Journal of Electroanalytical Chemistry

Published by Elsevier
The adsorption of ethylene glycol has been studied from 0.1 M aqueous solutions of NaF, KCl, KBr, and KI using the method of the capillary electrometer. Drop time measurements have also been carried out in the case of NaF solutions.The results are considered from the point of view of possible effects of the presence of specifically adsorbed ions on the thermodynamic analysis carried out both on the surface pressure curves and directly on surface excess data. Particular aspects of the congruence of the adsorption isotherm, and of the model of the adsorption layer are discussed in detail. The extension of the error, which is introduced by an intrinsic failure of the electrocapillary method at anodic charges due to change in the contact angle of mercury, is analysed.
The electrochemical response of both thiourea and formamidine disulphide dissolved in aqueous 0.5 M sulphuric acid on polycrystalline platinum electrodes is investigated at 298 K, using voltammetry and rotating ring-disk electrode techniques. The electro-oxidation of thiourea on platinum involves two stages occurring in different potential windows. The first thiourea electro-oxidation stage for E<0.7 V (vs. SCE), yielding soluble formamidine disulphide as the main product, behaves as an electrochemical process under intermediate kinetics. Kinetic parameters for the electro-oxidation of thiourea are estimated. The voltammetric electroreduction of formamidine disulphide dihydrochloride to thiourea is inhibited gradually by the adsorption of byproducts from the electrochemical reactions. The second thiourea electro-oxidation stage for E>0.7 V involves the oxidation of thiourea and adsorbed residues competing with the oxide monolayer formation on platinum.
Adsorption of 1-butanol and 1-pentanol on dropping mercury and gallium electrodes from aqueous 0.5 M Na2SO4 at 32° C was determined on the basis of capacitive charge measurements carried out by a computerized chronocoulometric apparatus. Adsorption measurements on Ga were carried out both in neutral solutions and in acidic solutions of pH ≈ 3.5; in the latter medium, which avoids the formation of a submonolayer of Ga oxides induced by alcohol adsorption, the capacitive charge was corrected for the appreciable faradaic contribution due to hydrogen evolution by a special procedure. Adsorptivities of both alcohols are lower on Ga than on Hg, especially at less negative charge densities. However, even at charge densities as negative as −12 σ C cm−2, differences in the adsorptivity of the same alcohol on the two metals remain appreciable. Since at these negative charge densities both anionic specific adsorption and metal surface oxidation can be ruled out, the above differences in adsorptivity strongly suggest a certain coupling between the orientation of adsorbed water molecules and electron spillover, whose extent is expected to be greater on Ga than on Hg at the same negative charge density.
The kinetics of the underpotential deposition (upd) of H and the hydrogen evolution reaction (her) have been found to be extremely sensitive to the surface geometry of Pt single-crystal surfaces. The order of electrocatalytic activity at the low-index planes was found in our previous studies to be (100)<(111)<(110). The study of the kinetics of the her at Pt(hkl) electrodes in 0.5 M H2SO4 is found to be complicated by the influence of H2 diffusion away from the surface. However, increase of the pH decreases the kinetic facility of the hydrogen electrode reaction, i.e. abstraction of H to form H2 is more difficult from H2O than from H3O+; at such lower rates, diffusive effects can be totally eliminated by electrode rotation. In the present paper we show that in 0.5 M NaOH the order of reactivity is identical with that found in acidic media, viz. (100)<(111)<(110). The slower rates arising in alkaline media allow a larger potential range to be probed and, unlike the situation in acid, quantitative rate information for each of the Volmer, Heyrovsky and Tafel steps of the her was obtained by using electrochemical impedance spectroscopy (EIS) and steady-state dc methods. As well as rate information, the charge, q1, corresponding to maximum fractional coverage by the overpotentially deposited H species (opd H), can be obtained and will be discussed in terms of the results previously found in acid, related to surface geometry and coverage by upd H. The results of the present work will also be compared with those of other recent studies conducted in alkaline media.
Adsorption of thiourea on dropping mercury and gallium electrodes from aqueous 0.5 M Na2SO4 at 32°C was determined on the basis Of capacitive charge measurements carried out by a computerized chronocoulometric apparatus. Adsorption measurements on Ga were carried out both in a neutral solution and in an acidic solution of pH ≈ 4; in the latter medium, which avoids Ga surface oxidation produced by thiourea adsorption, the capacitive charge was corrected for the faradaic contribution due to hydrogen evolution by a special procedure. Adsorptivity of thiourea is higher on Ga than on Hg even at charge densities as negative as —12 μC cm−2, where both anionic specific adsorption and metal surface oxidation can be ruled out. The above difference in adsorptivity is explained tentatively by a certain coupling between the orientation of adsorbed water molecules and electron spillover, whose extent is expected to be greater on Ga than on Hg at the same negative charge density.
The anodisation of copper electrodes in aqueous thiourea (TU) containing 0.5 M sulphuric acid is studied by electrochemical impedance spectroscopy (EIS) combined with rotating disk electrode and ring-disk electrode, SEM and EDAX techniques. For E<−0.35 V (vs. MSE), Nyquist plots show two time constants. The first one, which appears at high frequencies, involves the contribution of the double layer capacity and the charge transfer resistance related to the electro-oxidation of TU to formamidine disulphide (FDS). The second time constant, which is observed at low frequencies, is related to complex electrochemical and chemical processes following TU electro-oxidation to FDS. Depending on the applied potential, this time constant can be assigned to either an electroadsorption process from TU electro-oxidation products or an electrochemical process under diffusion through an anodic film. At high TU concentration, the low frequency time constant corresponds to the typical response of a pure capacitor due to the formation of a thick anodic film under a diffusion controlled process. In this case, for E≥−0.35 V, Nyquist plots exhibit at least three time constants and an inductive loop. The inductive loop is due to surface relaxation associated with copper pitting. The formation of different types of anodic films is confirmed by SEM observations. EIS results are consistent with the complex reaction pathway previously proposed for the anodisation of copper in aqueous TU-containing acid solutions.
