scholarly journals Kinetics and Mechanism of the Reaction between Chromium(III) and 3,4-Dihydroxy-Phenyl-Propenoic Acid (Caffeic Acid) in Weak Acidic Aqueous Solutions

2008 ◽  
Vol 2008 ◽  
pp. 1-7 ◽  
Author(s):  
Vladimiros Thoma ◽  
Konstantinos Tampouris ◽  
Athinoula L. Petrou
Keyword(s):  
Aqueous Solutions ◽  
Metal Ion ◽  
Activation Parameters ◽  
Ligand Molecule ◽  
Propenoic Acid ◽  
Protonation Equilibrium ◽  
Three Stages ◽  
Ph Decrease ◽  
Rapid Protonation ◽  
Mechanism Of The Reaction

Our study of the complexation of 3,4-dihydroxy-phenyl-propenoic acid by chromium(III) could give information on the way that this metal ion is available to plants. The reaction between chromium(III) and 3,4-dihydroxy-phenyl-propenoic acid in weak acidic aqueous solutions has been shown to take place by at least three stages. The first stage corresponds to substitution (Idmechanism) of water molecule from the Cr(H2O)5OH2+coordination sphere by a ligand molecule. A very rapid protonation equilibrium, which follows, favors the aqua species. The second and the third stages are chromium(III) and ligand concentration independent and are attributed to isomerisation and chelation processes. The corresponding activation parameters areΔH2(obs)≠= 28.6±2.9 kJmol−1,ΔS2(obs)≠=−220  ±10 JK−1mol−1,ΔH3(obs)≠= 62.9±6.7 kJmol−1andΔS3(obs)≠=−121  ±22 JK−1mol−1. The kinetic results suggest associative mechanisms for the two steps. The associatively activated substitution processes are accompanied by proton release causing pH decrease.

scholarly journals Kinetics and Mechanism of the Reaction between Chromium(III) and 2,3-Dihydroxybenzoic Acid in Weak Acidic Aqueous Solutions

2010 ◽  
Vol 2010 ◽  
pp. 1-8 ◽  
Author(s):  
Athinoula L. Petrou ◽  
Vladimiros Thoma ◽  
Konstantinos Tampouris
Keyword(s):  
Aqueous Solutions ◽  
Activation Parameters ◽  
Entropy Change ◽  
Hydroxyl Group ◽  
Positive Entropy ◽  
Dihydroxybenzoic Acid ◽  
Three Stages ◽  
Ph Decrease ◽  
Two Stages ◽  
Mechanism Of The Reaction

The reaction between chromium(III) and 2,3-dihydroxybenzoic acid (2,3-DHBA) takes place in at least three stages, involving various intermediates. The ligand (2,3-DHBA)-to-chromium(III) ratio in the final product of the reaction is 1 : 1. The first stage is suggested to be the reaction of[Cr(H2O)5(OH)]2+with the ligand in weak acidic aqueous solutions that follows anIdmechanism. The second and third stages do not depend on the concentrations of chromium(III), and their activation parameters areΔH≠=61.2±3.1 kJmol−1,ΔS≠=−91.1±11.0 JK−1mol−1,ΔH≠=124.5±8.7 kJmol−1, andΔS≠=95.1±29.0 JK−1mol−1. These two stages are proposed to proceed via associative mechanisms. The positive value ofΔS≠can be explained by the opening of a four-membered ring (positive entropy change) and the breaking of a hydrogen bond (positive entropy change) at the associative step of the replacement of the carboxyl group by the hydroxyl group at the chromium(III) center (negative entropy change in associative mechanisms). The reactions are accompanied by proton release, as shown by the pH decrease.

scholarly journals Reaction of Chromium(III) with 3,4-Dihydroxybenzoic Acid: Kinetics and Mechanism in Weak Acidic Aqueous Solutions

2008 ◽  
Vol 2008 ◽  
pp. 1-8 ◽  
Author(s):  
Kimon Zavitsanos ◽  
Konstantinos Tampouris ◽  
Athinoula L. Petrou
Keyword(s):  
Aqueous Solutions ◽  
Coordination Sphere ◽  
Activation Parameters ◽  
Experimental Results ◽  
Ligand Concentration ◽  
Water Molecules ◽  
Dihydroxybenzoic Acid ◽  
Proton Release ◽  
Three Stages ◽  
Ph Decrease

