Kinetics of oxidation of iron(II) by vanadium(V) under the conditions where VO2+ and decavanadate coexist

The kinetics of the reaction between iron(II) and vanadium(V) have been investigated in the pH range 2.6–4.2 where decavanadates and VO₂⁺ coexist in equilibrium. Under these conditions, the observed kinetic pattern is radically different from the one reported for the reaction in strong acid medium. In the pH range employed, the reaction rate is not appreciably altered by variation in the stoichiometric vanadium(V) concentration due to the operation of the equilibrium between the reactive species, VO₂⁺, and the unreactive species, decavanadates. The reaction, however, obeys first‐order kinetics with respect to Fe(II). In the presence of salicylic acid, which imparts considerable reactivity to iron(II) by reducing the reduction potential of iron(III)/iron(II) couple by forming a stronger complex with iron(III) than iron(II), the kinetic results provide evidence for the participation of decavanadates in the electron transfer. The mechanism under both conditions is discussed. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 535–541, 2000     

Effect of specific adsorbed anions on the electrode kinetics of the V(III)/V(II) and Eu(III)/Eu(II) couples

Electrochemical kinetic parameters of the V(III)/V(II) and Eu(III)/Eu(II) couples in sulfuric, perchloric, hydrochloric, and hydrobromic acids were measured by potentiostatic and double pulse galvanostatic methods. The ₂ potentials in these solutions were calculated from electrocapillary measurements and the effect of the ₂ potentials on the electrode kinetics was discussed. The kinetic data after the Frumkin correction was applied show a very good agreement in H₂SO₄, HClO₄, and HCl solutions, if we assume that the non-complexed ion, which is partially supplied by the dissociation of complex ions, participates in the electrode reaction. The corrected rate constants in the bromide solution were about ten times larger than those to be expected from the ₂ potentials in the case of the V(III)/V(II) couple and a small acceleration effect was observed for the Eu(III)/Eu(II) couple. The greater reaction rate in the bromide solution is explained by the bridging effect.     

Solvent effects on the electrode kinetics of simple redox couples: Investigations on the electrochemical behaviour of the V(III)/V(II) system in water + acetonitrile mixtures

Parallel optical and electrochemical studies on the V(III)/V(II) system in H₂O + acetonitrile (AN) + CF₃SO₃H mixtures have been performed. It was found, on the basis of the spectra of vanadium ions in the visible range, that V(III) was totally hydrated in mixtures up to x_AN ⋍ 0.6 while V(II) was specifically solvated by AN molecules, even at a molar fraction of acetonitrile in H₂O + AN mixtures as low as 0.02. In agreement with this, the formal potentials of the V(III)/V(II) system expressed versus the ferrocene electrode move to less negative potentials with an increase in AN concentration.Straightforward correlations of the electrode kinetics of the V(III)/V(II) system at a mercury electrode in H₂O + AN mixtures with both the electrode surface coverage by AN molecules and the resolvation of vanadium ions in the bulk solution were found.     

Confirmation of vanadium complex formation using electrospray mass spectrometry and determination of vanadium speciation by sample stacking capillary electrophoresis

Capillary zone electrophoresis (CZE) with UV detection was used to determine vanadium species. Nitrilotriacetic acid (NTA), hydroxyethylethylenediaminetriacetic acid (HEDTA), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), ethylene glycol-bis(2-aminoethylether)-tetraacetic acid (EGTA) and 2,6-pyridinedicarboxylic acid (PDCA) were investigated to determine whether these ligands formed stable anionic complexes with vanadium. Of all the ligands studied HEDTA was the most suitable ligand because it gave the largest UV response with reasonable migration time. Electrospray mass spectrometry (ES-MS) was used to confirm the formation of [VO₂(HEDTA)]²⁻ and [VO(HEDTA)]¹⁻ in solution. An electrolyte containing 25 mM phosphate, 0.25 mM tetradecyltrimethylammonium bromide (TTAB) at pH 5.5 was optimum for the separation of these anionic vanadium complexes. Sample stacking techniques, including large-volume sample stacking (LVSS) and field-amplified sample injection (FASI), were tested to improve the sensitivity. Best sensitivity was obtained using FASI, with detection limits of 0.001 μM, equivalent to 0.4 μg L⁻¹, for [VO₂(HEDTA)]²⁻ and 0.01 μM, equivalent to 3.4 μg L⁻¹ for [VO(HEDTA)]¹⁻. The utility of the method for the speciation of V(IV) and V(V) was demonstrated using ground water samples.     

