The electrode reactions of Fe(CN)63− and Fe(CN)64− ions studied at temperatures below the melting point of stoichiometric electrolytes

The electrode reactions of Fe(CN)₆^3? and Fe(CN)₆^4? ions have been studied at temperatures below the melting point of stoichiometric electrolytes. Tetramethylammonium cation hydrates: (CH₃)₄NOH·nH₂O(n = 5, 7.5, 10) and (CH₃)₄NF·4H₂O were selected as electrolytes. The redox active ion with higher electric charge, Fe(CN)₆^4?, is more stable at temperatures below as well as above the melting point of these electrolytes. However, the temperature dependence of the redox potential of the Fe(CN)₆^3?/Fe(CN)₆^4? couple is more pronounced in these conditions. In the limited temperature range below the electrolyte melting point the kinetics of the reaction are controlled by the rate of reactant transport towards the electrode surface. The apparent diffusion coefficient of redox active ions does not change substantially at temperatures around the electrolyte melting point. The activation energy of reactant transport is twice as large in frozen than in liquid electrolyte. It has been concluded that the motion of the redox active ions is restricted to a limited volume—the intergrain space of the electrolyte. This conclusion is supported by results of experiments performed in a cell filled with chemically inert beads and liquid electrolyte.     

Microgravimetric monitoring of transport of cations during redox reactions of indium(III) hexacyanoferrate(III,II): Radiotracer evidence for the flux of anions in the film

We demonstrate here that, primarily, electrolyte cations but also, to some extent, anions are capable of penetrating indium hexacyanoferrate films during redox reactions. We find from electrochemical quartz crystal microbalance measurements that the electrolyte cation (K⁺) undergoes sorption and desorption during the system's reduction and oxidation, respectively. The formal potential, which has been determined from the system's well-defined voltammetric peaks recorded with the use of an ultramicroelectrode, decreases ～40 mV per decade of decreasing K⁺ concentration. The latter value is lower than the 60 mV change expected for the involvement of a cation in the reaction mechanism according to the ideal Nernstian dependence. We also demonstrate, using 35-S labelled sulfate, that anion penetrates the reduced film and its concentration markedly increases during oxidation. Careful examination of cyclic voltammetric responses of the system shows that, in addition to the well-defined peaks, capacitance-like currents appear during oxidation. During reduction anion is largely expelled from the film. This complex ionic penetration and transport in indium hexacynoferrate may be explained in terms of formation of two forms: ‘soluble’, KIn^III[Fe^II(CN)₆], and ‘insoluble’ (‘normal’), In^III₄[Fe^II(CN)₆]₃, during the electrochemical growth or potential cycling of the films. These forms would require cations and anions, respectively, to provide charge balance during reactions. Regardless of the actual mechanism, penetration of anions cannot be neglected completely in the discussion of charging of indium hexacyanoferrate.     

Electrocatalytic oxidation of styrene by a high valent ruthenium porphyrin cation radical

A sterically-hindered carbonylruthenium(II) porphyrin Ru^II(CO)(TMP) (where TMP=meso-tetramesitylporphyrinato dianion) has been synthesized. Chemical oxidation of Ru^II(CO)(TMP) by m-chloroperbenzoic acid (m-CPBA) gives the dioxoruthenium(VI) porphyrin (Ru^VI(O)₂TMP). Cyclic voltammograms show that Ru^VI(O)₂TMP is reversibly oxidized at E_1/2=+1.24 V in CH₂Cl₂. Thin-layer absorption spectra for oxidation of Ru^VI(O)₂TMP at +1.32 V indicates that the product is a porphyrin cation radical (Ru^VI(O)₂TMP⁺). Electrogenerated Ru^VI(O)₂TMP⁺ reacts selectively with styrene to give phenylacetaldehyde (96%) and benzaldehyde (4%). We report the first case of styrene oxidation by high valent ruthenium porphyrin under electrochemical conditions. An electrocatalytic oxidation reaction scheme is proposed.     