The interactions between chromium(III) and 3,4-dihydroxybenzoic acid (3,4-DHBA) were studied resulting in the formation of oxygen-bonded complexes upon substitution of water molecules in the chromium(III) coordination sphere. The experimental results show that the reaction takes place in at least three stages, involving various intermediates. The first stage was found to be linearly dependent on ligand concentrationk1(obs)_=k0+k1(obs)[3,4-DHBA], and the corresponding activation parameters were calculated as follows:ΔH1(obs)≠=51.2±11.5 kJ mol−1,ΔS1(obs)≠=−97.3±28.9 J mol−1 K−1(composite activation parameters) . The second and third stages, which are kinetically indistinguishable, do not depend on the concentrations of ligand and chromium(III), accounting for isomerization and chelation processes, respectively. The corresponding activation parameters areΔH2(obs)≠=44.5±5.0 kJ mol−1,ΔS2(obs)≠=−175.8±70.3 J mol−1 K−1. The observed stages are proposed to proceed via interchange dissociative (Id, first stage) and associative (second and third stages) mechanisms. The reactions are accompanied by proton release, as is shown by the pH decrease.

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.

scholarly journals Kinetics and Mechanisms of the Chromium(III) Reactions with 2,4- and 2,5-Dihydroxybenzoic Acids in Weak Acidic Aqueous Solutions

2010 ◽  
Vol 2010 ◽  
pp. 1-10 ◽  
Author(s):  
Kimon Zavitsanos ◽  
Athinoula L. Petrou
Keyword(s):  
Hydrogen Bonding ◽  
Aqueous Solutions ◽  
Activation Parameters ◽  
Intramolecular Hydrogen Bonding ◽  
Dihydroxybenzoic Acid ◽  
Proton Release ◽  
Second Stage ◽  
Ph Decrease ◽  
Intramolecular Hydrogen ◽  
Two Stages

The reactions of 2,4- and 2,5-dihydroxybenzoic acids (dihydroxybenzoic acid, DHBA) with chromium(III) in weak acidic aqueous solutions have been shown to take place in at least two stages. The first stage of the reactions has an observed rate constantk1(obs)=k1[DHBA]+Cand the corresponding activation parameters areΔH1(2,4)≠=49,5 kJ/mol−1,ΔS1(2,4)≠=−103,7J mol−1K−1,ΔH1(2,5)≠=60,3 kJ/mol−1, andΔS1(2,5)≠=−68,0 J mol−1K−1. These are composite activation parameters and the breaking of the strong intramolecular hydrogen bonding in the two ligands is suggested to be the first step of the (composite) first stage of the reactions. The second stage is ligand concentration independent and is thus attributed to a chelation process. The corresponding activation parameters areΔH2(2,4)≠=45,13 kJ/mol−1,ΔS2(2,4)≠=−185,9 J mol−1K−1,ΔH2(2,5)≠=54,55 kJ/mol−1, andΔS2(2,5)≠=−154,8 J mol−1K−1. The activation parameters support an associative mechanism for the second stage of the reactions. The various substitution processes are accompanied by proton release, resulting in pH decrease.

The kinetics, isotope effects, and mechanism of the reaction of 2,2-di(4-nitrophenyl)-1,1,1-trifluoroethane with alkoxide bases in alcohol solvents

1985 ◽  
Vol 63 (3) ◽  
pp. 576-580 ◽  
Author(s):  
Arnold Jarczewski ◽  
Grzegorz Schroeder ◽  
Wlodzimierz Galezowski ◽  
Kenneth T. Leffek ◽  
Urszula Maciejewska
Keyword(s):  
Rate Constants ◽  
Isotope Effects ◽  
Activation Parameters ◽  
Tert Butanol ◽  
Deuterium Isotope Effects ◽  
Alcohol Solvents ◽  
Multistep Reaction ◽  
Mechanism Of The Reaction

The reaction between 2,2-di(4-nitrophenyl)-1,1,1-trifluoroethane and the alkoxide bases ŌCH3, ŌC2H5, ŌnC4H9, ŌCH(CH3)2, and ŌC(CH3)3 in their corresponding alcohol solvents is a multistep reaction with several intermediates: 2,2-di(4-nitrophenyl)-1,1-difluoro-1-alkoxyethane (A), 2,2-di(4-nitrophenyl)-1-fluoro-1-alkoxyethene (B), 2,2-di(4-nitrophenyl)-1,1-dialkoxyethene (C), 2,2-di(4-nitrophenyl)-1,1-difluoroethene (D), and 4,4′-dinitrobenzophene (E). Rate constants and activation parameters have been measured for the appearance of the two stable products B and C. The kinetic deuterium isotope effects for the appearance of B fell in the range of kH/kD = 1 to 2 at 25 °C for the primary and secondary alkoxides, whereas kH/kD = 5.4 at 30 °C for the appearance of D with tert-butoxide. Exchange experiments showed that H/D exchange took place between the substrate and solvent to the extent of 100% with methoxide, 50% with ethoxide and isopropoxide, and 0% with tert-butoxide. It is concluded the HF elimination from the substrate follows an (ElcB)R mechanism with methoxide/methanol, changing to (ElcB)I or E2 with tert-butoxide/tert-butanol.