The spectrophotometric determination of vanadium after extraction as vanadium(III) ferronate by tribenzylamine

Vanadium(III) obtained by dithionite reduction of vanadium(V) can be extracted as its ferron complex with tribenzylamine in chloroform from 0.05 M sulphuric acid. Vanadium (0–5 μg ml^-1) is determined spectrophotometrically at 430 nm with a sensitivity of 0.0028 μg V cm^-2. Al(III), Co(II), Ni(II), Fe(II, III), Hg(II), Si(IV), Be(II), Mg(II), Ca(II), Sr(II), Ba(II), Cr(VI, III), W(VI), Zn(II), U(VI), Mn(II). Pb(II), Cu(II), Cd(II) and Th(IV) do not interfere; only Mo(VI), Ti(IV), Zr(IV). Bi(V) and Sn(II) interfere. A single determination takes only 7 min. The extracted complex is V^III (R-3H.TBA)₃ where R = C₉H₄O₄NSI. The method is satisfactory for the determination of vanadium in steels, alum and other samples without preliminary separations.     

Electroprecipitation of Ag(II)/Ag(III) tetraphenylsulfonate porphyrin and electrocatalytic behavior of the films

The electrochemical precipitation on glassy carbon and gold electrodes of Ag(II) tetraphenylsulfonate porphyrin (Ag(II)TPPS) from aqueous HClO₄ solutions, is reported. Electrochemical quartz crystal microbalance (EQCM) results indicate the possible formation of an Ag(II)–Ag(III) porphyrin dimer species. This species is oxidized and reduced in two consecutive steps: oxidation at +0.31 and +0.36 V (vs. SCE) and reduction at +0.11 and +0.07 V. The films show catalytic behavior toward O₂ reduction in 10⁻² M HClO₄ at relatively low potentials (E<−0.1 V) but catalyze NO reduction at relatively high-reduction potentials (E<0.4 V). The electrochemical results seem to indicate that the catalytic cycle in the case of NO involves formation of Ag(II)TPPS–Ag(II)TPPS(NO)⁺ and its electroreduction to regenerate Ag(II)TPPS–Ag(III)TPPS and NO-reduction products.     

Radiation-induced reduction of vanadium(IV) to vanadium(II) in aqueous picolinate-formate solutions

Quantitative reduction of V(IV)(pic) to V(III)(pic)_n and then to V(II)(pic)_n(1 ⩽ n ⩽3) occurs when N₂O-saturated formate solutions (pH 4.2–6.3) containing V(IV) and picolinic acid (2-carboxypyridine) are irradiated. Pulse radiolysis measurements show that CO^-₂ reacts with picolinate only when the N-atom is protonated (k = 2.7 × 10⁸ dm³ mol^-1 s^-1). Reduction of V(IV)(pic) and V(III)(pic)_n is effected by the electron adduct of the protonated picolinate (picH)^. with rate constants at pH 4.2 of (3.5 ± 0.2) × 10⁷ dm³ mol^-1 s^-1 for V(IV)(pic) and (6.9 ± 0.4) × 10⁸ dm³ mol^-1 s^-1 for V(III)(pic)_n. No reduction of V(II)(pic)_n is observed.     

Electrochemical Behavior of Iron(III)/Iron(II) Couple in Dimethylformamide

The electrochemical behavior of the iron(III)/iron(II) couple was investigated in both complexing (Cl⁻) and noncomplexing (ClO⁻₄) media in dimethylformamide (DMF), and the results were compared with the results obtained in aqueous solutions. The diffusion coefficients for iron(III) and iron(II) in DMF are larger in complexing medium than in noncomplexing medium, contrary to the results obtained in aqueous solutions. The heterogeneous electron transfer rate constants for the iron(III)/iron(II) couple obtained in DMF were found to be smaller in DMF solution as a result of the specific adsorption of DMF. The formal potential of the Fe(III)/Fe(II) couple in DMF is about 0.2 V less positive in noncomplexing medium as a result of the greater stabilization of iron(III) by the strongly cation-solvating DMF. The formal potential of the same couple in complexing medium (Cl⁻) was found to be 0.5 V less positive due to a combination of solvation and complexation effects. Cyclic voltammetric investigations show a quasi-reversible electron transfer without any coupled chemical reaction.     

