Metal-ion oxidations in solution. Part XXI. Kinetics and mechanism of the reaction of ascorbic acid, hydroquinone, and catechol with 12-tungstocobaltoate(III)

1977 ◽  
Vol 55 (20) ◽  
pp. 3581-3586 ◽  
Author(s):  
Zahid Amjad ◽  
Jean-Claude Brodovitch ◽  
Alexander McAuley
Keyword(s):  
Ascorbic Acid ◽  
Ionic Strength ◽  
Dissociation Constant ◽  
Metal Ion ◽  
Outer Sphere ◽  
Hydrogen Ion ◽  
Stopped Flow ◽  
Acid Media ◽  
Temperature Range ◽  
Mechanism Of The Reaction

The mechanisms of oxidation of the three di-hydroxy substrates in the title reactions have been investigated in acid media [HClO4] = 0.04–1.00 M, at an ionic strength of 1.0 M [LiClO4] over the temperature range 5–35 °C using the stopped-flow method. In contrast to the reactions of hydroquinone (k2(25 °C) = 1.43 × 104 M−1 s−1, ΔH≠ = 3.6 ± 0.4 kcal mol−1, ΔS≠ = −34 ± 8 cal deg−1 mol−1) and catechol (k2(25 °C) = 9.5 × 102 M−1 s−1, ΔH≠ = 5.9 ± 0.9 kcal mol−1, ΔS≠ = 27 ± 8 cal deg−1 mol−1) where no hydrogen ion dependence is observed over the range studied, the rate variations of oxidation of ascorbic acid (H2A) are consistent with two reactions involving H2A (k5 = 77.4 M−1 s−1, ΔH≠ = 4.5 ± 0.6 kcal mol−1, ΔS≠ = −35 ± 5 cal deg−1 mol−1) and HA− (k6 = 2.41 × 105M−1 s−1, ΔH≠ = 2.0 ± 1 kcal mol−1, ΔS≠ = −17 ± 6 cal deg−1 mol−1). The dissociation constant of ascorbic acid has been re-determined over the temperature range studied. Solvation effects are considered important in these outer-sphere systems, which may be discussed in terms of the Marcus relationship. Comparisons with related reactions are discussed.

Kinetics and Equilibrium of Binding of Fe3+ by a Fulvic Acid: A Study by Stopped Flow Methods

1975 ◽  
Vol 53 (20) ◽  
pp. 2979-2984 ◽  
Author(s):  
Cooper H. Langford ◽  
Tahir R. Khan
Keyword(s):  
Ionic Strength ◽  
Complex Formation ◽  
Fulvic Acid ◽  
Metal Ion ◽  
Equilibrium Constants ◽  
Acid Dissociation ◽  
Stopped Flow ◽  
Solution Ph ◽  
Sulfosalicylic Acid ◽  
Flow Measurements

The first report of a rate of binding of a metal ion (Fe3+) by a soluble fulvic acid is derived from stopped flow measurements. The rate of complex formation is normal in Wilkins' sense and similar to that for sulfosalicylic acid. Dissociation is slow (t1/2 > 10 s). The binding of Fe3+ by the fulvic acid in acid solution, pH = 1–2.5, was investigated by kinetic analysis in which the reaction of free Fe3+ with sulfosalicylic acid was followed by stopped flow spectrophotometry on a time scale short compared to release of Fe3+ by fulvic acid. Conditional equilibrium constants found were 1.5 ± 0.3 × 104 at pH = 1.5 and 2.5, and 2.8 ± 0.3 × 103 at pH = 1.0 at 25 °C (ionic strength 0.1).

Kinetics and Mechanism of the Reaction of Dichlorotetraaquaruthenium(III) and Thiols

2012 ◽  
Vol 65 (2) ◽  
pp. 113 ◽  
Author(s):  
Suprava Nayak ◽  
Gouri Sankhar Brahma ◽  
K. Venugopal Reddy
Keyword(s):  
Electron Transfer ◽  
Ionic Strength ◽  
Equilibrium Constants ◽  
Activation Parameters ◽  
Kinetics And Mechanism ◽  
Intermediate Complex ◽  
Mechanism Of Formation ◽  
Temperature Range ◽  
Temperature Reduction ◽  
Mechanism Of The Reaction