〈001〉-oriented nanometer sized straight pores without branches were formed for the first time on (001) n-type InP surfaces by photoelectrochemical anodic reaction in HCl. The pore diameter, wall thickness and pore length could be changed in the ranges of 110–250 nm, 16–50 nm and 17–80 μm, respectively, by changing the anodizing overpotential and time. Based on the diffusion limited model for porous Si formation, the formation of 〈001〉-aligned straight pores without branches on n-type InP was explained in terms of the short lifetime of holes combined with the presence of a strong electric field at the pore tip. Contrary to the expectation of blue-shifted photoluminescence (PL) emission due to quantum confinement at pore walls, porous (001) InP samples exhibited intense red-shifted PL peaks. This was explained by formation of a set of well defined new surface state levels on the anodized wall surfaces of pores.
Electrochemical impedance spectroscopy has been employed for a quantitative study of dodecyl sulfate anion adsorption kinetics at the Bi ∣ 0.05 M Na2SO4 aqueous solution interface. Analysis of the impedance data demonstrates that the adsorption process of the dodecyl sulfate anion at high negative surface charge density (i.e. in the region of adsorption–desorption peaks) is limited by the rate of the diffusion and heterogeneous adsorption steps (mixed kinetics) of the organic anion to the electrode surface. In the region of maximal adsorption the slow diffusion stage seems to be the rate-determining step of adsorption. At small ac frequency and higher dodecyl sulfate concentrations, two-dimensional association of the dodecyl sulfate anions is possible in the adsorption layer. Nonlinear regression analysis has been used for fitting the experimental complex plane (Z″, Z′) plots. It was found that, to a first approximation, the Frumkin–Melik-Gaikazyan equivalent circuit (where Ctrue is the double layer capacitance and ΔC is the adsorption capacitance, ZW is the Warburg-like diffusion impedance and Rel is an electrolyte resistance at ac frequency→∞) can be used for fitting the experimental complex plane plots for dilute dodecyl sulfate sodium salt solutions. For more concentrated solutions and within the region of maximal adsorption, other more complicated equivalent circuits (and elements) have been used for fitting the experimental impedance data. The parameters Ctrue, ΔC and diffusion resistance RD depend on the electrode potential as well as on the adsorbate concentration.
The electrochemical reduction of 1,1′-trimethylenebisthymine (Thy(C3)Thy) in dimethylsulfoxide was investigated using dc polarography, cyclic voltammetry, controlled potential electrolysis and spectroelectrochemistry. Thy(C3)Thy is reduced in a two-election step (E = −2.40 V) to a diradical dianion which abstracts protons from the parent Thy(C3)Thy to form a neutral free diradical and the Thy(C3)Thy dianion. The neutral free diradical is further reduced and protonated to form 1,1'-trimethylenebisdihydrothymine. The dianion forms insoluble mercury salts producing up to three oxidation peaks (Epa between −0.35 and −0.07V). The effect of added base on the electrochemical and spectroelectrochemical behavior is described. The number of layers of the mercury- Thy(C3)Thy salt adsorbed in the anodic and stripped in the cathodic processes was calculated. The mechanism of adsorption is discussed.
The anodic oxidation of 1,1-dimethylhydrazine has been studied in 0.5–1 N H2SO4. From potential-sweep measurements in the range −0.400 to +1.00 V (vs. mercury—mercurous sulfate) three oxidation peaks were obtained. The first peak is a well-defined irreversible peak which has a corresponding reduction peak.Tafel relationships, reaction orders, and pH effects, were studied and a mechanism proposed for the first oxidation step. This involves two fast charge-transfer steps which are in quasi-equilibrium and a slow non-activated desorption step which is the rate-determining step.
The distribution mechanism of protonated 1,10-phenanthroline and its derivatives between aqueous and 1,2-dichloroethane phases was elucidated by investigating the faradaic ion transfer across a liquid-liquid interface using current scan polarography at the ascending water electrode, as well as by programmed current chronopotentiometry. Neutral reagent distributes into the aqueous phase, where it is protonated near the interface. From there the protonated reagent transfers electrolytically to the organic phase. The hypothetical individual distribution constants of the protonated ions were determined to be 0.63, 1.8, 1.9, and 3.6 for 1,10-phenanthroline, 4,7-dimethyl-1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline, and 4,7-diphenyl-1,10-phenanthroline, respectively. They were found to correlate closely with the distribution constants of the respective neutral reagents.
A series of calculations on the energetics of complexation of alkaline metals with 1,10-phenanthroline are presented. It is an experimental fact that the ordering of the free energy of transfer across the water - 1,2-dichloroethane interphase is governed by the charge / size ratio of the diferent cations; the larger cations showing the lower free energy of transfer. This ordering of the free energies of transfer is reverted in the presence of 1,10-phenanthroline in the organic phase. We have devised a thermodynamic cycle for the transfer process and by means of ab-initio calculations have drawn the conclusion that in the presence of phen the free energy of transfer is governed by the stability of the PHEN/M $^{+}$complex, which explains the observed tendency from a theoretical point of view.
In this paper, we describe a simple procedure to make agar-gel microelectrodes by filling micropipettes. These microelectrodes were used to study K+ transfer across the agar–water ∣ 1,2-dichloroethane interface facilitated by dibenzo-18-crown-6 (DB18C6), and the transfer of tetraethylammonium (TEA+). The results observed were similar to those obtained at micro-liquid ∣ liquid interfaces. The effect of various amounts of agar in the aqueous phase was optimized and 3% agar was chosen based on the potential window and solidification time. The different shapes of micro-agar-gel electrodes were prepared in a similar way. The fabricated agar-gel microelectrodes obey the classical micro-disk steady-state current equation, which is different from the behavior of a normal micropipette filled with aqueous solution without silanization.