Simultaneous electrochemical and quartz crystal microbalance studies of non-conducting microcrystalline particles of trans-Cr(CO)2(dpe)2 and trans-[Cr(CO)2(dpe)2]+ (dpe = Ph2PCH2CH2PPh2) attached to gold electrodes

Simultaneous cyclic voltammetric double potential step and electrochemical quartz crystal microbalance (EQCM) experiments on water insoluble trans-Cr(CO)₂(dpe)₂ and trans-[Cr(CO)₂(dpe)₂]X (dpe = Ph₂PCH₂CH₂PPh₂, X = Cl^?, Br^? and I^?), attached as an array of microcrystals, have been employed to probe mechanistic aspects of the redox chemistry of the [trans-Cr(CO)₂(dpe)₂]^+/0 process at the electrode |solid| solvent (electrolyte) interface in a variety of aqueous electrolytes. EQCM experiments show that the oxidation of solid trans-Cr(CO)₂(dpe)₂ involves the slow incorporation of non-solvated anions from the electrolyte solution into the solid. Interestingly, on the reverse scan of cyclic voltammetric experiments, EQCM data reveal that some but not all the anions are rapidly expelled from the crystal lattice. Double potential step experiments with the neutral chromium compound confirm that the oxidation reaction is a relatively slow process. The conclusion reached from all experiments is that the reduction process predominantly expels the anions that are relatively close to the solid|solution interface. EQCM investigations of trans-[Cr(CO)₂(dpe)₂]X compounds in electrolytes containing a different anion to that in the compound show that the anion originally in the salt is rapidly replaced by the anion in the aqueous electrolyte at open circuit potential, presumably via a rapid ion exchange process. The anion from the electrolyte is then expelled and incorporated into the solid during the reduction and oxidation steps respectively.     

Electrochemical characteristics of a cobalt pentacyanonitrosylferrate film on a modified glassy carbon electrode and its catalytic effect on the electrooxidation of hydrazine

Electrochemically prepared thin films of cobalt pentacyanonitrosylferrate (CoPCNF) were used as surface modifiers for glassy carbon electrodes. The electrochemical behavior of a CoPCNF-modified glassy carbon electrode was studied by cyclic voltammetry; the modified electrode shows one pair of peaks with a surface-confined characteristic in 0.5 M KNO₃ as supporting electrolyte. The effect of different alkali metal cations in the supporting electrolyte on the behavior of the modified electrode was studied and the transfer coefficient (α) and charge transfer rate constant (k_s) for the electron transfer between the electrode and modifier layer were calculated. The experimental results show that the peak potential and peak current vary with different alkali metal cations, but anions such as Cl^?, NO₃^?, CH₃COO^?, H₂PO₄^?/HPO₄^2? and SO₄^2? at 0.5 M concentration have no effect on the peak potential and peak current. An extensive study showed that the response of the modified electrode is not affected within a pH range of 2–8. The CoPCNF films on glassy carbon electrodes show excellent electrocatalytic activity toward the oxidation of hydrazine in 0.5 M KNO₃. The kinetics of the catalytic reaction were investigated by using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and chronoamperometry. The average value of the rate constant for the catalytic reaction and the diffusion coefficient were evaluated by different approaches for hydrazine.     

Reply to the “Comments on the paper by M.-S. Zheng and S.-G. Sun entitled ‘In situ FTIR spectroscopic studies of CO adsorption on electrodes with nanometer-scale thin films of ruthenium in sulfuric acid solutions’ by C. Pecharromán,A. Cuesta and C. Gutiérrez”