Speciation of Cr(VI)/Cr(III) by electrothermal atomisation AAS after electrodeposition on a L'vov platform

Summary The pyrolysed graphite L'vov platform of a tube furnace is considered as an electrode for the electrodeposition and speciation of chromium by electrothermal atomisation atomic absorption spectrometry (ETA-AAS). Firstly, a preliminary study of the Cr(VI)/Cr(III) voltammetric behavior at pH 4.70 on a glassy-carbon electrode is carried out. Secondly, the L'vov platform is used as a cathodic macro-electrode for the selective preconcentration of Cr(VI)/Cr(III) on a mercury film. Speciation of Cr(VI)/Cr(III) is carried out on the basis of the electrolysis potential (E_e): at pH 4.70 and E_e=–0.30 V, only Cr(VI) is reduced to Cr(III) and accumulated as Cr(OH)₃ by adsorption on a mercury film; at E_e=–1.80 V both Cr(VI) and Cr(III) are accumulated forming an amalgam with added mercury(II) ions. Once the film has been formed, the platform is transferred to a graphite tube to atomise the element. The reliability of the method was tested for the speciation of chromium in natural waters and it proves to be highly sensitive thanks to the electroanalytical step. In all samples, the Cr(VI) concentration was less than the detection limit (0.15 ng ml^–1), and the concentration of Cr(III) agrees with those of total chromium. The analytical recovery of Cr(VI) added to water samples [3.50 ng ml^–1 of Cr(VI)] was 105±6.2%.     

Real and Hypothetical Intermediate-Valence Ag(II)/Ag(III) and Ag(II)/Ag(I) Fluoride Systems as Potential Superconductors

With the aim of gauging their potential as conducting or superconducting materials, we examine the crystal structures and magnetic properties of the roughly one hundred binary, ternary, and quaternary Ag(II) and Ag(III) fluorides in the solid state reported up to date. The Ag(II) cation appears in these species usually in a distorted octahedral environment, either in an [AgF](+) infinite chain or as [AgF(2)] sheets. Sometimes one finds discrete square-planar [AgF(4)](2-) ions. The Ag(III) cation occurs usually in the form of isolated square-planar [AgF(4)](-) ions. Systems containing Ag(III) (d(8)) centers are typically diamagnetic. On the other hand, the rich spectrum of Ag(II) (d(9)) environments in binary and ternary fluorides leads to most diverse magnetic properties, ranging from paramagnetism, through temperature-independent paramagnetism (characteristic for half-filled band and metallic behavior) and antiferromagnetism, to weak ferromagnetism. Ag(II) and Ag(III) have the same d-electron count as Cu(II) (d(9)) and Cu(III) (d(8)), respectively. F(-) and O(2-) ions are isoelectronic, closed-shell (s(2)p(6)) species; both are weak-field ligands. Led by these similarities, and by some experimental evidence, we examine analogies between the superconducting cuprates (Cu(II)/Cu(III)-O(2-) and Cu(II)/Cu(I)-O(2-) systems) and the formally mixed-valence Ag(II)/Ag(III)-F(-) and Ag(II)/Ag(I)-F(-) phases. For this purpose we perform electronic-structure computations for a number of structurally characterized binary and ternary Ag(I), Ag(II), and Ag(III) fluorides and compare the results with similar calculations for oxocuprate superconductors. Electronic levels in the vicinity of the Fermi level (x(2)-y(2) or z(2)) have usually strongly mixed Ag(d)/F(p) character and are Ag-F antibonding, thus providing the potential of efficient vibronic coupling (typical for d(9) systems with substantially covalent bonds). According to our computations this is the result not only of a coincidence in orbital energies; surprisingly the Ag-F bonding is substantially covalent in Ag(II) and Ag(III) fluorides. The electron density of state at the Fermi level (DOS(F)) for silver fluoride materials and frequencies of the metal-ligand stretching modes have values close to those for copper oxides. The above features suggest that properly hole- or electron-doped Ag(II) fluorides might be good BCS-type superconductors. We analyze a comproportionation/disproportionation equilibrium in the hole-doped Ag(II) fluorides, and the possible appearance of holes in the F(p) band. It seems that there is a chance of generating an Ag(III)-F(-)/Ag(II)-F(0) "ionic/covalent" curve crossing in the hole-doped Ag(II)-F(-) fluorides, significantly increasing vibronic coupling.     