In the present paper the interfacial behavior of a cationic dye, phenosafranin (PhS+), is voltammetrically analyzed. A sharp and narrow peak during the negative sweep was observed. The dependence of this peak on concentration, sweep rate and switch potential was a clear indication of an adsorption–desorption process similar to those observed on solid electrodes. It was determined that the adsorbed species results from the ion pairing between dicarbollylcobaltate (DCC−) and PhS+ at the interface as the process is observed only when the interface is polarized at the potential transfer of DCC−. The process fitted with a Frumkin isotherm and indicated a strong attractive interaction (g=−2.3) between the adsorbed species. This interaction was also analyzed spectrophotometrically.
Current–potential curves at the interface between the 1,2-dichloroethane (DCE) solution of Aerosol-OT (AOT, sodium diisooctyl sulfosuccinate, DOSS−Na+) and the aqueous solution of LiCi (W) show unusual current spikes, which are distinctly different from the curves predicted from the diffusion-controlled transfer of Na+ ions across the interface. After the initial negative current, presumably corresponding to the transfer of Na+ ions from DCE to W, the current reduces to the level of that of the base solution and concomitantly whitish emulsions become visible only in the DCE side of the interface. Then intermittent current spikes appear. The quantity of electric charge transferred by a single current spike far exceeds the charge that one emulsion particle bears, suggesting the avalanche-type transfer of many emulsion particles across the interface. Video-imaging of the interface demonstrates that the current spikes are triggered by the fusion to the interface of a large water droplet probably formed by the coalescence of W/O emulsion particles in the bulk of the DCE phase.
1,2-Benzophenoxazine-7-one (BPO) was absorbed on graphite to give a chemically modified electrode. The electrochemical redox reactions of BPO itself are fairly reversible at coverages less than 2 × 10−9 mol cm−2 with an E°′ of −210 mV (vs. SCE) at pH 7.0. The E°′ decreased 60 mV per pH unit from pH 1 to 11. The adsorbed BPO mediated electron transfer in the electrocatalytic oxidation of the dihydronicotinamide adenine dinucleotide (NADH). The kinetic reaction was studied with a rotating disk electrode modified with BPO for different coenzyme concentrations and pHs. The formation of a charge-transfer complex between NADH and the mediator was indicated. The rate of reaction increased as the pH decreased; the slope of a log k+2 vs. pH plot was −0.55, indicating a complex proton dependence. A reaction scheme with two parallel paths is discussed.
From ac impedance and galvanostatic pulse measurements the capacitance of the interface between solutions of LiCl in water and tetrabutylammonium tetraphenylborate in 1,2-dichloroethane has been evaluated at various electrolyte concentrations. The experimental data have been interpreted on the basis of the modified Verwey-Niessen model, according to which a layer of oriented solvent molecules (inner layer) separates two space charge regions (diffuse double layer). As for the water/nitrobenzene interface, the interfacial potential difference is spread mainly within the diffuse double layer. In the sequence nitrobenzene < 1,2-dichloroethane, the effects of the ion association, finite ion size and image forces, which modify the double-layer structure, become more pronounced. The latter two effects have been estimated by means of the weak-coupling and hypernetted chain theories, respectively. No evidence for the specific adsorption of ions or ion pairs was found.
In the first part of this paper the electrochemical transfer of alkali cations (M+) assisted by monensin (HX) across the water ∣ 1,2-dichloroethane (DCE) interface at pH<5, combined with a chemical exchange reaction at 5<pH<9, is proposed as the only mechanism responsible for the transfer of these cations. At pH>9 the current is voltammetrically negligible. An equation for the dependence of Δowφ1/2 on pH and Na+ concentration is developed. In the second part of the paper the transfer of alkaline earth cations assisted by the same ionophore is studied. The electrochemical reactions (Me(w)2++HX(o)⇄MeHX2+(o)) and (Me(w)2++X−(o)⇄MeX+(o)) are responsible for the peaks observed at pH<5.0 and at pH>9.0, respectively. As expected, in both cases ΔEp=0.030 V while at intermediate pH the electrochemical exchange reaction (Me2+(w)+HX(o)⇄MeX+(o)+H+(w)) is proposed. The net charge transfer of +1 at the interface accounts for ΔEp=0.060 V for the peak observed ΔEp in agreement with the hyper-Nernstian slope of 60 mV found for Ca2+ and Ba2+ ISE based on antibiotics with carboxylic groups at intermediate pH values. The higher selectivity for Ba2+ and the tendency in selectivity at alkaline pH found in the ISEs are also observed and thus explained according to the mechanism proposed.
Cyclic voltammetry with a frozen electrode has been used to investigate the ion transfer of tetramethylammonium (TMA+), tetraethylammonium (TEA+), and tetrapropylammonium (TPrA+) across the water/1,2-dichloroethane (DCE) interface at −15°C and to compare these results with the liquid/liquid system at 25°C. All the half-wave potentials of these ion transfers shift to negative potential in the frozen system, and the ion transfers exhibit less reversibility comparing transfers in the liquid system with the same composition at 25°C. These negative shifts indicate that the ionic solvations are unstable in the frozen phase. The degree of negative shifts increases in the order TMA+ < TEA+ < TPrA+. This order has been discussed in terms of the aqueous solvation of these ions. The diffusion coefficients of ions in the aqueous phase have been evaluated, and a very slow transport process found in the frozen system. In both frozen and liquid systems these values decrease in the order TMA+ > TEA+ > TPrA+. The apparent standard rate constants have also been evaluated for both systems. The values in the liquid system are larger than those in the frozen system, and both increase in the order TMA+ < TEA+ < TPrA+. That is, the ion transfer rate increases but the ion transport rate decreases with the Stokes radius of the ion in both systems.
