Kinetics and mechanism of the oxidation of hydrogen peroxide by the octacyanotungstate(V) ion in alkaline aqueous media

Summary The oxidation of H₂O₂ by [W(CN)₈]^3– has been studied in aqueous media between pH 7.87 and 12.10 using both conventional and stopped-flow spectrophotometry. The reaction proceeds without generation of free radicals. The experimental overall rate law, , strongly suggests two types of mechanisms. The first pathway, characterized by the pH-dependent rate constant k _s, given by , involves the formation of [W(CN)₈· H₂O₂]^3–, [W(CN)₈· H₂O₂·W(CN)₈]^6– and [W(CN)₈· HO]^3– intermediates in rapid pre-equilibria steps, and is followed by a one-electron transfer step involving [W(CN)₈·HO]^3– (k _a) and its conjugate base [W(CN)₈·O]^4– (k _b). At 25 °C, I = 0.20 m (NaCl), the rate constant with H _a =40±6kJmol^–1 and S _a =–151±22JK^–1mol^–1; the rate constant with H _b =36±1kJmol^–1 and S _b =–136±2JK^–1mol^–1 at 25 °C, I = 0.20 m (NaCl); the acid dissociation constant of [W(CN)₈·HO]^3–, K ₅ =(5.9±1.7)×10^–10 m, with and is the first acid dissociation constant of H₂O₂. The second pathway, with rate constant, k _f, involves the formation of [W(CN)₈· HO₂]^4– and is followed by a formal two-electron redox process with [W(CN)₈]^3–. The pH-dependent rate constant, k _f, is given by . The rate constant k ₇ =23±6m ^–1 s ^–1 with and at 25°C, I = 0.20 m (NaCl).     

Kinetics and mechanism of osmium(VIII)-catalyzed oxidation of hypophosphite by hexacyanoferrate(III) in aqueous alkali

The kinetics of osmium(VIII)-catalyzed oxidation of hypophosphite with hexacyanoferrate(III) in alkaline medium has been studied. The rate is independent of the concentration of the oxidant. The order with respect to hydroxide ion is variable. Rate law (1) conforms with the experimental observations.

Kinetics and mechanism of oxidation of hydroxylamine by tetrachloroaurate(III) ion

The oxidation of H₂NOH is first-order both in [NH₃OH⁺] and [AuCl₄ ^–]. The rate is increased by the increase in [Cl^–] and decreased with increase in [H⁺]. The stoichiometry ratio, [NH₃OH⁺]/[AuCl₄ ^–], is 1. The mechanism consists of the following reactions.

Binuclear cobalt(II/II) and cobalt(II/III) chelates with O<Subscript>2</Subscript>N<Subscript>2</Subscript> ligands: synthesis,structure and magnetic studies

Abstract  The title complexes and have been synthesized in excellent yields by reacting Co(OAc)₂·4H₂O with H₂L¹ and H₂L², respectively, in acetonitrile solution. Here, [L¹]²⁻ and [L²]²⁻ are the deprotonated forms of N,N-bis(2-hydroxybenzyl)-N′,N′-dimethylethylenediamine and N,N-bis(2-hydroxybenzyl)-2-picolylamine, respectively. The crystal structures of and were determined by x-ray crystallography. In , each cobalt atom has distorted trigonal bipyramid geometry, while in , each cobalt atom has distorted octahedral geometry. Variable temperature magnetic moment measurements show weak antiferromagnetic interaction in . The magnetic characterization for is in agreement with the presence of Co(II) and Co(III) centers. Graphical Abstract  The title complexes and have been synthesized in excellent yields by reacting Co(OAc)₂·4H₂O with dianionic N₂O₂ coordinating ligands. In complex 1, each cobalt atom has distorted trigonal bipyramid geometry, while in complex 2, each cobalt atom has distorted octahedral geometry. Variable temperature magnetic moment measurements show weak antiferromagnetic interaction in complex 1. The magnetic characterization for complex 2 is in agreement with the presence of Co(II) and Co(III) centers. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Luminescence Behavior of Polynuclear Alkynylcopper(I) Phosphines

A series of soluble trinuclear and tetranuclear copper(I) complexes containing ₃-^l acetylides , and have been synthesized and shown to exhibit rich photoluminescent behavior at room temperature. The electrochemistry of the trinuclear Cu(I) acetylide complexes and the excited-state redox properties of have been investigated. The X-ray crystal structures of and have been determined.     

