate of polymerization is reduced by increasing the total concentration of chloride ions. Termination velocity coefficients at 25°C. for the inner sphere complexes FeCl²⁺·5H₂O and FeSO₄⁺·4H₂O have been calculated to be 18.9 × 10⁴ and 7.98 × 10⁴ l./mole-sec., respectively. The dependence on the concentration of ferric chloride of the molecular weights of the polymers produced has also been considered.     

Square‐Wave Voltammetry of the Molybdenum‐1,10 Phenanthroline‐Fulvic Acids Complex: Redox Kinetics Measurements

The reduction process of molybdenum in the presence of fulvic acids and phenanthroline was investigated by square-wave voltammetry (SWV). The mixed-ligand complex of molybdenum exhibits a pronounced tendency to adsorb onto the mercury electrode surface. The electrode reaction proceeds as a surface process in which both components of the redox couple are firmly confined to the electrode surface. The kinetics of the electrode reaction is studied utilizing the properties of “split SW peaks” and “quasireversible maximum”. The kinetic parameters obtained with two different square-wave voltammetric methods are in good agreement. In 0.5 mol/L NaCl solution with pH 2.5 the kinetic parameters are: standard rate constant k_s=8±2 s⁻¹, cathodic electron transfer coefficient α=0.41±0.05, and number of exchanged electrons n=2. The SW kinetic measurements are confirmed by cyclic voltammetric method.     

Determination of electrochemical kinetic parameters by square-wave polarography: Polarographic reduction of Zn(II) in 1 M KCl,KBr, and KNCS solutions

A new method of determining electrochemical kinetic parameters by square-wave polarography was presented, in which the faradaic current at θ/2, θ being the half-period of superimposed square-wave voltage, was used for the analysis. The method gave the following kinetic parameters for the electrode reaction, Zn(II) + 2e(Hg), in aqueous solutions at 25° C: k_c^θ=0.0052 cm s^?1 and α_c=0.36 in 1 M KCl, k_c^θ=0.011 cm s^?1 and α_c=0.30 in 1 M KBr, and k_c^θ=0.020 cm s^?1 and α_c=0.52 in 1 M KNCS. Induced adsorption of Zn(II) on the dropping mercury electrode was suggested in solutions containing thiocyanate ions.     

In vitro study of the interaction of cysteine with a thiazide diuretic (hydrochlorothiazide) at different ph by voltammetric and spectroscopic techniques

The interaction of cysteine (RSH) with a thiazide diuretic, hydrochlorothiazide (HCTZ) was characterized by UV-Vis absorption spectroscopy and square-wave voltammetry in Britton-Robinson (B-R) buffer solution (with pH 5, 7 and 9). On the square-wave voltamogram of cysteine, the reduction peak current of mercurous cysteine thiolate (Hg₂(RS)₂) decreased and its peak potential shifted to more positive values with the addition of HCTZ. This results showed that the RSH interacted with HCTZ. The stoichiometry of HCTZ-RSH molecular complex was determined by voltammetric data with the result of 1: 1. By using linear regression analysis of the voltammetric data at pH 5, 7 and 9, the apparent formation constants of HCTZ-RSH complex were calculated to be 9.54 × 10³, 2.80 × 10⁴ and 2.55 × 10⁴ M^?1, respectively. At the same time, this interaction was also supported by UV-Vis spectroscopic measurements. According to the voltammetric and spectroscopic results, it was suggested that the interaction mode between RSH and HCTZ molecules might be a combination of hydrophobic interactions and hydrogen bonds.     

Determination of the surface acidity of a free-base corrole in a self-assembled monolayer

Mixed self-assembled monolayers (SAMs) containing a corrole moiety have been prepared to examine their electrochemical properties and surface acidity, with the eventual goal of biosensor development. Mixed SAMs consisting of 6-mercapto-hexanol (6-MHO) and 8-amino-1-octanethiol (8-AOT) in varying ratios were modified with a free-base corrole and characterized via Osteryoung square-wave voltammetry and contact angle measurements. The surface acidity of the free-base corrole was determined using an electrochemical titration method, with pK _a values established at 6.4 when using Fe(CN) ₆ ^4? /Fe(CN) ₆ ^3? as a redox probe and at 6.7 when using iodide, and assigned to the CorH₄ ⁺ ? CorH₃ + H⁺ equilibrium.     

