Bromate Anion Reduction at Rotating Disk Electrode in Steady State under Excess of Protons: Numerical Solution of the Convective Diffusion Equations at Equal Diffusion Coefficients of Components

Process of bromate anion reduction at rotating disk electrode in steady state occurring owing to catalytic cycle is analyzed theoretically. The cycle consists of bromine/bromide reversible redox-couple and comproportionation irreversible reaction. Because of the cycle autocatalytic character (EC''-mechanism: Electrochim. Acta, 2015, 173, 779), the passing current can be enormously large at bromate high bulk concentration (up to the bromate limited diffusion current to electrode surface) even when the bromine concentration in the bulk solution is negligibly small. Unlike the previous theoretical studies of the problem (Electrochim. Acta, 2015, 173, 779; Doklady Chemistry, 2016, 468, 141; Russ. J. Electrochem., 2016, 52, No. 10, 925), in this work the component concentration distributions during the process at a prescribed value of the passing current are calculated on the basis of the convective diffusion equations for bromine and the bromate and bromide anions for the first time. Under the assumption that the component diffusion coefficients are equal, exact interrelations between these concentration profiles are derived, which allows reducing the problem to the solving of nonlinear equation of second order for the bromide concentration with boundary conditions at the electrode surface and in the solution bulk. Thus, obtained concentration profiles of all components for a corresponding set of current densities are used to calculate of steady-state polarization curves, as well, as the maximal current dependence on the rotating disk electrode velocity.     

Identifying Diffusion Limiting Current to Unravel the Intrinsic Kinetics of Electrode Reactions Affected by Mass Transfer at Rotating Disk Electrode

Rotating disk electrode systems are widely used to study the kinetics of electrocatalytic reactions that may suffer from insufficient mass transfer of the reactants. Kinetic current density at certain overpotential calculated by the Koutecky-Levich equation is commonly used as the metrics to evaluate the activity of electrocatalysts. However, it is frequently found that the diffusion limiting current density is not correctly identified in the literatures. Instead of kinetic current density, the measured current density normalized by diffusion limiting current density (j/j_L) has also been frequently under circumstance where its validity is not justified. By taking oxygen reduction reaction/hydrogen oxidation reaction/hydrogen evolution reaction as examples, we demonstrate that identifying the actual diffusion limiting current density for the same reaction under otherwise identical conditions from the experimental data is essential to accurately deduce kinetic current density. Our analysis reveals that j/j_L is a rough activity metric which can only be used to qualitatively compare the activity trend under conditions that the mass transfer conditions and the roughness factor of the electrode are exactly the same. In addition, if one wants to use j/j_L to compare the intrinsic activity, the concentration overpotential should be eliminated.     

Oxygen Reduction Reaction on a Rotating Ag/GC Disk Electrode in Acidic Solution

Finding an appropriate mechanism of oxygen reduction reaction, without having kinetic parameters and intermediate species at hand, is an impossible task. In view of the fact that this reaction is one of the important reactions, it was tried to overcome this problem through computing the kinetic parameters by means of Ag/GC rotating disc electrode in an acidic solution. Using this electrode, two values were obtained for electron transfer coefficient and reaction order was calculated through relative equations. Based on this information, an ECE (electrochemical chemical electrochemical) mechanism has been proposed.     

Activationless Reduction of the Hexacyanoferrate Anion on a Mercury Electrode

Polarization curves for the electroreduction of [Fe(CN)₆]^3– on a mercury electrode in solutions containing different amounts of surface-inactive supporting cations are simulated on the basis of modern theory of charge transfer in polar media combined with quantum-chemical approaches. The conclusions about an activationless nature of the process in the overvoltage range 1.2 to 2.0 V accessible experimentally, which were drawn from the results of earlier calculations made within simpler models, are confirmed. The maximum contribution to the current is shown to be made by energy levels of metal that lie considerably (up to 1 eV) lower than the Fermi level. To establish the reasons for the anomalous behavior of the current in the activationless region at high overvoltages, the effect various factors sensitive to the electrode charge exert on the model curves at high negative charges of the electrode surface is analyzed. In connection with this, the stability of the results of a calculation of the electron overlap metal/reactant to a model of the interface is considered. The plausibility of the model proposed for the reaction layer and the approaches used for computing the activation energy and the preexponential factor is corroborated by good agreement between the temperature effect found for the region of a minimum current and its experimental value.     