Solubility of Rhodochrosite (MnCO3) in NaCl Solutions

The solubility of rhodochrosite (MnCO₃) at 25°C under constant carbon dioxide partial pressure p(CO₂) was determined in NaCl solutions as a function of ionic strength I. The dissolution of MnCO₃(s) for the reaction

Iodine Hydrolysis Equilibrium

The equilibrium constant for the hydrolytic disproportionation of I₂

Dissociation Constants for Citric Acid in NaCl and KCl Solutions and their Mixtures at 25 °C

The constants for the dissociation of citric acid (H₃C) have been determined from potentiometric titrations in aqueous NaCl and KCl solutions and their mixtures as a function of ionic strength (0.05–4.5 mol-dm^–3) at 25 °C. The stoichiometric dissociation constants (K_i^*)

A kinetic study of the aqueous phase oxidation of di-μ-oxo-bis(L-cysteine)-oxomolybdenum(V) by molecular oxygen

The molecular oxygen-mediated decomposition of the binuclear complex, prepared from oxomolybdate(V) and L-(+)-cysteine, was studied spectrophotometrically at pH 3.5–5.6. The formation of MoVI was detected. The effects of pH and [O2] on the decomposition kinetics are given by the equation:

Solid-Solute Phase Equilibria in Aqueous Solution. XII. Solubility and Thermal Decomposition of Smithsonite

The solubility constant of ZnCO₃, smithsonite, in aqueous NaClO₄ solutions hasbeen investigated as a function of temperature (288.15 T/K 338.15) atconstant ionic strength I = 1.00 mol-kg^–1. In addition, the solubility of zinccarbonate has been determined at 2.00 and 3.00 mol-kg^–1 NaClO₄ (298.15 K).The solubility measurements have been evaluated by applying the Daviesapproximation, the specific ion-interaction theory, and the Pitzer model, respectively.The thermodynamic interpretation leads to an internally consistent set ofthermodynamic data for ZnCO₃ (298.15 K): solubility constant log*K _p50 ⁰ = 7.25 ± 0.10,standard Gibbs energy of formation _i G (ZnCO₃) = (–777.3±0.6)kJ-mol^–1, standard enthalphy of formation _f H (ZnCO₃)= (–820.2±3.0) kJ-mol^–1,and standard entropy S (ZnCO₃) = (77±10)J-mol^–1 K.^–1. Furthermore, the DSCcurve for the thermal decarbonation of zinc carbonate has been recorded in orderto obtain the enthalpy of formation _fH (ZnCO₃) =(–820.2±2.0) from theheat of decomposition. Finally, our results are also consistent within theexperimental error limits with a recent determination of the standard entropy ofsmithsonite, leading to a recommended set of thermodynamic properties of ZnCO₃:      

Hydrogen peroxide as a reductant of hexacyanoferrate(III) in alkaline solutions: kinetic studies

The kinetics of hexacyanoferrate(III) reduction by hydrogen peroxide in strongly alkaline media leading to hexacyanoferrate(II) ion have been studied spectrophotometrically within the wavelength range 300–500 nm. The reaction obeys a simple pseudo-first-order rate expression under the applied conditions, namely, a large excess of the reductant and OH⁻ anion concentrations, and a low oxidant concentration. The linear dependences of the pseudo-first-order rate constant on OH⁻ and H₂O₂ concentrations are consistent with the rate law of the form: where and are the second- and the pseudo-third-order rate constants for the electron transfer from HO₂ ⁻ and O₂ ²⁻ to [Fe(CN)₆]³⁻, respectively. The apparent activation parameters determined at 0.4 M NaOH are as follows: ΔH ^# = (18.0 ± 1.0) kJ mol⁻¹ and ΔS ^# = (−155 ± 3.5) J K⁻¹ mol⁻¹. The possible mechanism of the reaction is discussed.     