Square-wave voltammetry of 4,4′-diisopropyl-2,2′-bithiazoline and its complex with copper(II)

(4S)-4′-diisopropyl-2,2′-bithiazoline (DPT) is an electroactive organic chiral compound giving two reduction responses in square-wave voltammograms at potentials about −0.2 and −0.4 V by forming a complex with mercury which deposits at the electrode surface. By the addition of copper(II) ion to the solution of DPT a third peak appears between them at about −0.3 V, which corresponds to the reduction of adsorbed Cu-DPT complex. Optimal pH for the investigation of those redox processes was found to be 2.8. By square-wave voltammetric measurements it was interpreted that these redox reactions were quasireversible with immobilized reactants. By plotting i_p/f vs. frequency a quasireversible maximum was obtained, and the apparent standard reaction rate constants were calculated: log (k_s)DPT=(0.91 ± 0.9) and 1 < k_s < 65_S⁻¹, log (k_s)_CuDPT= (0.35 ± 0.9) and 0.3 < k_s < 18 S⁻¹ in 0.55 M NaCl.     

Study of the base-catalysed oxidation of the anti-bacterial and anti-protozoal agent metronidazole by permanganate ion in alkaline medium

The mechanism of dismutation of MnO₄ ^2? via the complex [MTZ–MnO₄·OH]^2?, formed during the oxidation of metronidazole (MTZ), has been investigated spectrophotometrically at different temperatures. The stoichiometry of the reaction is 1:1, i.e. 1 mol MTZ reacts with 1 mol Mn(VII).The reaction is first order in permanganate, less than first order in [MTZ] and [alkali]. The effects of added products and the dielectric constant and ionic strength of the reaction medium were investigated. The main products were identified by spot test and FT-IR. A mechanism involving a free radical has been proposed. In the equilibrium step MTZ binds to the MnO₄ ^? species to form a complex (C). Investigation of the reaction at different temperatures enabled determination of the activation data for the slow step of proposed mechanism. The proposed mechanism and the derived rate laws are consistent with the observed kinetics.     

The standard molar enthalpy of formation of LnPO4(s) (Ln = La, Nd, Sm) by solution calorimetry

The standard molar enthalpy of formation of LaPO₄(s), NdPO₄(s), and SmPO₄(s) has been determined using an isoperibol solution calorimeter. The solution calorimeter vessel was held at 298.15 K. The precipitation reaction between aqueous solution of rare-earth chloride (LnCl₃(aq.)) and ammoniacal solution of ammonium dihydrogen phosphate (NH₄H₂PO₄(aq.)) was studied. The temperature of the calorimeter vessel was measured before, during, and after the reaction. The enthalpy change due to precipitation of LaPO₄(s), NdPO₄(s), or SmPO₄(s) from required solutions was measured at 298.15 K. Using these values and other auxiliary data from the literature, thermochemical reaction scheme were devised to calculate the standard enthalpy of formation of each phosphate compound i.e., LaPO₄(s), NdPO₄(s), and SmPO₄(s). The calculated values for LaPO₄(s), NdPO₄(s), and SmPO₄(s) at 298.15 K were found to be ?1947.5 ± 3.2, ?1938.3 ± 3.6, and ?1942.9 ± 3.4 kJ mol^?1, respectively.     

Kinetics of the oxidation of U (IV) by hydrogen peroxide in sulfuric acid medium

The kinetics of the reaction have been investigated in H₂SO₄ medium under different conditions. The observed bimolecular rate constant k_obs, has been found to depend on [H⁺]^?0.55 and to increase with the initial concentration ratio of the reactants R₀ = [H₂O₂]₀/[U (IV)]₀ above 0.49. The activation energy of the overall reaction has been determined as 13.79 and 14.3 kcal/mol at R₀ = 1 and 0.35, respectively. Consistent with experimental data, a detailed reaction mechanism has been proposed where the hydrolytic reaction (4) followed by the rate-controlling reaction (10) and subsequent fast reactions of U (V) and OH radicals are involved: A kinetic expression has been derived from which a graphical evaluation of (kK₄)^?1 and k^?1 has been made at R₀ = 1 as (12.30 ± 0.09) × 10^?3 M min, (6.23 ± 2.19) × 10^?4 M min; and at R₀ = 0.35 as (12.63 ± 2.13) × 10^?3 M min, (8.32 ± 6.62) × 10^?4 M min, respectively. Indications of some participation of a chain reactionat R₀ = 1 have been obtained without affecting thesecond-order kinetics as observed.     