Macrodistribution of Anodic Dissolution Rate at a Rotating Disk Electrode with Partially Insulated Surface

Macrodistribution of etching rates of a macroscopically nonuniform, partially dipped, rotating disk electrode of low-carbon steel in 1 M NaCl at pH 1 with photoresist insulation and the density of active rectangular holes being 1.6 holes/mm² is studied at dimensionless etching rates i ⁰ _avg /i _l = 0.2–1.1, where i ⁰ _avg is the average current density per active surface and i _l the limiting anodic current density determined by the ionic transport rate. The distribution of the average etching rates in the normal direction is determined by the primary current distribution and is insensitive towards the type of the dissolution rate distribution in a cavity.     

Intensification of the Nitrate Anion Reduction on a Membrane Palladium Electrode

Polarization curves of electroreduction of nitrate anions in sulfuric acid aqueous solutions on a Pd/Pd palladium electrode with the opposite side kept at different constant potentials in the region of Pd–H -phase formation are compared. The reduction rate of nitrate anions is shown to be substantially higher on a membrane electrode, provided the conditions of hydrogen supply from the membrane's opposite side to the reaction zone are realized. This phenomenon is caused by the reduction of a chemisorbed intermediate (a certain N(III) form) by hydrogen diffused through the membrane. It is shown that the measurements of hydrogen diffusion currents through a membrane can be used in plotting hydrogen sorption isotherms in the Pd–H -phase.     

Applicability of the Levich Equation for a Two-Phase Solution in the Rotating Disk Electrode System

The mass-transter characteristics of a two-phase system formed by mixing a fixed quantity (20 v/v%) of organic solvent with an aqueous ferricyanide electrolyte solution in an RDE system were studied. The Levich eqution, I₁ = 0.62 nFAC_bD^2/3v^?1/6_ω^1/2, was found to be applicable to the two-phase system with only a minor modification in the angular velocity (ω) at Reynolds numbers between 3–5 × 10⁴. The experimental results indicate that the interfacial tension is the most important variable for the two-phase system. One group of organic solvents with smaller interfacial tension, such as benzene or toluene, needs a modification of the Levich equation by replacing the observed angular velocity (ω_o) with the true angular velocity (ω_t) which was observed to be 1.1 times the observed angular velocity. For the other group with larger interfacial tension, such as n-hexane or cyclohexane, there is no need to modify the observed angular velocity. In other words, the Levich equation may be expressed as I₁ = 0.65 nFAC_bD^2/3v^?1/6ω^1/2 for two-phase solution if the interfacial tension is smaller than 37.0 dyne/cm.     

旋转玻碳电极上二茂铁的电化学阻抗行为及其与DNA的相互作用

应用旋转圆盘电极和电化学阻抗法研究了二茂铁在Tris-NaC l(pH=7.2)缓冲溶液中于旋转玻碳电极上的电化学阻抗行为及其与DNA的相互作用.结果表明,二茂铁于旋转电极的伏安曲线呈现明显的极限电流平阶,而其交流阻抗谱则出现两个电容弧.二茂铁与DNA的作用,若受扩散过程控制则其极限扩散电流随DNA浓度增大而减小,而在电化学控制过程中则表现为电化学反应电阻随DNA浓度增大而增大.根据旋转圆盘电极和电化学阻抗谱测试,表明由这两种方法数据拟合求得的二茂铁条件电位速率常数能够很好地相互吻合,但如存在DNA时,则其条件电位速率常数有一定程度的减小.     