scholarly journals Comparison between Different Keratin-composed Biosorbents for the Removal of Heavy Metal Ions from Aqueous Solutions

2002 ◽  
Vol 20 (4) ◽  
pp. 393-416 ◽  
Author(s):  
Fawzi Banat ◽  
Sameer Al-Asheh ◽  
Dheaya‘ Al-Rousan
Keyword(s):  
Aqueous Solutions ◽  
Human Hair ◽  
Metal Ion ◽  
Ph Value ◽  
Freundlich Isotherm ◽  
Operating Conditions ◽  
Ion Uptake ◽  
Chicken Feathers ◽  
Initial Ph ◽  
Metal Ion Uptake

This study examined and compared the ability of chicken feathers, human hair and animal horns, as keratin-composed biosorbents, for the removal of Zn2+ and Cu2+ ions from single metal ion aqueous solutions under different operating conditions. The three biosorbents investigated in this study were all capable of adsorbing Zn2+ and Cu2+ ions from aqueous solutions. The biosorbent showing the highest uptake of Zn2+ and Cu2+ ions was animal horns. Chicken feathers showed a higher Cu2+ ion uptake and a lower Zn2+ ion compared to human hair. Increasing the initial concentration of Zn2+ or Cu2+ ions, or increasing the initial pH value, increased the metal ion uptake. Such uptake decreased when the temperature was raised from 25°C to 50°C for all adsorbent/metal ion combinations except for Zn2+ ion/human hair where the uptake increased with temperature. It was demonstrated that the addition of NaCl salt to the metal ion solution depressed the metal ion uptake. The Freundlich isotherm model was found to be applicable to the adsorption data for Cu2+ and Zn2+ ions.

Protonation du N,N-diméthylformamide: étude par spectrométries vibrationnelle et électronique

1975 ◽  
Vol 53 (3) ◽  
pp. 442-447 ◽  
Author(s):  
P. Combelas ◽  
M. Costes ◽  
C. Garrigou-Lagrange
Keyword(s):  
Aqueous Solutions ◽  
Raman Spectra ◽  
Infrared And Raman Spectra ◽  
Protonation Equilibrium ◽  
Ionization Equilibria

Infrared and Raman spectra of acidic (hydrochloric or sulfuric) aqueous solutions of dimethylformamide indicate the existence of a single protonation equilibrium.[Formula: see text]The pK for this reaction was evaluated as −1.35 ± 0.05 from u.v. spectra; a less precise value (−1.1 ± 0.5) was obtained from Raman spectra. This result disagrees with earlier conclusions which favored the existence of two ionization equilibria for dimethylformamide. [Journal translation]

A multi-site mechanism model for studying Pb and Cu retention from aqueous solutions by Fe-Mg-rich clays

Clay Minerals ◽  
2018 ◽  
Vol 53 (2) ◽  
pp. 175-192 ◽  
Author(s):  
Z. Kypritidou ◽  
A. Argyraki
Keyword(s):  
Aqueous Solutions ◽  
Metal Ion ◽  
Methodological Approach ◽  
Synergetic Effect ◽  
Sorption Isotherms ◽  
Superior Performance ◽  
Isotherm Models ◽  
Sufficient Information ◽  
Retention Mechanisms ◽  
Clay Materials

ABSTRACTThe retention mechanisms of metal ions during interaction of clay with metal-rich aqueous solutions is usually investigated by sorption isotherms. Although classical isotherm models may provide sufficient information about the characteristics of the solid–liquid system, they do not distinguish among the various retention mechanisms. This study presents a methodological approach of combining batch experiment data and geochemical modelling for the characterization of the interaction of Mg-Fe-rich clay materials with monometallic solutions of Pb and Cu. For this purpose, a palygorskite clay (PCM), an Fe-smectite clay (SCM) and a natural palygorskite-Fe-smectite mixed clay (MCM) were assessed for their effectiveness as metal ion sorbents. The sorption capacity of the materials follows the order MCM > SCM > PCM and ranges between 27.6–52.1 mg g–1for Pb and 7.7–17.6 mg g–1for Cu. Based on the experimental results that allowed the speciation calculations, fitting of sorption isotherms and the investigation of relationships between protons, Mg and the metals studied we suggest that a combination of sorption mechanisms occurs during the interaction of clay materials with metal solutions. These involve surface complexation, ion exchange and precipitation of solid compounds onto the solid surface. A three-term isotherm model was employed to quantify the role of each of the above mechanisms in the overall retention process. The superior performance of mixed clay among the materials tested is attributed to the synergetic effect of exchange in the interlayer and specific sorption on the clay edges.

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