Cyclic voltammetric studies of a thionine coated pyrolytic graphite electrode

Cyclic voltammetry has been used to study the spontaneous adsorption and oxidative coating of thionine onto basal plane pyrolytic graphite electodes. Spontaneous adsorption leads to a monolayer of flat lying molecules together with some weakly attached upper layers. Oxidation of an adsorbed thionine monolayer results in several products which appear to include polymeric species and the sulphoxide of thionine. Similar products are obtained by an oxidative coating procedure.     

Pt电极表面共吸附层NO和CO氧化还原的CV及原位FTIR研究

应用电化学循环伏安方法（CV）和原位傅里叶变换红外反射光谱（in situ FTIRS）研究了酸性溶液中Pt多晶电极表面NO和CO的共吸附行为及吸附态CO对吸附物种NO氧化还原反应的影响．研究结果表明,0．20V（VS．SCE）时,CO和NO能同时稳定吸附在Pt电极表面,CO以线性吸附态（CO L）存在,NO以桥式吸附态（NOB）和线性吸附态（NO L）共存．CO L 的共存使得NO的还原电流峰电位负移约0．024V,并且促使不易被氧化的NO B在0．93V处被氧化．原位FTIRS研究进一步表明,NO可以置换预吸附在电极表面的CO,NO和CO在Pt多晶电极表面的吸附是一个竞争吸附的过程．在0．45V-1．2V电位区间,NO和CO都能转化为环境友好产物,分别为NO3-和CO2．且Pt电极表面共吸附物种CO的量直接影响NO B的氧化产物NO3-的生成量．     

Effect of Cs+ ions on the electrochemical nanogravimetric response of platinum electrode in acid media

Platinum electrodes have been investigated in sulfuric acid solutions in the presence and absence of Cs⁺ ions by electrochemical quartz crystal nanobalance (EQCN). An unusual potential dependence of the quartz crystal frequency response has been observed in the presence of Cs⁺ ions. The frequency decrease is more pronounced in the region of the underpotential deposition of hydrogen, and the frequency decrease in the double layer region diminishes as the concentration ratio of Cs⁺ and H⁺ ions increases. After immersion in Cs₂SO₄ solutions the frequency change was higher than that expected taking into account the density and viscosity. The effects observed can be explained by the specific adsorption of Cs⁺ ions on the Pt surface, which competes with the hydrogen adsorption. At more positive potentials than the potential of zero charge (pzc) a desorption of the Cs⁺ ions starts. In this potential region both Cs⁺ and HSO₄^? ions are adsorbed at the platinum surface. In the double layer region the mass change caused by the desorption of Cs⁺ ions and the starting adsorption of sulfate ions compensates each other.     

Electrochemical nanogravimetric studies of platinum in acid media

Electrochemical quartz crystal nanobalance (EQCN) is one of the most powerful tools to obtain information on the events occurring at the electrode surface. This method has been exploited to monitor the surface mass changes and hence to draw conclusions in respect of the formation and removal of adsorbed species and oxides as well as changes in the electrochemical double layer also in the case of platinum electrodes. However, the results that had been obtained so far are somewhat contradictory, and consequently diverse interpretations can be found in the literature. Therefore, it is worth to review the knowledge accumulated and to carry out systematic study in this respect. In this work smooth and platinized platinum electrodes in contact with acidic solutions were studied using EQCN technique. The effects of temperature, the nature of cations and anions, pH, concentrations, potential range were investigated on the electrochemical, and the simultaneously detected nanogravimetric responses. It is shown that in the underpotential deposition (upd) of hydrogen the adsorption/desorption of species from the solution phase is governed by the oxidative desorption/reductive adsorption of hydrogen; however, unambiguos conclusions cannot be drawn regarding the actual participation of anions and water molecules in the surface coverage. In the hydrogen evolution region a weak cation adsorption can be assumed and the potential of zero charge can be estimated. Cs⁺ cations affect the EQCN response in the hydrogen upd region. In some cases, e.g., in the case of upd of zinc the mass change can be explained by an induced anion adsorption. Two types of dissolution processes have been observed. A platinum loss was detected during the reduction of platinum oxide, the extent of which depends on the positive potential limit and the scan rate, and to a lesser extent on the temperature. The platinum dissolution during the electroreduction of oxide is related to the interfacial place exchange of the oxygen and platinum atoms in the oxide region. At elevated temperatures two competitive processes take place at high positive potentials: a dissolution of platinum and platinum oxide formation.     