The formation of an intermediate ruthenium(iii) thiolate complex by the interaction of thiols, RSH (R = glutathione and l-cysteine) and dichlorotetraaquaruthenium(iii), [RuIIICl2(H2O)4]+, is reported in the temperature range 25–40°C. The kinetics and mechanism of formation of the intermediate complex were studied as a function of [RuIIICl2(H2O)4]+, [RSH], pH, ionic strength and temperature. Reduction of the intermediate complex takes place slowly and results in the corresponding disulfides RSSR and [RuIICl2(H2O)4]+. The results are interpreted in terms of a mechanism involving a rate-determining inner-sphere one-electron transfer from RSH to the oxidant used in the present investigation and a comparison of rate and equilibrium constants is presented with activation parameters.

Adsorption of hexavalent chromium on dunite

2011 ◽  
Vol 63 (4) ◽  
pp. 818-824 ◽  
Author(s):  
Antri Demetriou ◽  
Ioannis Pashalidis
Keyword(s):  
Experimental Data ◽  
Ionic Strength ◽  
Metal Ion ◽  
Outer Sphere ◽  
Atmospheric Conditions ◽  
Adsorption Model ◽  
Wide Ph Range ◽  
Ph Range ◽  
Relative Adsorption ◽  
Outer Sphere Complexes

The paper presents and discusses the effect of various physicochemical parameters (e.g. pH, ionic strength, Cr(VI) initial concentration, amount of the adsorbent, temperature and contact time between metal ion and adsorbent) on the adsorption efficiency of Cr(VI) on dunite in aqueous solutions under atmospheric conditions. Evaluation of the experimental data shows that dunite presents increased affinity for Cr(VI) over a wide pH range and Cr(VI) concentration, and the experimental data are well fitted by the Kd adsorption model. The relative adsorption is pH dependent and decreases slightly (about 10%) with increasing pH, because of changes in the surface charge of the solid. The effect of the ionic strength is significant (particularly at low pH), indicating the predominance of outer-sphere complexes. Moreover, adsorption experiments at various temperatures, two different pH values (pH 3 and pH 8) and three different ionic strengths (0.0, 0.1 and 1.0 M NaClO4), indicate an endothermic but spontaneous entropy-driven processes.

Metal-ion oxidations in solution. Part XIX. Redox pathways in the oxidation of penicillamine and glutathione by chromium(VI)

1977 ◽  
Vol 55 (18) ◽  
pp. 3335-3340 ◽  
Author(s):  
Alexander McAuley ◽  
M. Adegboyega Olatunji
Keyword(s):  
Ion Exchange ◽  
Redox Reaction ◽  
Metal Ion ◽  
Hydrogen Ion ◽  
Stopped Flow ◽  
Ion Concentrations ◽  
Flow Method ◽  
Chromium Vi ◽  
Reaction Proceeds ◽  
Transient Intermediate

Three moles of penicillamine or glutathione are required to reduce chromium(VI) to chromium(III). The kinetics and mechanism of the redox reaction have been studied using the stopped-flow method. The reaction proceeds via the formation of a transient intermediate (K1) which decomposes either in a proton-catalyzed pathway or by reaction with a second mole of thiol. The rate law[Formula: see text]where n = 1 for penicillamine and 2 for glutathione has been shown to hold over a range of thiol and hydrogen-ion concentrations. At 25 °C k2 = 14.3 ± 1.0 M−1 s−1 for penicillamine (ΔH≠ = 9 ± 2 kcal mol−1, ΔS≠ = −33 ± 6 cal K−1 mol−1) and 12.1 ± 0.4 M−1 s−1 for glutathione (ΔH≠ = 7 ± 2 kcal mol−1, ΔS≠ = −40 ± 5 cal K−1 mol−1). Several chromium(III) products have been identified by ion-exchange methods. The significance of the second-order pathways in these reactions is discussed.