An absorption spectral and electrochemical study for the zinc(II) complexes of meso-tetraphenylporphyrin dianion (TPP), meso-tetramesitylporphyrin dianion (TMP) and meso-tetra(2,6-dichlorophenyl)porphyrin dianion (TDCPP) in the presence of nitrogeneous bases in CH₂Cl₂ solution is reported. The Soret and Q bands of the zinc porphyrins were red-shifted in the presence of imidazole bases, however, the formation constants (K_f) with imidazole or 2-methylimidazole titration were found to be of a similar magnitude. The K_f and the formal electrode potentials of zinc porphyrins are in agreement with the electron-donating–withdrawing properties of the substituents in the phenyl groups. K_f for complexation of the imidazole to the oxidized and reduced zinc porphyrins was obtained from the electrochemical study. The one-electron oxidation of zinc porphyrins showed greater affinity toward imidazole ligation than the other oxidation states of zinc porphyrins. ZnTDCPP⁺ was found to have the greatest affinity with K_f up to 1.35×10⁸. Spectroelectrochemical methods were used to obtain absorption spectra of the complexation of zinc porphyrin cation radical with imidazole. The results showed a slight spectral difference between the complexed and uncomplexed zinc porphyrin cation radical.     

SNIFTIRS studies of the electrochemical oxidation of 1,3-dimethoxybenzene on platinum in acetonitrile/tetrabutilammonium electrolytes

The oxidation of 1,3-dimethoxybenzene on a platinum electrode, in different electrolytic media, has been investigated using in situ Fourier transform infrared spectroscopy (FTIR). These studies shed light on the mechanisms of electroxidative polymerisations of 1,3-dimethoxybenzene and the subsequent formation of a polymeric film on the surface of the platinum electrode. Vibrations from doping anions as well as the corresponding cations of the electrolyte salt were observed in the spectra obtained. Evidence of adsorption of monomer and the formation of nBu₄NBF₄–CH₃CN adduct are also reported.     

Synthesis and polymerization of 2- and 3-substituted thiophene derivatives linked by polyether bridges

New compounds consisting of 2- and 3-thienyl units linked by polyether bridges have been synthesized and their electrochemical polymerization was performed via constant potential electrolysis (CPE) in an electrolytic solution containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) dissolved in CH₃CN. 2-Thienyl monomers (I and II), but not 3-thienyl monomers (III and IV), were also polymerized via chemical oxidation, which yielded broken π-conjugated polymer products. The polymers were characterized using ¹H NMR and FT-IR spectroscopic techniques. It was found that both chemical and electrochemical oxidation of 2-thienyl monomers gave mainly poly(2,2′-bithiophenemethylene) due to elimination of polyether chains during the polymerization reaction. On the other hand, electrochemical oxidation of 3-thienyl monomers resulted in corresponding polymers without any cleavage of polyether bridges. Spectroelectrochemical (SPEL) properties of the products were investigated using UV–Vis spectroscopic techniques.     

Two-electon-transfer redox systems: Part 6. Two-electron oxidation of hexakis(benzylthio)benzene—a study by electrolysis and cyclic voltammetry

The anodic oxidation of the oligo-thioether hexakis(benzylthio)benzene in a dichloromethane+NBu₄PF₆ electrolyte is investigated by several complementary techniques: cyclic voltammetry, ESR spectroscopy, and macroscopic electrolysis (including fractional electrolysis in combination with the determination of open circuit potentials, i.e. ‘potentiometric titration’). While ESR spectroscopy proves the formation of a π-delocalized radical cation, the electrochemical techniques indicate further oxidation to a dicationic stage, which undergoes follow-up reactions. Analysis of fractional electrolysis results clearly shows strong potential compression for the two electron transfer steps with a difference of formal potentials ΔE°=23 mV. This value is succesfully used to simulate the cyclic voltammograms of the starting compound.     

