Samy S. Anis

Kinetic studies on the reaction of ethylenediaminetetraacetatochromium(III) complex with hydrogen peroxide

The rate of reaction of [Cr(III)Y]aq (Y is EDTA anion) with hydrogen peroxide was studied in aqueous nitrate media [μ = 0.10 M (KNO3)] at various temperatures. The general rate equation, Rate = k1 + k2K1[H+]−11 + K1[H+]−1 [Cr(III)Y]aq[H2O2] holds over the pH range 5–9. The decomposition reaction of H2O2 is believed to proceed via two pathways where both the aquo and hydroxo-quinquedentate EDTA complexes are acting as the catalyst centres. Substitution-controlled mechanisms are suggested and the values of the second-order rate constants k1 and k2 were found to be 1.75 × 10−2 M−1 s−1 and 0.174 M−1 s−1 at 303 K respectively, where k2 is the rate constant for the aquo species and k2 is that for the hydroxo complex. The respective activation enthalpies (ΔH*1 = 58.9 and ΔH*2 = 66.5 KJ mol−1) and activation entropies (ΔS*1 = −85 and ΔS*2 = −40 J mol−1 deg−1) were calculated from a least-squares fit to the Eyring plot. The ionisation constant pK1, was inferred from the kinetic data at 303 K to be 7.22. Beyond pH 9, the reaction is markedly retarded and ceases completely at pH ⩾ 11. This inhibition was attributed in part to the continuous loss of the catalyst as a result of the simultaneous oxidation of Cr(III) to Cr(VI).

Kinetic and mechanism of complex formation between chromium(III) and N-(2-hydroxyethyl)ethylenediamine-N,N` ,N`-triacetic acid

The absorption spectra of both the violet and the blue forms of the N-(2-hydroxyethyl)ethylenediamine-N,N` ,N`-triacetic acid complex with chromium ion(CrTOH) and the dependence of these forms on the hydrogen ion concentration are reported. The reaction is first order with respect to chromium and inversely proportional to the hydrogen ion concentration. The activation enthalpy and activation entropy for the reaction have been calculated from the data on temperature dependence.

THE DECOMPOSITION KINETICS OF HYDROGEN PEROXIDE CATALYSED BY ETHYLENEDIAMINE-TETRAKIS(METHYLENEPHOSPHONATO)IRON(III)

Abstract The catalysed decomposition of hydrogen peroxide in the presence of ethylenediaminetetrakis(methylene-phosphonato)iron(III) was studied at various temperatures. The observed rate law is given by -d[H2O2]/dt = k1K/([H+] + K) [Fe(CENTMP)]. where K is the acid-dissociation constant of the aquo complex. A mechanism involving the aquation of the ternary hydroxo complex as the rate-determining step is suggested. The values of k1 and pK were determined at 35°C to be 0.62 × 10−2 and 10.09 repectively. Activation enthalpy and activation entropy for the aquation reaction were determined from Arrhenius plots and found to be ΔH‡ = 74.57 kJ mol−1 and ΔS‡ = 202 J K−1 mol−1. The results are compared with previously reported data on catalysis by the structurally related [Fe(EDTA)] species.

Kinetics and mechanism of the oxidation of nitrilotris(methylenephosphonato)chromium(III) by periodate

The kinetics of oxidation of nitrilotris(methylenephosphonato)chromium(III), CrIIINTMP, by periodate to yield CrVI have been studied spectrophotometrically over the 5.80–6.85 pH range at 22–33 °C. The reaction rate, which is first-order with respect to [CrIIINTMP] and [IO−4] and inversely dependent on [H+], obeys the rate law:-d[CrIIINTMP/dt=kKKh[IO-4] [CrIII]T/Kh+ [H+] +KKh[IO-4] The values of the intramolecular electron transfer, k, and the formation constant of the intermediate complex, K, were determined at various temperatures. The hydrolysis constant for CrIIINTMP, Kh, was determined spectrophotometrically and is in agreement with the value estimated from the kinetic data. The activation parameters were calculated from the temperature dependence of the specific rate constants. A mechanism is proposed in which the hydroxo complex, [CrHNTMP(OH)]3−, is the reactive species. The results support a mechanism where intramolecular electron transfer is the rate-determining step.