Characterization of novel vanadium(III)/acetate clusters formed in aqueous solution

We report the first structures of simple acetate complexes of vanadium(III) formed in aqueous solution. Paramagnetic (1)H NMR spectroscopy titration experiments indicate the formation of two major V(III)/acetate complexes in acidic aqueous solution for acetate/V(III) < or =4, pD 3.50. A novel tetranuclear cluster and a trinuclear cluster have been characterized by X-ray diffraction studies. Mass spectrometry measurements show these clusters retain their integrity in solution.     

Polarographic studies on bipyridine complexes: III. Polarography of tris(2,2′-bipyridine) complexes of chromium(I), chromium(0), vanadium(0), titanium(0) and molybdenum(0)

Polarograms and cyclic voltammograms for tris(2,2′-bipyridine) complexes of V(0), Cr(0), Cr(I), Ti(0) and Mo(0) in N,N-dimethylformamide are reported. The reversible half-wave potentials for the following redox systems in lower oxidation states are determined: Cr(?I)/Cr(?II), Cr(?II)/Cr(?III), V(I)/V(0), V(0)/V(?I), V(?I)/V(?II), V(?II)/V(?III), Ti(0)/Ti(?I), Ti(?I)/Ti(?II), Mo(?I)/Mo(?II) and Mo(?II)/Mo-(?III). On the basis of the half-wave-potential shift caused by the methyl substitution of ligands, it is concluded that each excess electron of the reductant species of the redox systems, V(bipy)₃^?/V(bipy)₃^2?, Cr(bipy)₃/Cr(bipy)₃^?, Cr(bipy)₃^?/Cr(bipy)₃^2? and Cr(bipy)₃^2?/Cr(bipy)₃^3? (bipy=2,2′-bipyridine), occupies a ligand π^*-orbital and that of the V(bipy)₃²⁺/V(bipy)₃⁺ and V(bipy)₃⁺/V(bipy)₃ systems a metal t_2g-orbital. The apparent π-character of the excess electron of the redox systems Cr(bipy)₃⁺/Cr(bipy)₃ and V(bipy)₃/V(bipy)₃^? is discussed. It is pointed out that the relative electron affinities of trisbipyridine complexes can be determined from the half-wave potential data. The lowest π^*-orbitals of V(bipy)₃^?, Cr(bipy)₃ and Fe(bidy)₃²⁺ become higher in this order. This suggests that the electrostatic interaction between a π^*-electron and the residual charge on the central metal ion predominantly accounts for the observed π^*-level shift.     

Electrode processes of the V(III)/V(II) and Eu(III)/Eu(II) systems in mixed water + acetone solvents

Rate constants of the electrode reaction V(III)+e⁻ → V(II) in water+acetone mixtures were determined. In the regions of irreversible and quasi-reversible behaviour we used polarographic and square-wave polarographic measurements, respectively. The values of the constant go through a minimum with increasing concentration of acetone. Following the published data for the Eu(III)/Eu(II) system (H. Elzanowska, Ph. D. Thesis, Warsaw, 1957), this behaviour was explained by the simultaneous reduction of differently solvated ions in the solution where, depending on the degree of electrode coverage, a partial resolvation at the electrode surface can occur. The calculated dependence of the rate constant on the solvent composition is in accord with experimental values.     