Kinetics and mechanism of the oxidation of benzenediols and ascorbic acid by bis(1,4,7-triazacyciononane)nickel(III) in aqueous perchlorate media

1985 ◽  
Vol 63 (6) ◽  
pp. 1198-1203 ◽  
Author(s):  
A. McAuley ◽  
Lee Spencer ◽  
P. R. West
Keyword(s):  
Electron Transfer ◽  
Ascorbic Acid ◽  
Exchange Rate ◽  
Rate Constants ◽  
Outer Sphere ◽  
Kinetics And Mechanism ◽  
Hydrogen Ion ◽  
Transfer Reactions ◽  
Radical Ions ◽  
Electron Transfer Reactions

The reactions of the outer-sphere electron transfer reagent, Ni(9-aneN3)23+, (bis(1,4,7-triazacyclononane)nickel(III) ion) with ascorbic acid, hydroquinone, catechol, and resorcinol have been investigated. The absence of any proton related equilibria with the oxidant provides a means of ascribing the observed inverse hydrogen ion dependences to reactions of the dissociated ascorbate or quinolate ions, (HA−). The data are consistent with the rate-determining one-electron transfer reactions:[Formula: see text]followed by rapid oxidation of the radical ions formed. In the reaction with ascorbic acid, k1 ~ 0 and k2 (T = 25° C) = 5.2 × 106 M−1 s−1 (ΔH≠ = 10.1 ± 2.5 kcal mol−1, ΔS≠ = 5.7 ± 5.1 cal mol−1 K−1). For hydroquinone, catechol, and resorcinol, k1 = 2.9 × 103, 2.8 × 102, and ~0 M−1 s−1and k2 = 6.9 × 109, 4.1 × 109, and 2.8 × 108 M−1 s−1, respectively. These data have been combined with those from other similar reactions leading, by use of a Marcus correlation, to self-exchange rate constants for the HAsc−/HAsc• couple of 3.5 × 105 M−1 s−1 and for the H2Quin0/+ and H2cat0/+ systems of 5 × 107 and 2 × 107 M−1 s−1, respectively. The importance of the effect of bond-reorganisation on electron transfer is discussed.

Oxidation of ascorbic acid by osmium(VIII)

1970 ◽  
Vol 48 (7) ◽  
pp. 1148-1150 ◽  
Author(s):  
U. S. Mehrotra ◽  
S. P. Mushran
Keyword(s):  
Ascorbic Acid ◽  
Ionic Strength ◽  
Hydrochloric Acid ◽  
Negligible Effect ◽  
Hydrogen Ion ◽  
Ion Concentration ◽  
Kcal Mole ◽  
Hydrogen Ion Concentration ◽  
Influence Of Temperature ◽  
Entropy Of Activation

Reduction of osmium(VIII) by ascorbic acid has been studied in the presence of hydrochloric acid. The reaction is found to be directly proportional to the concentration of ascorbic acid and osmium(VIII) and to the reciprocal of the hydrogen ion concentration. In the proposed mechanism, the reacting species have been shown to be perperosmic acid and ascorbate ion; this was supported by the negligible effect of the ionic strength. Influence of temperature was studied in the range 25–40 °C, and the energy and entropy of activation were calculated as 14.1 ± 0.2 kcal mole−1 and −8.6 ± 0.1 e.u., respectively. Osmium(IV) was found to be a product of the reaction.

Polymerization of acrylonitrile initiated by peroxomonosulphate-thiols redox systems

1990 ◽  
Vol 55 (8) ◽  
pp. 2001-2007
Author(s):  
Gurusamy Manivannan ◽  
Pichai Maruthamuthu
Keyword(s):  
Ionic Strength ◽  
Thermodynamic Parameters ◽  
Thermal Polymerization ◽  
Reaction Scheme ◽  
Degree Of Polymerization ◽  
Reducing Agent ◽  
Redox Systems ◽  
Temperature Range ◽  
Rate Of Polymerization

Aqueous thermal polymerization of acrylonitrile (AN) initiated by peroxomonosulphate (HSO5-, PMS)-thiolactic acid (TLA) and PMS-thiomalic acid (TMA) redox systems has been carried out in the temperature range 30-50 °C. The effect of concentration of monomer, initiator, reducing agent, H+, and ionic strength on rate of polymerization, Rp, has been investigated under deaerated conditions. The Rp has been found to depend on, Rp ~ [AN]01.5 [PMS]0.5 [TLA]0.5 in PMS-TLA system and, Rp ~ [AN]02.0 [PMS]1.0 [TMA]0 in PMS-TMA system. The degree of polymerization (Xn) values and thermodynamic parameters have been evaluated. Suitable reaction scheme has been proposed and expressions for Rp and Xn have been obtained.