Monomolecular films of a nickel(II) pentaazamacrocyclic complex for the electrocatalytic oxidation of hydrogen peroxide at gold electrodes

Self-assembled monolayers (SAMs) of a redox active nickel(II) pentaazamacrocyclic complex 1 and mixed monolayers of 1 with ethyl disulfide have been fabricated on a gold electrode, and their electrochemical behavior has been studied by cyclic voltammetry in aqueous solutions of Na₂SO₄ and NaNO₃. The results demonstrate that the redox behavior as well as the electrocatalytic activity towards the oxidation of H₂O₂ of the SAM and mixed monolayers of 1 largely depend on the electrolyte anions, i.e. SO₄^2? and NO₃^?: the formal potential in the SO₄^2? electrolyte is about 220 mV less positive than that in the NO₃^? electrolyte, and the SAM and mixed monolayers of 1 possess an efficient electrocatalysis for the oxidation of H₂O₂ in the NO₃^? electrolyte, but not in the SO₄^2? electrolyte. In addition, a unique cyclic voltammogram with a sharp peak of inverted ‘V’ shape has been observed in the cathodic scan for the electrocatalytic oxidation of H₂O₂, largely depending on the concentration of the NO₃^? electrolyte anion and the solution pH. These electrolyte anion-dependent redox behaviors have been discussed on the basis of different coordinating tendencies of SO₄^2? and NO₃^? to the nickel(III) centre of the complex and a possible reaction mechanism for the observed electrocatalytic reaction.     

Electrochemical behavior of iodide at a nickel pentacyanonitrosylferrate film modified aluminum electrode prepared by an electroless procedure

The surface of an aluminum disk electrode was modified by a thin film of nickel pentacyanonitrosylferrate and used for electrocatalytic oxidation of iodide. The cyclic voltammogram of the modified Al electrode showed surface redox behavior due to the [Ni^IIFe^III/II(CN)₅NO]^0/1? redox couple. The modifying layer shows excellent catalytic activity toward the oxidation of iodide. Different supporting electrolytes containing different alkali metal cations affected the apparent formal potential of the redox films and thus, changed the thermodynamic tendency and kinetics of the modifying film toward the catalytic oxidation of iodide. This was explained by including the concept of a surface coverage normalized-catalytic current. The kinetics of the catalytic reaction were investigated by cyclic voltammetry and rotating disk electrode voltammetry in a suitable supporting electrolyte. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The heterogeneous rate constant for the catalytic reaction, k, diffusion coefficient of iodide in solution, D, and transfer coefficient, α, were found to be 5.8 × 10² M^?1 s^?1, 1.3 × 10^?5 cm² s^?1 and 0.66, respectively. In addition the effect of electrode surface coverage on the dynamic range of a calibration curve was investigated. Under optimum conditions a linear calibration graph was obtained over an iodide concentration range of 2–100 mM.     

The high speed channel electrode applied to heterogeneous kinetics: the oxidation of 1,4-phenylenediamines and related species in acetonitrile

The application of the high-speed microband channel electrode to the study of the heterogeneous electron transfer kinetics of the oxidation of some N-substituted phenylenediamines is described. Experiments to investigate the standard electrochemical rate constant, k₀, of the oxidation of 1,4-phenylenediamine (PPD), N,N-dimethyl-1,4-phenylenediamine (DMPD), and N,N-diethyl-1,4-phenylene-diamine (DEPD) in acetonitrile solutions containing 0.10 M tetrabutylammonium perchlorate (TBAP) are reported for 2.5 and 5 μm platinum microband electrodes using a range of centre-line velocities from 12 to 25 m s^?1. The measured values of k₀ for PPD, DMPD, and DEPD are 0.84±0.16, 3.15±0.30 and 1.64±0.25 cm s^?1, respectively. The respective formal oxidation potentials are also found to be 0.287±0.002, 0.245±0.001, and 0.208±0.003 V (all measured vs. Ag). Experiments are also presented using “fast scan” cyclic voltammetry to obtain measurements of the heterogeneous rate constants for PPD, DMPD and DEPD to compare between the steady-state channel electrode and the ‘established’ transient methodologies. Scan rates in the range 10²–10⁴ V s^?1 were used to measure peak separations with the resulting k₀ values of 0.51±0.05, 1.89±0.10, and 1.28±0.20 cm s^?1, respectively. The use of steady-state voltammetry obviates the need for capacitative corrections, perhaps suggesting a greater reliability in the resulting data.     