Analysis of a quasi-reversible two-electron cyclic voltammetric wave for an organometallic ir(III)/ir(I) couple at platinum and mercury electrodes

^*CpIr(η-C₆Me₆)^2+/0 (^*Cpη⁵-C₅Me₅) displays Nernstian two-electron voltammetry at a Hg electrode, but quasi-reversible charge transfer kinetics at solid electrodes. Cyclic voltammetry (CV) peak shapes and separations change drastically from one solvent to another at Pt, ΔE_p values being as small as 170 mV in acetone and as large as 350 mV in CH₃CN (v = 0.03 V/s). These variations arise from changes in the relative E° values of the one-electron Ir(III)/Ir(II) and Ir(II)/Ir(I) couples, and from changes in charge-transfer rates. It is concluded that the Ir(II)/Ir(I) couple has a significantly lower charge-transfer rate than the Ir(III)/Ir(II) couple at platinum electrodes. The sensitivity of the CV curves to the relative E° values allows the approximate determination of the individual E° values for each one-electron process. In contrast, Nernstian conditions allow only the average of the two one-electron E° potentials to be determined. Solvents with higher solvating power are shown to facilitate the thermodynamics of the two-electron transfer process by moving E°₂ positive with respect to E°₁. Possible reasons for the abnormally slow charge transfer rates at Pt electrodes are discussed.     

CO2 to CO Electroreduction,Electrocatalytic H2 Evolution,and Catalytic Degradation of Organic Dyes Using a Co(II) meso-Tetraarylporphyrin

The meso-tetrakis(4-(trifluoromethyl)phenyl)porphyrinato cobalt(II) complex [Co(TMFPP)] was synthesised in 93% yield. The compound was studied by ¹H NMR, UV-visible absorption, and photoluminescence spectroscopy. The optical band gap Eg was calculated to 2.15 eV using the Tauc plot method and a semiconducting character is suggested. Cyclic voltammetry showed two fully reversible reduction waves at E_1/2 = −0.91 V and E_1/2 = −2.05 V vs. SCE and reversible oxidations at 0.30 V and 0.98 V representing both metal-centred (Co(0)/Co(I)/Co(II)/Co(III)) and porphyrin-centred (Por²⁻/Por⁻) processes. [Co(TMFPP)] is a very active catalyst for the electrochemical formation of H₂ from DMF/acetic acid, with a Faradaic Efficiency (FE) of 85%, and also catalysed the reduction of CO₂ to CO with a FE of 90%. Moreover, the two triarylmethane dyes crystal violet and malachite green were decomposed using H₂O₂ and [Co(TMFPP)] as catalyst with an efficiency of more than 85% in one batch.     

Stoichiometry of superconducting YBa2Cu3Oy. Determination of Cu(III)/Cu(II) ratio and oxygen content

The electrical properties of the high T_c superconductor YBa₂Cu₃O_y depend on its oxygen content. The oxygen content is indirectly determined by iodimetric measurement of the oxidation state of copper. A combination of two titrations, with and without addition of KI, prior to dissolving the sample permits the determination of the two species Cu(II) and Cu(III). A simplified automatic titration with potentiometric detection of the end-point is described. The method is suitable for rapid and reliable determinations of the Cu(III)/Cu(II) ratio and total copper content and for controlling the stoichiometry of the compound.Samples of the superconductor were analysed and the stoichiometric coefficient y for oxygen was determined with excellent results. Typically, y=6.811 ± 0.0063 (s.d.) (n=5). A comparison of the total copper content (found by direct analytical determination) with the copper concentration calculated from the stoichiometric formula gives an evaluation of possible deterioration of the sample.     

On Mn(Hg)/Mn(II) equilibria and reactions in aqueous solution

The title subject has been studied through galvanostatic single-pulse and chronopotentiometric measurements on the Mn(Hg)/Mn(II) electrode and equilibrium measurements on the same and the Ag/AgCl electrode, all in x MMnCl₂+(0.5?x)M MgCl₂ solutions of pH 4.3–4.9 at 25°C. The Mn(Hg)/Mn(II) reactions are found to occur in two consecutive steps, an unsymmetric (α_c near 0.8) ion-transfer step Mn(Hg)/Mn(I) and an essentially symmetric (α_c near 0.5) electron-transfer step Mn(I)/Mn(II). Besides charge transfer, no sluggishness other than diffusion is observed, but the dispersed precipitate Mn₂Hg₅ of saturated amalgam serves as an ageing-dependent source of anodic reactant Mn(Hg). Quantitative kinetic and thermodynamic data are presented and discussed. Comparisons are made to corresponding reactions for the succeeding elements iron, cobalt, nickel, copper, and zinc.     