Cyclic voltammetric studies of the redox behavior of iron(III)-vitamin B6 complex at carbon paste electrode

Electrochemical redox behavior of Fe-vit B₆ complex is investigated in HEPES buffer in the pH range 5.1–13.1 using cyclic voltammetry. Well-defined anodic and cathodic peaks are observed in the voltammograms at pH 13.1. At pH 8.0, only one cathodic peak and at pH 5.1, only one anodic peak are found. At all the pH values, the peak potential separation is much higher than that of a reversible electrochemical reaction. The peak current ratio (i _pa/i _pc) is less than unity and decreases with the scan rate. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 6, pp. 691–697. The text was submitted by the authors in English.     

Electrochemical nanogravimetric studies of sulfur/sulfide redox processes on gold surface

Electrochemical quartz crystal microbalance, combined with cyclic voltammetric, chronoamperometric, and potentiostatic measurements, was used to study electrodeposition/dissolution phenomena at a gold electrode in solutions containing Na₂S. Spontaneous, open-circuit deposition processes as well as dissolution of the deposits in sulfide-free solutions have also been investigated. The potential range, scan rate, sulfide concentration, and pH have been varied. The results of the piezoelectric nanogravimetric studies are elucidated by a rather complex scheme involving underpotential deposition of sulfur at approximately −0.85 V vs. sodium calomel electrode, reductive dissolution of the deposited sulfur-containing layer at potentials more negative than approximately −0.9 V, and formation of a sulfur-containing multilayer at potentials more positive than −0.2 V. During the reduction of sulfur deposited on Au, a mass increase due to the formation of polysulfide species in the surface layer, accompanied by incorporation of Na⁺ counterions, can be observed that starts at approximately −0.4 V. This is a reversible process, i.e., during the reoxidation, counterions leave the surface layers. Frequency excursions during the electroreduction and reoxidation processes reveal existence of several competitive dissolution–deposition steps. Spontaneous interaction between Au and HS⁻ species results in a surface mass increase at the open-circuit potential, and it also manifests itself in the substantial decrease of the open-circuit potential after addition of Na₂S to the supporting electrolyte.     

Mn3+/Mn2+氧化还原体系的旋转圆盘电极法性能

氧化还原液流电池的独特性能受到关注[1～4]。本文综合运用循环伏安法,旋转圆盘电极法研究锰离子浓度较高(0 25～0 35mol·L-1、转速范围较宽(400～4200rpm)、同时考虑过电位的条件下Mn(Ⅲ)/Mn(Ⅱ)电对在RDE铂盘电极上的电极过程动力学并确定有关的动力学参数,了解其影响因素,为该电对作为氧化还原液流电池正极活性材料提供动力学依据。1　实验部分铂电极使用前在铬酸洗液中浸10min,水冲洗干净,蒸馏水淋洗,然后在6 3mol·L-1H2SO4中超声清洗10min,再用CHI660电化学工作站(美国CH仪器公司)控制,以0 05V/s的扫速在-1 0～1 2V之间扫…     

Cyclic voltammetric studies of the antipsychotic chlorpromazine using an alkylthiol/phospholipid-modified gold electrode

Self-assembled monolayers of alkanethiols on gold have been reported to be highly stable for voltammetry experiments in aqueous electrolyte. In this work a gold electrode has been modified by first depositing one layer of an alkylthiol (S-C₁₈) and then coating by phospholipid multilayers. Voltammetric oxidation of the antipsychotic chlorpromazine at this two-step modified electrode was followed by means of cyclic voltammetry measurements. The results give important information concerning the behaviour of the pharmacological agent at the lipid-water interface. Measurements made using the pre-concentration method allow good sensitivity improvement after 5 min accumulation time. The ability of chlorpromazine to penetrate inside the phospholipid multilayers has also been investigated under different conditions such as the nature of the phospholipid and the pH of the medium. The accumulation process seems to be closely related to the charge carried by the phospholipid and by the molecule, while the incorporation process seems to be independent of the charge carried by the phospholipid and dependent of the degree of fluidity of their hydrocarbon chains. We found through this work that the acid-base equilibrium of chlorpromazine together with its amphiphilic properties (as compared with the results of similar studies on phenothiazine) could be responsible for governing the principal aspects of the drug's behaviour toward biological membranes. Received: 6 January 1997 / Accepted: 27 February 1997     