On the facilitating effect of neutral macrocyclic ligands on the ion transfer across the interface between aqueous and organic solutions: Part III. Competitive facilitated ion-transfer

A theoretical equation of the reversible current–potential curves is derived for the competitive ion-transfer of two kinds of cations present in an aqueous (w) phase simultaneously facilitated by a macrocyclic ligand (L) present in an organic (o) phase. Especially, for the two limiting cases, i.e. case (A): c*_Mj (j=1, 2)?c*_L and case (B): c*_L?c*_Mj (where c*_Mj and c*_L denote the bulk concentrations of cation M_j in the w-phase and of L in the o-phase, respectively), simple explicit expressions are derived and a method analyzing the facilitated waves is presented. Furthermore, the main part of the theoretical predictions obtained is verified experimentally at 25 °C by using the ion-transfer-polarographic method with the electrolyte dropping electrode, for the following four combinations: (i) competitive cations: protonated alanine and H⁺, L: benzo-18-crown-6 ether; (ii) Na⁺ and Li⁺, benzo-15-crown-5 ether; (iii) K⁺ and Na⁺, dibenzo-18-crown-6 ether; and (iv) Na⁺ and Ba²⁺, dibenzo-24-crown-8 ether.     

Factors influencing the ion-exchange preconcentration and voltammetric behaviour of redox cations at polyestersulfonated ionomer coated electrodes in acetonitrile solutions

Glassy carbon electrodes coated with the poly(estersulfonate) Kodak AQ55 were used in acetonitrile solutions of various supporting electrolytes to study the ion-exchange voltammetric behaviour of some electroactive cations, namely the monocharged cation (ferrocenylmethyl)trimethylammonium (FA⁺) and the dications Ru(bpy)₃²⁺ (bpy=2,2′-bipyridine), methylviologen, butylviologen and heptylviologen. For all the electroactive cations, it was found that the success in their ion-exchange is determined by the size of the supporting electrolyte cation. In particular, lowering the steric hindrance of the supporting electrolyte cation results in a more significant ion-exchange competition with the electroactive cation. As far as different redox analytes are concerned, it was observed that the preconcentration efficiency increases with increasing charge and for electroactive species with the same ionic charge, parallels the order of decreasing steric hindrance. In the case of the viologens, this last factor seems to overcome the hydrophobic one, where the small (but less hydrophobic) methylviologen is preconcentrated more efficiently than the large (but more hydrophobic) heptylviologen.     

Solid polymer electrolyte membrane composite microelectrode investigations of fuel cell reactions. II: voltammetric study of methanol oxidation at the nanostructured platinum microelectrode|Nafion® membrane interface

Electrochemical oxidation of methanol occurring at the high surface area nanostructured platinum film|Nafion^® membrane interface has been studied using voltammetry at various temperatures. The effect of mobile anions on the electrode processes usually encountered in aqueous electrolyte is avoided in this composite microelectrode configuration. Both the methanol oxidation reaction (mor) and the carbon monoxide oxidation reaction have been studied at the platinum|Nafion^® membrane interface. The mor is found to be similar to that seen in aqueous acid electrolytes, although there are some interesting differences. Methanol adsorbs on platinum over the entire potential range studied. When the potential is greater than 0.3 V (RHE), a proton is stripped from the adsorbed methanol to form CH₂OH_ad. The direct electrooxidation of methanol to CO₂ commences at potentials more positive than 0.60 V vs. RHE where OH_ad is produced. During the negative-going scan an oxidation peak appears at potentials where the coverage of the surface hydroxyl is about 0.5. In contrast, during the positive-going scan, the peak oxidation current is seen at potentials where the hydroxyl coverage is smaller than 0.5, possibly owing to reaction intermediates blocking available methanol and hydroxyl adsorption sites. Importantly, we have found that surface sites which are not in contact with the proton-conductive Nafion^® membrane participate in electrochemical reactions. The surface area accessible to the hydrogen and CO adsorption/stripping processes is about twice that available to the methanol process, suggesting that some of the intermediates in the mor have low surface diffusion coefficients compared to H_ad and CO_ad. The presence of these surface diffusion processes have important implications regarding the performance of DMFC anodes and new anode electrocatalysts.     