Electrochemical properties and square-wave voltammetric determination of pravastatin

The electrochemical reduction and adsorptive voltammetric behaviour of pravastatin have been studied by means of cyclic and square-wave voltammetry at a hanging mercury-drop electrode in electrolytes of different pH. Within the entire pH range (2.0–9.0) in Britton–Robinson buffer, pravastatin gave rise to a single voltammetric peak in the potential interval from −1.22 to −1.44 V, depending on pravastatin concentration. It was found that the reduction of pravastatin proceeds via a relatively stable intermediate, which is transformed to the final electroinactive product by a coupled chemical reaction or can be re-oxidized back to pravastatin. The rate of chemical transformation is controlled by the proton concentration. The electrode mechanism has the properties of a surface redox reaction. A sensitive analytical method for trace analysis of pravastatin based on the adsorptive stripping technique has been developed. The calibration plot was linear in the range 8×10⁻⁸–5×10⁻⁷ mol L⁻¹. Application of the square-wave voltammetric method to determination of pravastatin in a pharmaceutical dosage form, without sample pretreatment, resulted in acceptable deviation from the stated concentration.     

Copper perchlorate hexahydrate: a highly efficient catalyst for the cyanosilylation of aldehydes

Cu(ClO₄)₂·6H₂O has been found to be an efficient catalyst for cyanosilylation reaction of aldehydes in THF at room temperature with low catalytic loading (1.0 mol%) in short reaction time (mostly within 10 min). Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis and Characterization of Poly(Methyl Methacrylate) Grafted from Acacia Gum

Graft polymerization of methyl methacrylate onto acacia gum has been studied in detail. The grafting was found to be optimal under the following reaction conditions: gum at 0.4 g/dL, monomer at 7.52 × 10^?;2 mol/dL, ceric ammonium sulfate at 15.81 × 10^?;4 mol/dL, H₂SO₄ at 0.037 mol/dL, temperature at 50 °C and time at 3.0 h. Fourier transform infrared spectroscopy was sed for the confirmation of grafting. Thermal and physical properties of the copolymer were studied. A probable mechanism of polymerization has been suggested based on reaction kinetics.     

Study On The Catalytic Reaction Of β-Cd-Hemin Using 2, 3, 4-Trichlorophenol As Substrate And Analytic Application

ABSTRACT|The catalytic activity of various mimetic enzymes instead of the peroxidase have been investigated by 4-aminoantipyrine (4-AAP) and 2, 3, 4-trichlorophenol (TCP) to form a dye utilizing hydrogen peroxide as hydrogen acceptor. The different Chlorophenolic derivatives, which act as a substrate in β-CD-hemin-H₂O₂-4-AAP catalytic reaction, have been systematically studied.|Meanwhile, the relationship of structure-effect for the β-CD-hemin as catalyst, and chlorphenols as substrate has been respectively discussed. The mechanism of catalytic reaction has been investigated. The results showed that β-CD-hemin was the best mimetic enzyme for peroxidase among those tested and TCP was a good substrate for the determination of hydrogen peroxide with β-CD-hemin. The method for the determination of hydrogen peroxide was proposed using 4-AAP-TCP system with β-CD-hemin as catalyst. A linear calibration graph was obtained over the H₂O₂ concentration of 4.8×10-^?8-7.7×10-^?5M, and the relative standard deviation at a H₂O₂ concentration of 2.8×10-^?5M was 2.5%. The apparent molar absorptivity of the chromogenic reaction for H₂O₂ was 1.54× 10⁴ L.mol-^?1.cm^?1. Satisfactory results were obtained in the determination of H₂O₂ in synthetic samples by this method.