The transfer of benzodiazepine derivatives (diazepam, bromazepam, alprazolam, oxazepam, nitrazepam, clonazepam, chlordiazepoxide, flunitrazepam, midazolam and lorazepam) across the water ∣ 1,2-dichloroethane interface was studied using cyclic voltammetry. The partition coefficients of ionic species of benzodiazepines, log PXH+, were calculated from the transfer potentials measured at pH<pKa. These values were compared with the partition coefficient of neutral species, log PX. The difference between log PXH+ and log PX was related to the degree of charge delocalization, which depends markedly on the presence of electron acceptor substituents in the molecule. These electron acceptor groups, in turn, affect the biological activity of these drugs. The results indicated that the difference Δlog PXH+=log PXH+−log PX can be used in structure–activity correlations as it takes into account the effects of substituents on the main positions within the molecule.
In N, N-dimethylformamide, 1,2-dithiole-3-thiones undergo a reversible 1-electron addition. The resulting thiyl radicals are subsequently converted into ring-fused thiyl radicals which dimerize. Controlled potential electrolysis affords a convenient route to symmetrical disulphides. The rate constants of the intramolecular ring closure which have been deduced from the voltammetric and chronoamperometric measurements depend on the nature of the substituents at C-4 and C-5. The findings are discussed from a pharmacological standpoint.
When the interface formed between an aqueous solution containing ofloxacin, a fluorquinolone antibiotic complexed with Fe(III), and an organic solution of a supporting electrolyte in 1,2-dichloroethane is potentiodynamically polarized, interesting electroadsorption phenomena are observed. The narrow negative voltammetric peak observed at much lower potentials than those corresponding to the diffusional transfer of a triply charged species, the linear response of Ip with v, the difference in the potentials of the positive and negative processes and the shift of this peak towards more negative potentials with sweep rate as coverage is increased, provide clear evidence of an irreversible adsorption–desorption process displaying attractive interaction between the adsorbate molecules at high coverage. These results are reinforced by the appearance of an important charge transfer resistance value in ac measurements.
The preparation of trans-1,2-diiodocyclohexane by reaction of cyclohexene and iodine has been characterized. Equilibrium favors the diiodide in chloroform or cyclohexene but dissociation occurs in dimethylformamide (DMF). Solutions in cyclohexene were introduced into cold DMF for study in the latter solvent. 1H and 13C NMR were used to confirm that the trans isomer is formed and to obtain relative populations of the conformers with axial iodines (aa) and equatorial iodines (ee). The ratio of concentrations of aa to ee is 4.0 in DMF at −55° C. Platinum microdisc electrodes were used to investigate the electrochemical reduction by cyclic voltammetry. At low temperatures and high scan rates in DMF solvent, the reduction peak for trans-1,2-diiodocyclohexane splits into two peaks. The peak appearing at less negative potential was assigned to the reduction of aa and the second peak to ee. Digital simulations were fit to the background-corrected voltammograms to obtain the conformational equilibrium and rate constants for temperatures in the range of 0 to −40° C.
Heterogeneous electron transfer (ET) reactions at the polarised water ∣ 1,2-dichloroethane (DCE) interface are studied by in situ UV-Visible spectroscopy in total internal reflection mode. The reduction of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and the oxidation of 1,1′-dimethylferrocene (DMFc) by the hexacyanoferrate redox couple are considered. The generation of products in the organic phase is monitored spectroscopically and correlated to the simultaneous current response. Both systems exhibit a good correlation between optical and electrochemical responses, highlighting the ideal behaviour of these redox couples for electron transfer studies at liquid ∣ liquid interfaces. The kinetics of ET between hexacyanoferrate and TCNQ is analysed also by chronoabsorptometry and potential modulated reflectance spectroscopy. The potential dependence of the ET rate constant is discussed within the framework of the ET models for ITIES.
Cyclic voltammetric and chronoamperometric experiments were performed on the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and its oxidative metabolites, 1-methyl-4-phenyl-2,3-dihydropyridinium ion (MPDP+) and 1-methyl-4-phenylpyridinium ion (MPP+). In neutral phosphate buffer electrolyte, MPTP underwent a 2-electron, 1-proton electrooxidation to MPDP+ according to an ECE mechanism where U2° < U1°. Further oxidation could be achieved in alkaline solution. MPDP+ could be reduced to the radical state (U° <- −0.560 V versus SHE) followed by dimerization and other side reactions. It was also shown that MPDP+ gradually disproportionates to produce MPTP and MPP+. With an U°=−1.067±0.010 V, MPP+ underwent a 1-electron reduction, followed by dimerization. The redox potentials of MPP+ and possibly even MPDP+ were beyond the limits of known physiological reducing potentials; therefore, the primary cytotoxic action of MPP+ does not proceed via redox cycling of its reduction radical, as do other pyridinium-based toxins such as methyl viologen.
On a mercury electrode in DMF as solvent the electrochemical reduction of FeII(PDT)32+ occurred in four steps of which the first three, involving iron based molecular orbitais, were one-electron, whereas the fourth was a three-electron reduction of the three ligands PDT. The electrochemical behaviour of FeII(PDT)2(SCN)2 was quite similar. Protonation of coordinated PDT in Fe(PDT)32+ led to tuning the site of the first electrons transferred to the complex, either to iron based orbitals, or to protonated PDT, depending on the pKa of the protic acid added.