Kinetics and mechanism of dissociation of copper(II) complexes of biguanide and someN 1-substituted biguanides in aqueous acetate buffer media

The kinetics fo dissociation of thebis complexes [Cu(LH)₂]²⁺ formed by Cu^II with biguanide andN ¹-substituted methyl, phenyl, dimethyl and diethyl biguanides into the mono biguanide complexes in aqueous NaOAc-HOAc buffer media have been studied by stopped-flow spectrophotometry. The results, under pseudo-first-order conditions, indicate k_obs=k_o+k_H[H⁺]. For the different complexes k_o values are comparable, but k_H values differ appreciably; log k_H versus log K _d ^H is linear withca. unit slope K _d ^H being the equilibrium constant for the process:

Synthesis of flexible ligands for assembling two metals in close proximity. Magnetic,electrochemical and spectral properties of binuclear copper (II) complexes with different exogenous bridging motifs

Binuclear Cu^II complexes having new flexible heptadentate ligands 2,6-bis{[bis(3,3-N,N-dimethylaminopropyl)amino]methyl}-4-bromophenol [HL¹], 2,6-bis(3,3-N,N-dimethylaminopropyl)amino]methyl}-4-methylphenol [HL²], and 2,6-bis{[bis(3,3-N,N-dimethylaminopropyl)amino]methyl}-4-methoxyphenol [HL³], capable of assembling two copper ions in close proximity have been synthesized. Comparisons of the charge-transfer (CT) features, observed in electronic spectra of these complexes, are correlated with the electronic effect on the aromatic ring of the ligand systems. Cyclic voltammetry has revealed the existence of two reduction couples,

Kinetics of osmium(VIII) catalysis of periodate oxidation of DMF in aqueous alkaline medium

The osmium(VIII) catalysed IO₄ ⁻ oxidation of DMF in aqueous alkaline medium follows the rate law:

Simple and rapid catalytic spectrophotometric determination of superoxide anion radical and superoxide dismutase activity in natural medical vegetables using phenol as the substrate for horseradish peroxidase

The coupling reaction of 4-aminoantipyrine (4-AAP) with phenol using the superoxide anion radical ( ) as oxidizing agent under the catalysis of horseradish peroxidase (HRP) was studied. Based on the reaction, produced by irradiating vitamin B₂ (V_B2) was spectrophotometrically determined at 510 nm. Under the optimum experimental conditions, the relationship between A ₅₁₀ and concentration was linear in the range 9.14×10^–6–1.2×10^–4 mol L^–1. The detection limit was determined to be 1.37×10^–6 mol L^–1. A possible reaction mechanism was discussed. The effect of interferences and surfactants on the determination of was also investigated. The proposed method was applied to determine superoxide dismutase activity in garlic, scallion, and onion with satisfactory results.     

First and second thermodynamic mixing functions of ethylbenzene+n-nonane, +n-decane,and+n-dodecane at 25 and 45°C

Isothermal compressibilities _T and isobaric thermal expansion coefficients _p have been determined for mixtures of ethylbenzene+n-nonane, +n-decane, and +n-dodecane at 25 and 45°C in the whole range of composition. The excess functions and have been obtained at each measured mole fraction. The first one is zero for ethylbenzene +n-nonane, positive for ethylbenzene +n-decane, and +n-dodecane and increases with chain length n of the n-alkane. The function is positive for the three studied systems and nearly constant with n. Both mixing functions increase slightly with temperature. From this measurement and supplementary literature data of molar heat capacities at constant pressure C _P , the isentropic compressibilities _S, the molar heat capacities at constant volume C _V and the corresponding mixing functions have been calculated at 25°C. Furthermore, the pressure dependence of excess enthalpy H ^B , at zero pressure and at 25°C has been obtained from our experimental results of and experimental literature values for excess volume V ^E .     

Potentiometric Study of the Association of Magnesium and Sulfate Ions at 25°C in High Ionic Strength Media

The association constant for the reaction: has been measured at 25°C using magnesium ion-selective electrode (Mg–ISE) potentiometry in aqueous solutions of ionic strength (I) ranging from 0.25 to 6 M in CsCl and in 1 M (Me₄NCl). The value of log (MgSO₄) = 0.98 ± 0.02 in 1 M (Me₄NCl) was significantly higher than that of 0.75 ± 0.01 obtained in 1 M(CsCl). This difference can be explained by a weak association between Cs⁺and SO ₄ ^2- , with log (CsSO ₄ ^- ) = –0.11 ± 0.03, which is also qualitatively consistent with the absence of an increase in (MgSO₄) at high ionic strength in CsCl media. Extrapolation of the results in CsCl media gave an infinite dilution value of log ° (MgSO₄) = 2.38 ± 0.03 that was rather dependent on the nature of the extrapolation function. The performance of the Mg–ISE in various media is also briefly described.     