Generation and electrochemical nanogravimetric response of the third anodic hydrogen peak on a platinum electrode in sulfuric acid media

Platinum electrodes have been investigated in sulfuric acid solutions in the hydrogen adsorption–desorption region by electrochemical quartz crystal nanobalance (EQCN). It was found that a well-developed peak (the so-called third peak) between the two main peaks appeared when, following the cycling in the oxide region, the electrode was kept at potentials just more positive than the potential of hydrogen evolution under the same conditions. The extent of this third peak and its ratio to oxidation peaks of the strongly and weakly adsorbed hydrogen depend on the waiting time at potentials mentioned above as well as on the scan rate. Similarly to the other two peaks, the simultaneous EQCN response shows a slight mass increase which can be assigned to adsorption of HSO₄ ^? ions at the platinum surface. Because the third peak appears only after a potential excursion in the oxide region, it is related to the formation of specific surface sites on which hydrogen can be adsorbed with an energy which falls between the energies of the weakly and strongly bound hydrogen. The waiting time effect indicates that this adsorption is a slow process, and it is the very reason that it cannot be observed during the second cycle. The scan rate dependence can be elucidated by the transformation of this type of adsorbed hydrogen to the other two forms.     

Cyclic voltammetric study of glucose oxidation on an oxide-covered platinum electrode in the presence of an underpotential-deposited thallium layer

A cyclic voltammetric study of the oxidation of glucose on an electrochemically formed platinum oxide layer of a platinum electrode was carried out in both the absence and presence of an underpotential-deposited thallium or lead layer. An enhanced effect on the oxidation of glucose was observed in oxide-covered platinum electrodes compared to bare platinum electrodes. This enhancement remained in the presence of an underpotential-deposited thallium or lead layer. The presence of the underpotential-deposited metal layer significantly increased the anodic peak current and the response to changes in glucose concentration over the concentration range of 0 to 10800 mg/100 cm³ (0 to 0.6 M). The presence of the underpotential-deposited thallium layer appears to alter the mechanisms of the electrochemical oxidation of glucose on the platinum electrode. This is reflected by the results-the anodic peak current is directly proportional to the voltage scan rate applied in the cyclic voltammetric study on a bare platinum electrode, whereas it is directly proportional to the square root of the voltage scan rate in the presence of an underpotential-deposited thallium layer.     

Cyclic voltammetric and computational study of a 4-bromophenyl monolayer on a glassy carbon electrode

A glassy carbon (GC) surface modified with monolayer of 4-bromophenyl was examined as voltammetric electrode for some redox systems. The modified electrode exhibited very slow electron transfer in comparison to the unmodified surface by factors which varied with the redox systems. However, after scanning the modified electrode in 0.1 M tetrabutylammonium tetrafluoroborate (TBABF₄) in acetonitrile from 0.4 to −1.1 V vs. Ag/AgCl for 20–25 cycles, the modified electrode showed much faster electron transfer kinetics, e.g., the results for Fe(CN)₆ ^3−/4− were approaching those observed with unmodified surfaces. The effect is attributed to an apparently irreversible structural change in the 4-bromophenyl monolayer, which increases the rate of electron tunneling. The transition to the conducting state is associated with electron injection into the monolayer and causes a significant decrease in the calculated HOMO-LUMO gap for the monolayer molecule. Once the monolayer is switched to the conducting state, it supports rapid electron exchange with the redox system, but not with dopamine, which requires adsorption to the electrode surface. A conductive surface modified electrode may have useful properties for electroanalytical applications and possibly in electrocatalysis. Correspondence: Abbas A. Rostami, Department of Chemistry, Faculty Basic of Science, University of Mazandaran, Babolsar, Iran.     