Cyclic voltammetry and controlled-potential electrolysis have been employed to examine the electrochemical reduction of a series of di-, tri-, and tetrahalobenzenes at carbon cathodes in dimethylformamide containing tetramethylammonium perchlorate. Cyclic voltammograms for the reduction of 1,2,4,5-tetrabromo-, 1,2,3,4-tetrachloro-, and 1,2,4,5-tetrachlorobenzene exhibit four irreversible waves, those for 1,2,4-tribromo-, 1,3,5-tribromo-, 1,2,3-trichloro-, 1,2,4-trichloro-, ad 1,3,5-trichlorobenzene show three irreversible waves, and those for 1,3-dibromo-, 1,4-dibromo-, 1,2-dichloro-, 1,3-dichloro-, and 1,4-dichlorobenzene reveal two irreversible waves. Products derived from the electrolytic reduction of 1,2,4,5-tetrabromobenzene are 1,2,4-tribromo-, 1,3-dibromo-, and 1,4-dibromobenzene, along with 1,3,5-tribromobenzene, and the appearance of the last compound indicates that an electrolytically induced halogen dance takes place. For all of the other polyhalobenzenes investigated, except 1,2-dibromobenzene, the distributions of electrolysis products are consistent with expectations based on their voltammetric behavior.
Electrochemical oxidation of catechol (1a), 3-methylcatechol (1b) and 3-methoxycatechol (1c) in the presence of 1,3-diethyl-2-thiobarbituric acid (3) as a nucleophile in aqueous solution has been studied using cyclic voltammetry and controlled-potential coulometry. The results indicate that (1a–1c) participating in a 1,4 (Michael) addition reaction converts to dispirothiopyrimidine derivatives (6a–6c). The electrochemical synthesis of 6a–6c has been successfully performed in an undivided cell in good yield and purity.
This paper presents polarographic (dc and DP) and voltammetric studies of the electroreduction of the s-triazine derivative simazine (2-chloro-4,6-di(ethylamino)-1,3,5-triazine) on mercury electrodes. The study is performed in the acidity range 2.25 M H2SO4 to pH 5. Above this last pH value no signals were obtained. In DP polarography, two main reduction peaks were observed, accompanied by a pre-peak, at less negative potentials, and a post-peak, at more negative potentials, due to the adsorption of simazine on the electrode. The main peaks corresponded to two-electron reduction processes. At pH below the protonation pK of the triazine ring (ca. 1.7), the results showed that in the first stage, simazine suffers a cleavage of the Cl atom via a CEC process to yield a dechlorinated intermediate, which is reduced through an irreversible two-electron process, the rate-determining step being the second electron transfer. At pH>pK, a protonation of the triazine ring precedes the reduction process, this reaction being also responsible for the observed decrease in limiting current. Linear-sweep voltammetry showed that simazine is adsorbed on the electrode with the triazine ring parallel to the electrode surface, as can be inferred from the value of the calculated area covered by one molecule. At pH values where both the protonated and unprotonated forms of simazine coexist, both forms are co-adsorbed on the electrode.
The electrochemical oxidation of 1,3,7,9-tetramethyluric acid at the pyrolytic graphite electrode has been studied in aqueous solution over a wide pH range. At all pH values a single voltammetric oxidation peak is observed at potentials considerably more positive than are observed for other uric acid derivatives. The pH-independent voltammetric peak is due to a primary 2 e−1 oxidation of tetramethyluric acid to a very unstable quinonoid dication. Rapid attack by water leads to formation of the 4,5-epoxide of tetramethyluric acid. This then slowly hydrates further giving the corresponding 4,5-diol which in turn rapidly fragments into numerous products. At pH < 3 the latter diol decomposes to an interesting spiro compound, 3,1′,3′-trimethyloxazolidine-2,4-dione-5-spiro-5-hydantoin. The 4,5-diol also appears to undergo two other reactions. At pH > 3 it cleaves across the C(4)N(3) bond to yield, ultimately, 2,4-dimethyl-5-hydroxyhydantoin-5-N-methylcarboxamide. The remaining products formed between pH 2 and 9 suggest that the 4,5-diol undergoes a ring contraction reaction leading to 1-hydroxy-5-carbo-hydroxy-2,4,6,8-tetraaza-2,4,6,8-tetramethyl-3,7-dioxo-bicyclo-(3.3.0)-octane which then decomposes to tetramethylallantoin (pH > 3), 2,4-dimethylalloxanic acid, 2,4-dimethyl-5-carboxy-5-N-methylaminohydantoin, 2,4-dimethyl-5-hydroxyhydantoin, dimethylurea and N-methylcarbamic acid.
The use of differential pulse polarography for the analysis of 1,4-benzodiazepines in body fluids is well documented. Although not as sensitive as electron-capture gas-liquid chromatography, the technique yields results of comparable accuracy, precision and sensitivity to spectrophotometric, spectrofluorometric, and other chromatographic methods. The major drawback of the technique is the need for a thin layer chromatographic separation prior to polarographic measurement to ensure specificity in the measurement of a 1,4-benzodiazepine in the presence of its metabolites. The recent introduction of an on-stream dropping mercury electrode detector (operated in the differential pulse amperometric mode) for high performance liquid chromatography enhances specificity to allow for the rapid simultaneous assay of the 1,4-benzodiazepines and their respective metabolites.
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.
A series of modified carbon paste electrodes were constructed by incorporation of 1,4-naphthoquinone and some of its derivatives in a graphite powder-mineral oil matrix. It has been shown by cyclic voltammetry, rotating disk electrode voltammetry and chronoamperometry that these electrodes can catalyze the reduction of O2 to H2O2 in aqueous medium (pH > 2). It has been found that in the optimum condition, the reduction of dioxygen at the surface of such electrodes occurs at potentials about 350 to 550 m V less negative than at an unmodified carbon paste electrode. The heterogeneous rate constants for the reduction of O2 at the surface of these modified carbon paste electrodes were determined by use of Koutecky-Levich plots. In addition, the apparent diffusion coefficients of modifiers in mineral oil, O2 in solution and the effective area of carbon paste electrodes have also been estimated.