Molecular Structure and Conformation of p-Bis(trimethylsilyl)benzene: A Study by Gas-Phase Electron Diffraction and Theoretical Calculations

The molecular structure and conformation of p-bis(trimethylsilyl)benzene have been investigated by gas-phase electron diffraction, ab initio MO calculations at the HF/6-31G*, MP2(f.c.)/6-31G*, and B3LYP/6-31G* levels, and MM3 molecular mechanics calculations. The calculations indicate the syn- and anti-coplanar conformations, with two bonds in the plane of the benzene ring, to be energy minima. The perpendicular conformations, with two bonds in a plane orthogonal to the ring plane, are transition states. The two coplanar conformers have nearly the same energy with a low interconversion barrier, 0.3–0.5 kJ mol^–1. The calculated lengths of the and bonds differ by only a few thousandths of an angstrom, in agreement with electron diffraction results from molecules containing either or bonds. The geometrical distortion of the benzene ring in p-bis(trimethylsilyl)-benzene may be described by superimposing independent distortions from each of the two SiMe₃ groups. The electron diffraction intensities from a previous study (Rozsondai, B.; Zelei, B.; Hargittai, I. J. Mol. Struct. 1982, 95, 187) have been reanalyzed, imposing constraints from the theoretical calculations, and using a model based on a 1:1 mixture of the two coplanar conformers. The effective torsion angles of the SiMe₃ groups may indicate nearly free rotation. Important geometrical parameters from the present electron diffraction analysis are , and . While the mean bond lengths are virtually the same from the previous and present analyses, the new ipso angle is in better agreement with the MO calculations [HF, 116.9° MP2(f.c.), 117.1° B3LYP, 116.9°].     

Effect of charge on the standard partial molar volumes and heat capacities of organic electrolytes in methanol and water

Previously developed additivity schemes for nonelectrolytes have been used to estimate and for tetraalkyl and tetraphenyl methanes in methanol and water. Corrections have been applied to the thermodynamic values of these model compounds to account for a variation in size of the central atom, and these were used to ascertain the effect of charge on and of alkyl and phenyl quaternary ions having N, P and B as central atoms. Investigations of R₄NBr, (R=methyl to heptyl) salts show that the charge effect on and of R₄N⁺ ions is large and relatively independent of ion size suggesting that the solvent molecules penetrate the ions. The ability to estimate and of the quaternary ions in the bromide salt solutions has made it possible to make ionic assignments with some confidence; (Br^–) has been evaluated as 19.7±2 and 30.2±7 cm³-mol^–1 and (Br^–) as –83±7 and –68±30 J-K^–1-mol^–1 in methanol and water, respectively. The use of organic ions for making ionic assignments of and is critically examined and comparisons with other assignments are made. The scaled particle theory is employed to divide the heat capacities of electrolytes into cavity and interaction contributions.     

Characterization of Potassium Chloride as an Equitransferent “Intersolvental” Salt Bridge

The electromotive forces (emf) E _A and E _C of the following concentration cells with transference: respectively, together with the emfs E _MAX of the corresponding double cell without transference: have been measured at KCl molalities m (m ₁ fixed and m ₂ varied, with m ₂>m ₁) approximately up to the KCl solubility limit in 12 solvent mixtures for the three aqueous–organic solvent systems (ethylene glycol+water), (acetonitrile + water), and (1,4-dioxane + water) up to 0.8 mass fraction of organic component. For all the cases explored, the E _A vs. E _MAX relation is linear over the whole KCl molality range. The ionic transference numbers t of KCl determined therefrom show a curvilinear dependence on the mass fraction of the organic component of the relevant solvent mixture and are found to fall in the range 0.52–0.48, viz., within ±4% of exact equitransference (t ₊ = t _– = 0.5). In particular, KCl becomes exactly equitransferent (i.e., an ideal salt bridge) in aqueous mixtures with the following mass fractions of organic component: 0.4 ethylene glycol and 0.09 acetonitrile, as well as 0.12 methanol, and 0.08 and 0.34 ethanol from our recent work. Even if use of KCl as a salt bridge would be somewhat restricted by its limited solubility in high mass fractions of dioxane and acetonitrile and pending extension of investigation to other mixed-solvent systems, the above figures characterize KCl as a fairly good intersolvental salt bridge in electrochemistry, electroanalysis, and corrosion science.