Preparation,characterization, and electrocatalytic properties of copper hexacyanoferrate film and bilayer film modified electrodes

Copper(II)hexacyanoferrate films have been prepared from various electrolyte aqueous solutions using consecutive cyclic voltammetry. The cyclic voltammograms recorded the direct deposition of copper(II)hexacyanoferrate films from the mixing of Cu²⁺ and Fe(CN)₆^3? ions from solutions of ten cations: Li⁺, Na⁺, K⁺, Rb⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, H⁺ and Al³⁺. An electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth of the copper(II)hexacyanoferrate films. The copper(II)hexacyanoferrate film showed a single redox couple that exhibited a cation effect (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) and an anion effect (F^?, Cl^? and Br^?) in the cyclic voltammograms and formal potential of the redox couple. The electrochemical and EQCM properties of the film indicate that the redox process was confined to the immobilized copper(II)hexacyanoferrate, and the interaction between the copper(II)hexacyanoferrate film with K⁺ (monovalent cation) and Ca²⁺ (divalent cation). The electrocatalytic oxidation properties of NADH, NH₂OH, N₂H₄, SO₃^2? and S₂O₃^2? were also determined. The electrocatalytic reduction properties of SO₅^2? and S₂O₈^2? by monolayered iron, nickel, and cobalt hexacyanoferrate films, and by bilayered metal–copper hexacyanoferrate films were determined. Two-layered modified electrodes and hybrid films composed of a copper(II)hexacyanoferrate film with iron(II)hexacyanoferrate, cobalt(II)hexacyanoferrate, or nickel(II)hexacyanoferrate film were prepared, and these films caused the electrocatalytic reduction of SO₅^2? and S₂O₈^2?.     

Speciation of a water-soluble chromium porphyrin by spectral and electrochemical methods

A water-soluble chromium porphyrin, [Cr^IIITF₄TMAP]⁵⁺ (chromium(III) meso-tetrakis(2,3,5,6-tetrafluoro-N,N,N-trimethyl-4-anilinium)yl) porphyrin) was synthesized and the spectral and electrochemical properties were examined. In aqueous solution, [Cr^IIITF₄TMAP]⁵⁺ exhibits two pK_as at 7.04 and 10.07. The cyclic voltammogram showed that a reversible reduction wave appeared at ?0.84 V in pH 5.0 buffer solution, which is assigned as the Cr^III/II redox couple. The oxidation formal potentials were obtained by spectroelectrochemistry. With suitable control of the oxidation potentials and solution conditions, the high valent oxo-Cr^IV and oxo-Cr^V porphyrins were obtained. The ligand trans to the oxygen atom is either H₂O or OH^?, depending on the pH of the solution. In this paper, oxo-Cr^VTF ₄TMAP is thus the first reported water-soluble Cr^V porphyrin that can be generated electrochemically and chemically. Electrogenerated oxo-Cr^V porphyrin species catalyzed the oxidation of cyclopent-2-ene-1-acetic acid to cyclopent-2-ene-4-one-1-acetic acid in the presence of dioxygen and returned to oxo-Cr^IV porphyrin, which is not reactive toward the substrate.     