The oxidation potential of 1,4-diaminobenzene has been calculated theoretically using a combination of ab-initio calculations and molecular dynamics simulations. An experimental determination of the same quantity is presented. Data obtained from cyclic voltammetric experiments, at a platinum electrode, were consistent with an EC electrode reaction mechanism. Cyclic voltammograms recorded at high voltage scan rates (v ⪢ 500 mV s−1), such that the following homogeneous kinetics were outrun, indicated the oxidation to be quasi-reversible and this enabled the standard oxidation potential to be estimated. Comparison of the theoretical and experimental values for the oxidation potential showed agreement to within 25 mV. The level of agreement between theory and experiment was considered to be highly satisfactory.
1,4-Dialkoxybenzenes (from dimethoxy to didecyloxy) can be oxidized reversibly into stable radical cations in a CH2Cl2 + CF3COOH mixture containing 0.2 M NBu4BF4. The effects of the alkoxy substituents on the shift of the redox potentials (from the respective Taft substituent constants) as well as the variations of diffusion coefficients D and the charge transfer rate constants ks are examined. In particular, the D values are found to decrease from about 1.0 × 10−5to 0.4 × 10−5cm2s−1 with the lengthening of ether chains, while the ks values are around 2 × 10−2cms−1. In another investigation medium, such as dry CH3CN, the oxidation of 1,4-didecylozybenzenes leads to the deposition onto the electrode surface of the corresponding p-doped polyphenylenes, except for the 1,4-didecyloxybenzene which does not electropolymerize. The doping level decreases with the lengthening of ether chains and polymers globally exhibit poor film properties. The apparent instability of their electrochemical response and probably the low conductivity can be attributed to a powdery structure, a poor attachment onto the electrode surface and a lack of cohesion of the deposit.
The electrochemical lithium insertion reaction into the vanadium pentoxide xerogel V2O5·.1.6 H2O (noted VXG) in a propylene carbonate solution has been investigated by structural, thermodynamic and kinetic studies. This material, obtained via a sol-gel process, is a lamellar compound whose high anisotropic structure is characterized by the stacking of ribbons in the c direction. The presence of propylene carbonate in the starting material leads to a basal distance of 21.6 Å (1 Å = 10−10 m). About 1.8 Li+ ions can be accomodated between the ribbons of this lamellar compound at the same energetic level of ~ 3.1 V vs. Li/Li+. Entropy measurements, X-ray diffraction experiments and particle size determinations have given evidence for the existence of two one-phase regions for the composition ranges 0 < x < 0.1 and 0.2 < x < 1.8, and a two phase region for the narrow lithium content 0.1 < x < 0.2. Between x = 0.1 and x = 0.2, lithium insertion causes the removal of the propylene carbonate from the inter-ribbon spacc to give rise to a collapsed VXG form. With a theoretical faradaic capacity of 250 Ah/kg, the reversibility of the lithium insertion process being proven, VXG constitutes a promising intercalation material. Nevertheless, even with a high diffusion coefficient DLi > 5sx 10−11 cm2 s−1, its low electronic conductivity hinders utilisation of high current densities. Cycling experiments have shown a satisfactory behaviour since for a current density j = 0.05 mA cm−2, about 70% of the initial capacity, ie. 70 Ah/kg, is recovered after the 30th cycle.
The electrogenerated chemiluminescence of 9,9′-bianthryl and 10,10′-dimethoxy-9,9′-bianthryl was studied in acetonitrile electrolyte solutions using the triple-step method. In the electrogenerated emission spectra, only the twisted intramolecular charge transfer (TICT) band was observed. The Feldberg plot analysis indicates that the emitting species (excited TICT state) are formed directly by the electron transfer between the electrogenerated radical anion and radical cation (S-route). The emission efficiencies (ØECL up to 0.03) and the yields of excited state formation (ØES up to 0.14) were evaluated in the range between room temperature and the melting point of acetonitrile. The (ØES values obtained have been interpreted in terms of Marcus theory.
The design of reliable reference electrodes for molten fluorides is a major problem, and therefore internal reference systems obtained by in-situ generation of a redox couple were investigated. The , , and couples were examined using convolution voltammetry, and the last of these was selected. Galvanostatic pulses were used to generate ferrous ions at the interface between a solid iron electrode and the electrolyte, and then the current was switched off and the system allowed to relax. The redox system was reversible, the theoretical equations were obeyed and the electrode potential determinations were very reproducible. The potential measured at a given time could be used to check the potential of a classical reference or pseudo-reference electrode in molten NaF at 1025°C within ±2 mV.
Three-dimensional superstructures, multilayer-arrays consisting of Au-nanoparticles (13±1 nm) and crosslinked by microperoxidase-11 (MP-11), were assembled on transparent ITO conductive glass supports. The structures of the assemblies were confirmed by spectroscopic and electrochemical analyses, which revealed that each Au-particle is associated with ca. 50 MP-11 molecules. The multilayered MP-11/Au-particle systems act as electrocatalysts for the reduction of H2O2. The number of MP-11 ∣ Au-particle layers associated with the electrode controls the resulting electrocatalytic currents. The controllable number of MP-11 ∣ Au-particle layers associated with the electrode enables the control of the effectiveness of the electrocatalytic process and tuning of the sensitivity of the H2O2-sensing interface.