Flow injection analysis for oxidation state speciation of vanadium(IV) and vanadium(V) in natural water.

A sensitive and simultaneous spectrophotometric flow injection method for the determination of vanadium(IV) and vanadium(V) is proposed. The method is based on the effect of ligands such as 2,4,6-tris(2-pyridyl)-1,3,5-triazine (TPTZ) and diphosphate on the conditional redox potential of iron(III)/iron(II) system. A four-channel flow system is assembled. In this flow system, diluted hydrochloric acid (1.0 x 10(-2) mol dm(-3)) as a carrier for standard/sample, acetate buffer (pH 5.5) as a carrier for diphosphate solution, an equimolar mixed solution of iron(III) and iron(II) and a TPTZ solution are delivered, so that the baseline absorbance can be established by forming a constant amount of iron(II)-TPTZ complex (lambda(max) = 593 nm). Vanadium(IV) and/or vanadium(V) (400 microL) and diphosphate (200 microL) solutions are simultaneously introduced into the flow system; in this system the diphosphate solution passes through a delay coil. The potential of the iron(III)/iron(II) system increases in the presence of TPTZ, and therefore vanadium(IV) is easily oxidized by iron(III) to vanadium(V) to produce an iron(II)-TPTZ complex (a positive peak for vanadium(IV) appears). On the other hand, the potential of the redox system decreases in the presence of diphosphate, so that vanadium(V) can be easily reduced by iron(II) to vanadium(IV). In this case, the amount of iron(II) decreases according to the amount of vanadium(V). As a result, the produced iron(II)-TPTZ complex decreases (a negative peak for vanadium(V) appears). In this manner, two peaks for vanadium(IV) and vanadium(V) can be alternately obtained. The limits of detection (S/N = 3) are 1.98 x 10(-7) and 2.97 x 10(-7) mol dm(-3) for vanadium(IV) and vanadium(V), respectively. The method is applied to the simultaneous determination of vanadium(IV) and vanadium(V) in commercial bottled mineral water samples.     

Ethylenediaminetetraacetato-monoperoxo complexes of vanadium(V)

Summary The monoperoxo complexes, M₂[VO(HEDTA)(O₂)]· 4H₂O, where M is K⁺ or NH ₄ ⁺ and H₄EDTA is ethylene-diaminetetraacetic acid, were prepared and characterized by Raman and i.r. spectra in the solid state and in aqueous solution. The single crystal X-ray study revealed a pentagonal bipyramidal anion structure with a tetradentate HEDTA(3—) ligand. The decomposition of complexes in aqueous solution to blue vanadium(IV) complexes as end products proceeds via a nonperoxo complex of vanadium.     

A Comparative Study of Functionalized High‐Purity Carbon Nanotubes towards the V(IV)/V(V) Redox Reaction Using Cyclic Voltammetry and Scanning Electrochemical Microscopy

Functionalized high purity carbon nanotubes (CNTs) with various amounts of oxygen containing surface groups were investigated towards the relevant redox reactions of the all‐vanadium redox flow battery. The quinone/hydroquinone redox peaks between 0.0 and 0.7 V vs. Ag|AgCl|KCl_sat. were used to quantifying the degree of functionalization and correlated to XPS results. Cyclic voltammetry in vanadyl sulfate‐containing 3 M H₂SO₄ as a common supporting electrolyte showed no influence of the amount of surface groups on the V(IV)/V(V) redox system. In contrast, the reactions occurring at the negative electrode (V(II)/V(III) and V(III)/V(IV)) are strongly affected by oxygen surface groups. However, under modified experimental conditions, SECM experiments detecting the consumption of VO²⁺ molecules by CNT thin films in pH=2 solution show improved onset potentials with increased surface oxygen content up to ∼ 3 at%. Further increase in surface oxygen up to 8 at% led to minor improvement. These dissimilar results under different experimental conditions are rationalized by suggesting that oxygen functional groups do not form the active site for the V(IV)/V(V) reaction but wetting of the catalyst layer is of high importance.