XPS and electrochemical studies of ferrocene derivatives anchored on n- and p-Si(1 0 0) by Si–O or Si–C bonds

High-coverage functionalization of H-terminated n- and p-Si(1 0 0) with vinylferrocene (VFC) and ferrocenecarboxaldehyde (FCA) has been obtained by wet chemistry methods, via the formation of a covalent bond between silicon atoms and the carbon (VFC) or the oxygen (FCA) termination of the molecules. The resulting functionalized electrodes have been analyzed by XPS, before and after cyclic voltammetry and capacitance–voltage measurements in an electrochemical cell. The very high quality of the hybrid species Si–CH₂–CH₂–C₅H₄–Fe²⁺–C₅H₅, resulting from VFC, has been certified by the negligible presence of silicon oxide and ferrocenium ions, and by the correct carbon/iron atomic ratio, accounting for the molecular species. The hybrid produced from FCA, Si–O–CH₂–C₅H₄–Fe²⁺–C₅H₅, presents a higher amount of silica, carbon and ferrocenium, as a consequence of the higher temperature of the functionalization procedure and of the higher reactivity of FCA. The use of two closely similar redox molecules has allowed to compare the behaviour of the Si–C–Y vs. Si–O–Y bond (Y being the redox moiety), with respect to their electrochemical reactivity in an organic solution. Both hybrids behave similarly on n- and p-Si substrates, in terms of redox potentials, stability to more than 1000 voltammetric cycles, linearity of the current intensity with the scan rate. The vinyl derivative, however, showed a faster and more reversible electron transfer kinetics than the carboxaldehyde derivative and an enhanced stability to long-term electrochemical experiments. VFC/p-Si is the first reported ferrocene derivative anchored to monocrystalline silicon via a C–Si bond. It is also the only large-area hybrid capacitor, to date, to be frequency-independent up to several hundred Hz, having a redox capacitance of 10^?4 F cm^?2, the highest reported so far for a monolayer on a Si(1 0 0) face.     

Novel studies on the electrochemical oxidation of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) in acetonitrile at platinum electrodes

The electrochemical oxidation mechanism of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) is studied in a 0.1 M N(C₄H₉)₄ClO₄ + acetonitrile (ACN) reaction medium by cyclic (CV) and square wave voltammetries (SWV) as well as by controlled potential bulk electrolysis at platinum electrodes. The primary radical cation formed by the one electron oxidation of DPMB undergoes a deprotonation process, which is the rate-determining step, followed by a radical–radical coupling. On the other hand, an initial quasi-reversible monoelectronic charge transfer mechanism is inferred from cyclic and square wave voltammograms recorded at scan rates and frequencies higher than 0.4 V s^?1 and 40 Hz, respectively. Diffusion coefficients of DPMB at different temperatures were calculated from the quasi-reversible convoluted cyclic voltammograms. DigiSim® and COOL software were used to fit the quasi-reversible cyclic and square wave voltammetric responses, respectively. Formal potentials, formal rate constants and positive transfer coefficients at different temperatures were evaluated from the fitting of cyclic voltammograms. The experimental activation parameters were also determined. The effects of the analytical concentration of the reagent and the temperature, as well as the addition of trifluoracetic acid and a strong base such as lutidine on the electrochemical responses are discussed. A general reaction mechanism as well as probable structures for dimeric products are proposed.Besides, the presence of an acid–base equilibrium in DPMB solutions is also studied by employing UV–Vis spectroscopic measurements at different trifluoracetic acid concentrations. An apparent value of (1.5 ± 0.2) × 10³ M^?1 was estimated for the DPMB basic constant at 20.0 °C     

Voltammetric studies of the interaction of the lithium cation with reduced forms of the Dawson [S2Mo18O62]4− polyoxometalate anion

Multiple electron transfer steps observed in the reduction of [S₂Mo₁₈O₆₂]^4?, when LiClO₄ is used as electrolyte, are accounted for by assuming that Li⁺ acts as a moderately strong Lewis acid. For example, in the 95:5 CH₃CN+H₂O solvent mixture (0.1 M NBu₄ClO₄), the voltammetric behavior obtained on addition of Li⁺ can be simulated according a reaction scheme involving an extensive series of reversible potentials and equilibrium constants. Precipitation of highly reduced polyoxometalate salts occurring at the electrode surface complicates the voltammetry at very negative potentials when 0.1 M LiClO₄ is used as the electrolyte.