Microperoxidase-11, MP-11, is made by proteolytic digestion of cytochrome c, cyt. c. It consists of a polypeptide of 11 amino residues attached covalently to the heme. Given that MP-11 has a more exposed heme than the complete protein, it would seem that electron transfer, ET, between immobilized MP-11 and electrodes would be at least as fast as for intact cyt. c. However, while the maximal heterogeneous ET rate for immobilized cyt. c is around 1000 s−1, that reported previously for immobilized MP-11 does not exceed 20 s−1. This work attempts to understand this difference in measured ET rates. The MP-11 was immobilized on gold electrodes using several protocols: (electrode A) the immobilization was done following a previously published carbodiimide based recipe yielding ET rates of the order of 20 s−1; (B) MP-11 was bound to gold electrodes by Lomant’s reagent and gave an ET rate close to 4000 s−1; (C) physisorbed MP-11 on gold electrodes with a self assembled monolayer, SAM, of alkane thiols gave an ET rate approaching 2000 s−1 for the shortest length alkane thiol. Inspection of the immobilization chemistries suggests that the procedure employed in producing electrodes B and C are likely to lead to a monolayer or less of immobilized MP-11 while the procedure employed for electrode A may lead to a film comprised of a multilayer of MP-11. The presence of such a film on electrode A complicates the ET process since the MP-11 in the layer adjacent to the electrode could have fast ET rates while the MP-11 in the outer layers may have significantly slower ET rates. The net result would be an apparent ET rate constant which is much smaller than the value for the first layer. The measurements and calculations are presented in support of such an interpretation.
The heme-containing undecapeptide microperoxidase-11 was immobilized as a monolayer on gold wire electrodes by carbodiimide coupling of the peptide to a cystamine monolayer. Direct communication between the electrode and the heme center was observed in acetonitrile and ethanol. The observed formal potential for the FeIII/FeII couple for the immobilized microperoxidase-11 was −0.290 V vs. SCE in acetonitrile and −0.286 V in ethanol. These electrodes were applied as amperometric sensors for organic peroxides in acetonitrile and ethanol. The electrocatalytic reduction of cumene hydroperoxide, t-butyl hydroperoxide and hydrogen peroxide was demonstrated. In acetonitrile, kinetic parameters (Km and Imax) were evaluated using a modified Michaelis-Menten analysis for the electrocatalysis of the reduction of cumene hydroperoxide, t-butyl hydroperoxide and hydrogen peroxide.
The geometrical arrangement of Ir sites favourable for the main oxidation reaction and the poison formation reaction in formic acid oxidation is determined on Ir (111), (100) and (110). The order of catalytic activity is (111) > (100) > (100) for the main oxidation and that for the poison formation is (111) < (110) < (100). This order is different from that on Pt single crystal planes, the order of which is (111) > (100) > (110) for the main oxidation and (111) < (100) < (110) for poison formation. Since the order is different on Pt and Ir, the chemical property determines the order. On both Ir and Pt, poison formation occurs independently of the existence of adsorbed hydrogen.
The kinetics of HCOOH oxidation via a reactive intermediate on Pt(100), Pt(110), Pt(111), Pt(510) and Pt(911) single crystal electrodes were studied quantitatively. The difficulty due to the self-poisoning involved in HCOOH oxidation has been overcome successfully by designing a programmed potential step procedure, which is based on results of kinetic studies of dissociative adsorption of HCOOH. The data processing method of integration transform of j∼t transient data was developed for extracting kinetic parameters. The rate constant (kf), the apparent activation energy (ΔH≠°) and the transfer coefficient (β) for HCOOH oxidation on different Pt single crystal electrodes have been evaluated. The values of ΔH≠° obtained on the 5 Pt single crystal electrodes vary from 10.1±0.1 to 32.7±0.2 kJ mol−1. The results demonstrated that the well-defined Pt(110) electrode possesses the lowest value of ΔH≠° amongst the 5 Pt single crystal electrodes studied, signifying a higher electrocatalytic activity for HCOOH oxidation. According to the values of ΔH≠°, the electrocatalytic activity of the three basal planes of Pt single crystals can be arranged in the ascending order of Pt(110)>Pt(111)>Pt(100). The values of ΔH≠° of the two stepped surface are close to that of Pt(100) (32.2±0.5 kJ mol−1), which is in good accord with the configuration of surface structure of the two stepped surfaces since the majority surface sites are of (100) symmetry. It has been found that the transfer coefficient β does not vary with the reaction temperature, and manifests a similar variation as that of ΔH≠° versus the orientation of the Pt single crystal electrode. The small values of β (ranging from 0.102±0.004 to 0.251±0.008) may suggest a stepwise transfer of two electrons and imply the unavoidable interaction of HCOOH with surfaces of the Pt single crystal electrodes. The variation of ΔH≠° and β versus the orientation of Pt single crystal electrode demonstrated, from a kinetic point of view and for the first time, the effects of surface atomic arrangement towards HCOOH oxidation.
The voltammetric behaviour of oxalic acid on basal planes of platinum in sulphuric and perchloric media has been studied. Oxidation takes place at potentials higher than 0.80 V (RHE) on the three planes. Specific adsorption of hydrogenoxalate appears to occur on the three orientations, affecting hydrogen adsorption. The strength of this anionic adsorption is greater than that of HSO−4 and close to that of Cl− in the same range of concentrations. As in Cl− containing solutions, there is experimental evidence of a reconstruction of Pt (100) (1 × 1) towards a (5 × 20) structure, probably induced by hydrogen oxalate adsorption. No irreversibly adsorbed species were obtained from dissociative adsorption experiments, thus showing the stability of the C-C bond in oxalic-like adspecies. Nevertheless, blocking of the Pt (110) surface at low potentials was observed, due to dissociative adsorption from products formed by oxalic acid reduction on this plane. Results obtained with stepped surfaces are in accordance with the existence of specific anionic adsorption, as well as with the existence of an increase in hydrogen adsorption charge on surfaces containing (100) terraces. Blocking of hydrogen adsorption is observed only in electrodes containing (110) sites.