Kinetic study of the hydrogen peroxide reaction with the 1-hydroxyethylidene-1,1-diphosphonatoiron(III) complex

Abstract The rate of reaction of [Fe(HEDP)] with H2O2 in 0.10 M KNO3 solution was investigated at various temperatures. The observed rate law is given by the expression where k1 and k2 are the rate constants in the presence of [FeHL−] and [FeL2-] complex species, respectively, and K1 is the ionisation constant of [FeHL−]. The rate constants k1 and k2 were determined at 30°C to be 0.024 ± 0.002 and 0.189 ± 0.005 M−1s−1, respectively. The corresponding activation enthalpies (ΔH1 = 73.8 ± 10.5 and ΔH2 = 58.9 ± 0.8 kJ mol−1) and activation entropies (ΔS1 = −33 ± 33 and ΔS2 = −64 ± 2 J K−1 mol−1) were calculated from a least-squares fit to Arrhenius plots. The results are compared with previously reported data for other complexes.

Kinetics and mechanism of complex formation between chromium(III) and ethylenediamine-N,N′-diacetic acid

Abstract The kinetics of complex formation between chromium(III) ion and ethylenediamine- N , N′ -diacetic acid (EDDA) have been studied spectrophotometrically. The reaction proceeds by a path which is first-order with respect to chromium(III) and EDDA. The rate increases with the increase of pH and temperature. The results are best accounted for by outer-sphere complexation equilibrium involving [Cr(H 2 O) 5 OH] 2+ and EDDA. A rate equation is established which involves k and K OS (the interchange rate constant and outer-sphere complexation). The hydrolysis constant of the complex was determined and the activation parameters for the reaction have been calculated.

DECOMPOSITION KINETICS OF SODIUM PERBORATE CATALYZED BY ETHYLENEDIAMINETETRA-ACETATOMANGANESE(III)

Abstract The decomposition of sodium perborate in the presence of ethylenediaminetetraacetato-manganese(III) was studied in aqueous solution over the pH range 8.3–10.8 at 50–65°C. The reaction was shown to exhibit second-order kinetics, first-order in each of the reactants. The rate law is where Y denotes the form of the ligand. Values of k1 and k2 were determined at various temperatures. Corresponding activation parameters are ΔH1* = 36.7 kJ mol−1, ΔS1* = 182 JK−1 mol−1, ΔH2* = 32.8 kJ mol−1, ΔS2* = 194 J K−1 mol−1. Comparison is made with hydrogen peroxide decomposition by manganese complexes.

Kinetics and mechanism of the catalysed decomposition of sodium perborate by the ethylenediaminetetraacetatoiron(III) complex

The rate of reaction of [FeIIIY] (Y = EDTA anion) with NaBO3 was studied in aqueous 0.1 M NaNO3 at various temperatures. The observed rate constant,kobs = kKKK1[FeEDTA(H2O)-]/{[H+] + KK1[FeEDTA(H2O)-]}applies over the pH range studied. The monohydroxy complex, [FeEDTA (OH)]2− is the catalyst which reacts with the peroxy ion to produce a violet intermediate complex. The composition of perborate confirms to Michaelis–Menten kinetics, the rate-determining step involving breakdown of the intermediate complex. The activation enthalpy and activation entropy were calculated.

Kinetics of interaction and complexation of the palladium(II) ion with new para-substituted-phenylazo-R-acid chromophores

The kinetics of complex formation between the PdII ion and the new phenylazo-R-acid chromophore [HL] and it is para-substituted derivatives {X = H, Cl, CO2H, and SO2NH2}, [p-X-HL], have been studied at various temperatures. The observed rate constant is given by:kobs=/k1 K1/K[H+]+k2 K2}[p-X-HL]A mechanism involving initial formation of a 1:1 complex between the PdII ion and [p-X-HL], followed by aquation as the rate-determining step, is proposed. The activation enthalpy and entropy for the aquation were determined. The rate of formation is greatly effected by electron-withdrawing substitutents in the para position of the phenylazo-R-acid ligand. The solid complex of p-SO2NH2-phenylazo-R-acid has been prepared and characterized by elemental analysis, and FT-IR spectra. The p-substituted-phenylazo-R-acid ligands are very active against C. albicans fungi, so that these compounds can be used for the chemotherapy of candidiasis and other fungal skin diseases.

Kinetic study of hydrogen peroxide reaction with hydroxonitrilotri(methylenephosphonato)iron(III) complex

SummaryThe decomposition of hydrogen peroxide in the presence of hydroxonitrilotri(methylenephosphonato)iron(III), [Fe(NTMP)(OH)4−], was studied in nitrate media (μ=0.10–0.26 M) over the 0.2–0.5 mM concentration range for the iron complex and the temperature range 26–40°C. The rate law;