白杨素在汞电极上的电化学行为及其消除超氧阴离子自由基能力研究

The electrochemical behavior of chrysin in pH 2.0-9.0 Britton-Robinson （B-R） buffer solutions was studied by the means of linear sweep voltammetry and cyclic voltammetry at a static mercury drop electrode. In different pH range of B-R buffer solutions, chrysin could cause four reduction waves. In pH 2.0-5.8 B-R buffer solutions, wave P1 yielded by chrysin is a one-electron reduction wave, and wave P1 caused by further reduction of the products of wave P1 in pH〈3.0 B-R buffer solution is also a one-electron reduction wave. But in 3.0〈pH〈5.8 B-R buffer solution wave P1 was overlapped by the hydrogen wave. Between pH 5.8 and 9.0, chrysin could yield two reduction waves P2 and P3- The former is an irreversible adsorptive wave of ionized chrysin involving one electron and the latter is also an irreversible adsorptive wave of reduction intermediate radical of chrysin involving one electron and one proton. And a linear relationship between ip3 and the concentration of chrysin can be established from 1.0×10^-6 to 4.0×10^-5 mol·L^-1 （r=0.9924） with the detection limit of 5×10^-7 mol·L^-1. In addition, the antioxidant ability of chrysin was investigated by linear sweep voltammetry （LSV）. The determination result of IC50 of chrysin showed that chrysin is a good antioxidant.     

磷钼杂多酸修饰电极催化还原测定IO_3~-与BrO_3~-的液相色谱电化学研究

本文介绍了1:12磷钼杂多酸(PMo_(12))修饰电极的制备及其对IO_3~-、BrO_3~-的催化还原作用。本文还研究了在涂敷十六烷基三甲基溴化铵的C_(18)键合固定相上IO_3~-与BrO_3~-的分离方法,并采用修饰电极色谱电化学的方法对IO_3~-与BrO_3~-进行定量测定。IO_3~-、BrO_3~-的量分别在8.0×10(-10)～1.0×10~(-7)mol和1.6×10~(-9)～3.0×10~(-7)mol范围内与峰高呈良好的线性关系,检测限分别为4.0×10~(-10)mol与1.0×10~(-9)mol。用本方法测定IO_3~-和BrO_3~-重现性好、线性范围大,是测定IO_3~-与BrO_3~-的一种实用方法。     

Enhancement of Bioelectrocatalytic Processes by the Rotation of Mediator‐Functionalized Magnetic Particles on Electrode Surfaces: Comparison with a Rotating Disk Electrode

The rotation of redox‐functionalized magnetic particles (MPs) by means of an external magnet is a common practice for enhancing bioelectrocatalytic processes and for the amplification of biosensing events. The current densities generated by rotating redox‐functionalized MPs in two bioelectrocatalytic systems are compared to the current densities generated by rotating disc electrodes (RDE) functionalized with similar redox functionalities. The bioelectrocatalytic systems consist of pyrroloquinoline quinone (PQQ)‐functionalized MPs that oxidize NADH, and ferrocene‐functionalized MPs that mediate the bioelectrocatalyzed oxidation of glucose in the presence of glucose oxidase. The results reveal that only ca. 1% of the area of the redox‐functionalized MPs are electrically contacted with the electrode. Also, the current densities generated by the rotating MPs at high rotation speeds are lower than theoretically expected, presumably due to lose of electrical contact between the MPs and the electrode, and incoherent rotation of the particles on the electrode, due to insufficient magnetization. The comparison of the current densities in the bioelectrocatalytic systems in the presence of the rotating redox‐functionalized MPs to the analogous RDE systems allows us to elucidate the kinetics of electron transfer at the redox‐active MPs.     

银在玻碳旋转圆盘电极表面的化学镀分离研究

以水合肼为还原剂,在2.25 mol/L NH3-0.56 mol/L HAe(pH 10～10.5)的基本镀浴中,用化学镀方法将镀浴中的微量Ag 离子选择性地沉积于玻碳旋转圆盘电极表面,从而将Ag 与其它共存金属离子分离.用开路电位-时问谱技术(OCP-t)、循环伏安法(CV)和微分脉冲伏安法(DPASV)表征了该化学镀分离银的机理和效果.证明在多种金属离子和常见阴离子共存的复杂溶液体系中,可将银进行有效分离;电极表面化学镀富集银量的OCP-t曲线与镀浴中还原剂NH2NH2的浓度在0.1～1.5mol/L范围内有线性关系;而CV法溶出峰电量与镀浴中Ag 浓度在1～10 mmoL/L范围内呈线性关系.电极表面富集的银溶出峰电流与Ag 浓度在6～20μmol/范围内有很好的线性关系.该方法已应用于粗铅样中银的还原、分离和测定,分析结果与电感耦合等离子体发射光谱法(ICP/AES)基本一致.     