By combining electrochemistry with electronic methods of surface analysis it is possible to obtain valuable information about the metal-electrolyte interface. This implies a transfer of the electrode from the UHV to the electrochemical cell and vice versa without extensive loss of the surface quality. A glove box system has been developed with an ultra purification system for the atmosphere (O2 < 1 μg g−1) and used in the case of Au (110). No irreversible electrochemically induced surface modification of the Au (110) surface has been found in post electrochemical LEED patterns.
The adsorption of trifluoracetate anions on Pt(100), Pt(110) and Pt(111) has been studied by means of cyclic voltammetry and FTIR spectroscopy. For all three surfaces, adsorbed trifluoracetate shows the same characteristic band around 1215 cm−1 assigned to the stretching vibration of the CF3 group. The potential dependence of the integrated band intensity and band center frequency varies with the single crystal plane used. Lateral interactions within the adsorbed layer are the main origin of the band frequency shift observed. Trifluoracetate is less strongly bonded to the metal surface than its parent acetate. This effect may be due to the presence of the −CF3 group, which is known to cause a withdrawal of electrons from the COO− group.
The surface characterization of Pt electrodes begun in Part I of this is extended to Pt(110) and Pt(111). Flame annealing and quenching in aqueous electrolyte yields the reconstructed Pt(110)-(1×2) surface, which gives two distinct hydrogen adsorption-desorption waves in dilute perchloric acid. The (1×2) structure is distinguished from other possible surface structures by comparison of the voltammetry with that of a vicinal (111) surface, namely Pt(331), and also by a detailed consideration of the effects of anions and cations on the hydrogen adsorption-desorption profile. The surface of a Pt(111) electrode pretreated by flame annealing and quenching in aqueous electrolyte is either highly defective or possesses a high degree of surface lattice strain, resulting in an unusually strong binding of adsorbed hydrogen. Our pretreatments also produce high binding energy hydrogen adsorption sites on (100), but not on (110) or any of the higher Miller index surfaces studied. The distinguishing feature is considered to be the low density of monoatomic steps at (111) and (100).
We report two roles of the adsorbed anions in the underpotential deposition (upd) of zinc ions giving cooperative and competitive interaction with the upd metals under different circumstances. The effect of specifically adsorbed anions on Zn upd was investigated systematically at Pt(111), Pt(100), and Pt(110) in solutions of pH 1–4.6 by cyclic voltammetry, where the anions were (bi)sulfate, phosphate, chloride, bromide, and iodide anions. Zn upd hardly occurred on Pt(111) in acidic solutions of pH 1. However, the growth of sharp Zn upd voltammetric waves was observed on Pt(111) in phosphate solutions with pH increase in the pH range of 2–4.6. In sulfate and perchlorate solutions, such behavior was not observed on Pt(111) with pH increase. On the other hand, at Pt(110), the Zn upd was clearly observed in phosphate solution of pH 1. In 0.1 M KH2PO4 (pH 4.4) with 10−3 M halides, the onset potential of Zn upd on Pt(111) shifted negatively according to the order of the adsorption strength of Cl−<Br−<I−. The negative shift of 0.02 V by Cl− adsorption on Pt(111) was smaller than that of 0.10 V on Pt(100). These results are discussed and correlated to the strength of anion adsorption at the onset potentials of Zn upd, in terms of cooperative and competitive interaction of the adsorbed anion with the upd Zn; adsorbed phosphate anions on Pt facilitate the Zn upd kinetically by the desorption action, but halides tightly adsorbed at the Pt surface obstruct the initiation of Zn upd.
The influence of underpotentially deposited Cu adlayers on the electrocatalytic reduction of oxygen at Pt(111) has been studied in 0.05 M H2SO4 solutions using hanging meniscus rotating-disk (HMRD) voltammetry and electrochemical scanning tunneling microscopy (STM). Oxygen reduction at clean bare Pt(111) proceeds by a direct four-electron transfer with the formation of H2O as the primary reaction product. After the formation of the first underpotentially deposited Cu adlayer with (√3 × √3)R30° structure, the oxygen reduction current decreases to a steady-state value which is almost half that observed at bare Pt. This partial inhibition provided by the Cu adatoms can be explained by a change in the oxygen adsorption mechanism from the bridged orientation, favoring a four-electron transfer, to the end on orientation, favoring a two-electron transfer. The effect of coadsorbed halides on underpotential deposition (UPD) as well as the oxygen reduction reaction, has also been examined. Oxygen reduction at Cu-modified Pt(111) in the presence of chloride was completely inhibited after the first UPD process. Further, oxygen reduction in a pure H2SO4 solution on bare Pt(111) was carefully studied using HMRD voltammetry. The oxygen reduction current at 0.02 V was almost half the constant limiting current observed in the potential region between 0.5 and 0.3 V. This result can also be explained by a change in the oxygen adsorption mechanism from the bridged to the end-on orientation.
The voltammetry of the formation of (√3×√3)R30° and p(2×2) overlayers on (111) electrodes is modeled by analytical and Monte Carlo techniques. Both ordered structures are formed by second-order order–disorder phase transitions that lead to sharply-peaked ‘butterfly’ features in the voltammogram. The butterflies for both systems are, however, distinctly different and resemble the voltammetry of Pt(111) in sulfuric and perchloric acid, respectively, even though the simulated adlayer structures are not exactly the same as the experimental ones. Some general features of butterfly peaks in voltammetry and their implications are discussed.
Top-cited authors
Koichi Jeremiah Aoki
  • University of Fukui
Claude Andrieux
  • Paris Diderot University
Hubert H Girault
  • École Polytechnique Fédérale de Lausanne
Keith Oldham
  • Trent University
Takeo Ohsaka
  • Tokyo Institute of Technology