Kinetics and mechanism of the reaction of iron(III) with 6-methyl-2,4-heptanedione and 3,5-heptanedione

1990 ◽  
Vol 55 (8) ◽  
pp. 1984-1990 ◽  
Author(s):  
José M. Hernando ◽  
Olimpio Montero ◽  
Carlos Blanco
Keyword(s):  
Aqueous Solution ◽  
Metal Ion ◽  
Activation Parameters ◽  
Enol Form ◽  
Kinetics And Mechanism ◽  
Kinetic Constants ◽  
Steric Factors ◽  
Kinetics Of ◽  
Mechanism Of The Reaction

The kinetics of the reactions of iron(III) with 6-methyl-2,4-heptanedione and 3,5-heptanedione to form the corresponding monocomplexes have been studied spectrophotometrically in the range 5 °C to 16 °C at I 25 mol l-1 in aqueous solution. In the proposed mechanism for the two complexes, the enol form reacts with the metal ion by parallel acid-independent and inverse-acid paths. The kinetic constants for both pathways have been calculated at five temperatures. Activation parameters have also been calculated. The results are consistent with an associative activation for Fe(H2O)63+ and dissociative activation for Fe(H2O)5(OH)2+. The differences in the results for the complexes of heptanediones studied are interpreted in terms of steric factors.

Kinetics and mechanism of the reaction between oxidized cytochrome c and ascorbic acid

1991 ◽  
Vol 56 (2) ◽  
pp. 478-490 ◽  
Author(s):  
Joaquin F. Perez-Benito ◽  
Conchita Arias
Keyword(s):  
Ascorbic Acid ◽  
Cytochrome C ◽  
Redox Reactions ◽  
Arrhenius Equation ◽  
Horse Heart Cytochrome ◽  
First Order ◽  
Acidic Form ◽  
Ph Range ◽  
Horse Heart Cytochrome C ◽  
Mechanism Of The Reaction

The reaction between horse-heart cytochrome c and ascorbic acid has been investigated in the pH range 5.5 – 7.1 and at 10.0 – 25.0 °C. The rate shows a first-order dependence on the concentration of cytochrome c, it increases in a non-linear way as the concentration of ascorbic acid increases, it increases markedly with increasing pH and, provided that the ionic strength of the medium is high enough, it fulfills the Arrhenius equation. The apparent activation energy increases as the pH of the solution increases. The results have been explained by means of a mechanism that includes the existence of an equilibrium between two forms (acidic and basic) of oxidized cytochrome c: cyt-H+ -Fe3+ + OH- cyt -Fe3+ + H2O, whose equilibrium constant is (6.7 ± 1.4). 108 at 25.0 °C, the acidic form being more reducible than the basic one. It is suggested that there is a linkage of hydrogenascorbate ion to both forms of cytochrome c previous to the redox reactions. Two possibilities for the oxidant-reductant linkage (binding and adsorption) are discussed in detail.

Protonation effects on dynamic flux properties of aqueous metal complexes

2009 ◽  
Vol 74 (10) ◽  
pp. 1543-1557 ◽  
Author(s):  
Herman P. Van Leeuwen ◽  
Raewyn M. Town
Keyword(s):  
Complex Formation ◽  
Metal Ion ◽  
Good Description ◽  
Minor Component ◽  
Outer Sphere ◽  
Metal Species ◽  
Central Metal ◽  
Inner Sphere ◽  
A Minor ◽  
Protonated Ligand

The degree of (de)protonation of aqueous metal species has significant consequences for the kinetics of complex formation/dissociation. All protonated forms of both the ligand and the hydrated central metal ion contribute to the rate of complex formation to an extent weighted by the pertaining outer-sphere stabilities. Likewise, the lifetime of the uncomplexed metal is determined by all the various protonated ligand species. Therefore, the interfacial reaction layer thickness, μ, and the ensuing kinetic flux, Jkin, are more involved than in the conventional case. All inner-sphere complexes contribute to the overall rate of dissociation, as weighted by their respective rate constants for dissociation, kd. The presence of inner-sphere deprotonated H2O, or of outer-sphere protonated ligand, generally has a great impact on kd of the inner-sphere complex. Consequently, the overall flux can be dominated by a species that is a minor component of the bulk speciation. The concepts are shown to provide a good description of experimental stripping chronopotentiometric data for several protonated metal–ligand systems.

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