电极扩散层组分分析取样技术的研究

以直径小于１００微米的毛细管作取样管,以负压和虹吸为动力将电极扩散层内的溶深吸取出来,采用电位法离线检测,通过测量电极扩散层内的Ｃｕ＾２＋浓度,对各种取样方式进行了实验评价。获得阴极扩散层Ｃｕ＾２＋浓度较本体溶液低３０％,阳极扩散层内Ｃｕ＾２＋浓度较本体溶液高１６０％的结果,证明了方法的可行性,为毛细管电泳分析及其它分析方法测量扩散层内各物质浓度及形态建立了基础。     

Performance of a Bismuth Bulk Rotating Disk Electrode for Heavy Metal Analysis: Determination of Lead in Environmental Samples

The bismuth bulk electrode is proposed here for the first time in the rotating configuration (BiB‐RDE) as the electrode of choice for voltammetric analysis of selected heavy metal ions. Optimization of chemical and instrumental parameters was carried out to develop a reliable and convenient method for the determination of Zn(II), Cd(II) and Pb(II) by SWASV. Appropriate detection limits were found for environmental monitoring applications in the medium – low µg/L range. The method was validated for Pb(II) determination by certified reference materials. Successful application to the determination of Pb(II) in samples of fortified rainwater and sewage sludge from a steel industry is described.     

离子液体BMImBF4中硝基甲烷在铜盘电极上的电化学行为

以铜盘电极为工作电极, 采用循环伏安和原位红外反射光谱技术研究了硝基甲烷在离子液体1-正丁基-3-甲基咪唑四氟硼酸盐(BMImBF4)中的电还原行为, 讨论了温度和扫描速度等因素对其电还原行为的影响. 研究结果表明, 硝基甲烷在离子液体BMImBF4中的还原反应是受扩散控制的不可逆过程, 其还原产物为亚硝基甲烷二聚体.     

The Influence of Viscosity on Diffusion at the Stationary Electrode

Abstract

The influence of viscosity on diffusion at the stationary electrode is studied. Relations between the maximum electrolysis time attainable without any disturbance by natural convection, the thickness of the diffusion layer and solution viscosity are presented and discussed in the case of constant current electrolysis at shielded electrodes. A re-examination of data existing in literature is made.     

旋转铂盘电极上Cu(phen)22+与6-巯基嘌呤的相互作用

在 Tris-NaCl(pH=7.2)缓冲溶液中,应用循环伏安法、微分脉冲伏安法、 旋转圆盘电极实验、交流阻抗法及其数据模拟等技术研究了Cu(phen)22+（phen=1,10-邻菲咯啉）与6-巯基嘌呤（6-MP）的相互作用.结果显示, Cu(phen)22+与6-MP无论在扩散控制过程或电化学控制过程都发生了相互作用. Cu(phen)22+及其与6-MP的作用产物于铂电极上均呈现一对氧化还原峰,但后者呈现的氧化还原峰负移,峰电流减小，交流阻抗结果显示,无论6-MP存在与否, Cu(phen)22+在交流阻抗谱上均呈现两个清晰的电容弧,但当6-MP存在时,电化学反应电阻和电化学吸脱附电阻均增大. Cu(phen)22+在不同转速下的阻抗拟合结果显示,随转速增大,电化学反应电阻和电化学吸脱附电阻均减小,双电层电容呈增大趋势,而吸脱附电容呈减小趋势;当6-MP存在时,仍然呈现此变化规律.     

利用K-L方程估算旋转圆盘电极体系反应动力学电流的误差来源分析

旋转圆盘电极（RDE）体系主要用于低溶解度反应物的电极过程动力学研究. 在利用RDE技术研究不可逆电极反应动力学时,人们常利用Koutecky-Levich方程排除传质的影响,从总电流估算反应的动力学电流. 由于K-L方程是建立在系统满足稳态扩散模型的基础上,实际运用时如果体系偏离稳态扩散,就有可能对估算的动力学参数造成很大误差. 本文以氧气在多晶铂电极上的还原反应为例系统地估算了不同氧气浓度与电极转速下的误差,结果表明低氧气浓度与低圆盘转速的情况不满足稳态扩散条件,若此时仍根据K-L方程利用外推法进行分析,误差可达30%. 因此作者建议,在RDE体系中利用K-L方程估算动力学参数时,最好忽略低浓度与低转速下的数据,直接使用较高浓度与较高转速下的数据进行计算与分析.