Cyclic voltammetric study of redox-active surfactant by hydrogel-modified electrode

The dissolved states of redox-active non-ionic surfactant (FPEG) in the swollen state of N-isopropyl acrylamide (NIPA) hydrogel have been studied by using a gel-modified electrode. The pronounced decrease in the peak current and the negative shift in the formal potential of CV at the gel-modified electrode, as compared with the normal GC electrode, was observed in the micelle-solution; this indicates that the diffusive FPEG molecules which form the micelle hardly penetrate into the NIPA gel. This result suggests that there exists an interaction between FPEG molecules and the NIPA gel in the vicinity of the surface of the NIPA gel in the micelle-solution. However, this also indicates that a small amount of FPEG molecules which can form micelles exist in the NIPA gel.     

Electrocatalytic oxidation of ascorbic acid and voltammetric determination with a ferrocene-modified platinum electrode

Ferrocene attached to the surface of a platinum electrode catalyses the electrochemical oxidation of ascorbic acid in acidic buffer solutions. The overpotential for ascorbic acid oxidation is decreased by 150 mV at pH 2.2 compared with reaction at bare platinum; and an increase in anodic current and decrease in cathodic current for the redox reaction of ferrocene occurs on addition of ascorbic acid to the solution. The ferrocene-modified electrode is useful for the voltammetric determination of ascorbic acid in natural fruit juices. The advantages result from the electrocatalytic effect and from the prevention of adsorption of inhibitory substances from solution.     

Cyclic voltammetry and electrochemical quartz crystal microbalance studies of a rhodized platinum electrode in sulfuric acid solution

The adsorption properties of a rhodized platinum electrode in sulfuric acid were studied through simultaneous cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) measurements. The processes occurring at the Rh surface during the potential scan between 0.04 and 1.3 V were analyzed in terms of the changes in mass and charge. The apparent molar masses of the species adsorbed on Rh in different potential regions were determined from the mass–charge correlation. The results obtained suggest that the desorption of hydrogen UPD initially occurs in conjunction with the incorporation of mass due to the adsorption of bisulfate/sulfate ions between 0.04 V and 0.12 V; the apparent molar mass (M ₁) in this potential region is consistent with that of bisulfate ion, assuming that one sulfate species occupies the site originally occupied by five H atoms. Between 0.12 V and 0.20 V, the apparent molar mass, M ₂, is less than M ₁. This result may be associated with a structural rearrangement of the adsorbed sulfate species. The apparent molar mass, M ₃, was determined in the potential region of 0.20–0.56 V; the value obtained for M ₃ suggests that the adsorbed species are hydrated bisulfate ions (HSO₄⁻·4H₂O). Additionally, it was found that Rh oxides formed at the surface between 0.56 V and 1.3 V could not be directly identified from the correlation between charge and mass.     

Cyclic voltammetric studies on gallium film electrodes in alkaline media

Using cyclic voltammetric techniques the electrochemical behaviour of gallium in alkali media has been investigated. Three anodic peaks, one cathodic peak, and a secondary anodic peak—an observation hitherto unreported—are obtained. Detailed analyses of these various stages of oxidation and reduction based on their peak potential and peak current characteristics have revealed the nature of the underlying electrochemical reactions.     

Electrochemical nanogravimetric study of the electropolymerization of 6-aminoindole and the redox transformations of the polymer formed in aqueous media

The electrochemical quartz crystal nanobalance was employed to study the electropolymerization of 6-aminoindole on gold electrodes in acidic media. Potentiostatic or potential cycling electrooxidation of 6-aminoindole below 0.5 V vs. saturated calomel electrode leads to the formation of multilayer polymeric films, while at higher positive potentials, further oxidation takes place resulting in a different material which remains attached to the metal surface but shows decreased or no redox activity. A mechanism of the redox transformations of poly(6-aminoindole) which involves protonation–deprotonation accompanying the electron transfer is suggested. In this potential range, a yellow-green reversible color change occurs. At higher positive potentials blue-purple, indigo-type compounds are formed, and bond-breaking leads to the decrease of the electroactivity.