Also, the method was coupled with the glucose oxidation reaction to determination glucose in human serum.     

Thermal studies on thorium(IV) complexes of 1-butyl-1-methylpiperazinium iodide

Thorium(IV) complexes of the type Th(NO₃)₄·3L·2C₂H₅OH, Th(SCN)₄·L·C₂H₅OH and Th(SO₄)₂·2L·2C₂H₅OH (L=1-butyl-1-methylpiperazinium iodide(I) have been synthesised. From thermogravimetric (TG) curves, the decomposition pattern of the compounds has been analysed. The order, activation energy and apparent activation entropy of the thermal decomposition reaction have been elucidated. The heat of reaction has been calculated from differential thermal analysis (DTA) studies.     

Kinetics and mechanism of the redox reaction between malachite green and iron(III) in aqueous and micellar media

The kinetics of the oxidation of malachite green (MG⁺) by Fe(III) were investigated spectrophotometrically by monitoring the absorbance change at 618 nm in aqueous and micellar media at a temperature range 20–40 °C; I = 0.10 mol dm^?3 for [H⁺] range (2.50–15.00) × 10^?4 mol dm^?3. The rate of reaction increases with increasing [H⁺]. The reaction was carried out under pseudo-first-order conditions by taking the [Fe(III)] (>10-fold) the [MG⁺]. A mechanism of the reaction between malachite green and Fe(III) is proposed, and the rate equation derived from the mechanism was consistent with the experimental rate law as follows: Rate = (k ₄ + K ₁ k ₅[H⁺]) [MG⁺][Fe(III)]. The effect of surfactants, such as cetyltrimethylammonium bromide (CTAB, a cationic surfactant) and sodium dodecylsulfate (SDS, an anionic surfactant), on the reaction rate has been studied. CTAB has no effect on the rate of reaction while SDS inhibits it. Also, the effect of ligands on the reaction rate has been investigated. It is proposed that electron transfer proceeds through an outer-sphere mechanism. The enthalpy and the entropy of the activation were calculated using the transition state theory equation.     

Photocatalytic Degradation of 2-Propanol and Phenol Using Au Loaded MnWO4 Nanorod Under Visible Light Irradiation

Single crystalline MnWO₄ nanorod has been prepared by low temperature hydrothermal reaction at 180 °C. The prepared MnWO₄ possesses band gap of 2.63 eV. Photochemical decomposition method has been followed to disperse Au nanoparticles onto MnWO₄ nanorod. The prepared Au loaded MnWO₄ nanorod demonstrated greatly enhanced photocatalytic activity in decomposing 2-propanol and evolving CO₂ in gas phase and phenol in aqueous phase compared to bare MnWO₄ and commercial TiO₂ nanoparticles (Degussa P25) under visible light (λ ≥ 420 nm) irradiation. The Au loading was optimized to 3.79 wt% for the highest efficiency. The enhanced photocatalytic activity originates from the absorption of visible light by MnWO₄ as well as the introduction of nanoparticulate Au on the surface of MnWO₄ as cocatalyst to impede the recombination of photogenerated charge-carriers.     

Kinetics and Mechanism of the Oxidation of 4-Oxo-4-arylbutanoic Acids by N-bromophthalimide in Aqueous Acetic Acid Medium

The kinetics of oxidation of a series of substituted 4-oxobutanoic acids (Y–C₆H₄COCH₂CH₂COOH: Y = H, OCH₃, CH₃, C₆H₅, Cl, Br or NO₂) by N-bromophthalimide have been studied in aqueous acetic acid medium at 30 °C. The total reaction is second-order, first-order each in oxidant and substrate. The oxidation rate increases linearly with [H⁺], establishing the hypobromous acidium ion, H₂O⁺Br, as the reactive species. A variation in ionic strength has no effect on the reaction rate. The order of reactivity among the studied 4-oxoacids is: 4-methoxy > 4-methyl > 4-phenyl > 4-H > 4-Cl > 4-Br > 3-NO₂. The effect of changes on the electronic nature of the substrate reveals that there is a development of positive charge in the transition state. The activation parameters have been computed from Arrhenius and Eyring plots. Based on the kinetic results, a suitable mechanism has been proposed.