Additive differential pulse voltammetry,instead of double differential pulse voltammetry

A new electrochemical double pulse potential technique called additive differential pulse voltammetry (ADPV) is proposed. This technique is inspired by the original idea of Birke et al. [Anal. Chem. 53 (1981) 852] of recording two differential pulse (DP) voltammograms and it consists of plotting the sum of these two signals versus the first pulse potential, although in this paper the proposal is to obtain the ADPV signal through just one experiment. ADPV behaves in an identical way to the triple-pulse technique double differential pulse voltammetry (DDPV) for reversible processes when diffusion coefficients are equal for spherical electrodes and for any value of diffusion coefficients in planar electrodes. In the case of reversible electrode processes with amalgamation of reaction product or other more complex processes, ADPV is more advantageous than DDPV. This is due, among other reasons, to the fact that, under these conditions, a double potential step is much simpler to analyse than a triple potential step.     

Voltammetry at microcylinder electrodes: Part I. Linear sweep voltammetry

An expression for reversible linear sweep voltammograms at stationary microcylinder electrodes is presented. From numerical calculations theoretical voltammograms are obtained for various values of the dimensionless parameter, p=(nFa²v/RTD)^{$\text{1}\text{2}$}, where a is the radius of the electrode, v the potential sweep rate and D the diffusion coefficient. The peak current and the peak potential are evaluated from the voltammograms as functions of p and are expressed by approximate equations with high precision. In order to examine the validity of the equations, an experimental study was made at platinum wire micro-electrodes (a=10?10) μm). The experimental voltammograms were in good agreement with the ones predicted theoretically for various values of the sweep rates and for several different radii of the electrodes.     

Electrochemical behaviour of tilmicosin at the hanging mercury drop electrode

The electrochemical behaviour of tilmicosin (TIM) was investigated using cyclic voltammetry and square-wave voltammetry in Britton-Robinson buffers (pH 2–12). The voltammograms of TIM showed one-irreversible peak which attributed to reduction of the >C=C< group in the entire pH values. However, an additional peak was shown over the pH range 2.00 to 4.00. Its peak potential is more positive potential from that of >C=C< group, and its potential was not practically dependent on the pH. On the other hand, the peak potential of >C=C< group was shifted to the more negative values with increasing pH. According to the obtained voltammetric data, the probable reaction mechanism for the reduction of TIM was proposed. Moreover, the adsorption of TIM on the mercury electrode surface by means of square-wave adsorptive stripping voltammetry measurements is studied at different pH values. The peak current variations with the deposition time and TIM concentration were observed.     

Chain fragmentation under liquid-state and solid-state conditions

Thermally Activated fragmentation of copolycarbonates PC(T_xA_1-x) of bisphenol A (unit CA) and the Heat-sensitive diol 1,1,2,2-tetraethyl-1,2-di-(p-hydroxy) phenylethane (unit CT) was studied in the bulk, i.e., in the pure copolymers and in their blends with the polycarbonates of bisphenol A (PCA) or tetramethyl bisphenol A (TMPC). Fragmentation proceeds via dissociation followed by disproportionation at the central C? C bond of the unit CT. The reaction has rates that are convenient to study near the glass transition temperature. The “chemical” time constants τ for the entire reaction and τ₂ for the disproportionation step compete with the “physical” time constants δ_α for segmental motion and δ_q for fragment diffusion. A cage effect is observed below τ₂ = δ_α and effects of delayed matrix response below τ = δ_α and τ = δ_q. Owing to the two latter effects, parameters such as the glass transition temperature and the structure factor of concentration fluctuations do not respond primarily to the fragmentation, but rather to subsequent relaxation and diffusion processes in the polymer matrix.     

Faradaic electrochemistry at microcylinder,band, and tubular band electrodes

Current-time relationships of faradaic processes at microcylinder, band, and tubular band electrodes have been evaluated. Microcylinder electrodes were fabricated from platinum wires (5 μm radius) sealed in glass capillaries. Band and tubular electrodes were constructed with platinum sheets (～ 20 μm width) or thin pieces of graphite (～ 5 μm width) sealed between insulating mateials. The temporal response of the current at a microcylinder electrode for the reduction of ferricyanide in aqueous potassium chloride solutions is in excellent agreement with that predicted by equations derived for heat flux to a cylinder. An estimation of the magnitude and temporal properties of the measured current at a band electrode can be obtained when a hemicylinder geometry is assumed. The current respone is identical at band and tubular band electrodes even for the smallest tubular radius investigated, 0.54 mm. Cyclic voltammograms at electrodes of all three geometries show significant contributions from radial diffusion at slow scan rates (< 20 mV s^?1). The current at a graphite tubular band electrode was found to be independent of flow of solution through the electrode at flow rates up to 3 ml min^?1.     

Improving Limits of Detection. Microdisc versus Microcylinder Electrodes

The specific apparent capacitance (Farads per unit geometric area) of commercial carbon microdisc electrodes and in‐house fabricated carbon microcylinders is studied through the use of cyclic voltammetry. Home‐made microcylinder electrodes exhibit lower specific apparent capacitances and as a result facilitate lower limits of detection. We further demonstrate practically how the electroanalytical limit of the detection of solution phase trace oxygen is significantly improved using such microcylinder electrodes. Finally we show that after the degassing of a solution by nitrogen purging even in the most commonly encountered ‘best’ case scenarios approximately tens of μM of oxygen remain present in the electrochemical cell, unobservable by the commercial microdisc electrodes, but detectable with microcylinder electrodes, testifying to their analytical superiority.     

Superparamagnetic blocking and superspin-glass freezing in ultra small δ-(Fe(0.67)Mn(0.33))OOH particles

The magnetic properties of ultra-small (~2 nm) δ-(Fe(0.67)Mn(0.33))OOH nanoparticles prepared by a microemulsion technique have been investigated by magnetization and ac susceptibility measurements at variable frequency. The results provide evidence of two different magnetic regimes whose onset is identified by two maxima in the zero-field-cooled susceptibility: a large one, centered at ~150 K (T(mh)), and a narrow one at ~30 K (T(ml)). The two temperatures exhibit a different frequency dependence: T(mh) follows a Vogel-Fulcher law τ = τ(0)exp[(E(a)/k(B))/(T-T(0))], indicating a blocking of weakly interacting nanoparticle moments, whereas T(ml) follows a power law τ = τ(0)(T(g)/T(mν)-T(g))(α), suggesting a collective freezing of nanoparticle moments (superspin-glass state). This picture is coherent with the field dependence of T(ml) and T(mh) and with the temperature dependence of the coercivity, strongly increasing below 30 K.     

Application of an amperometric sensor to in-line monitoring of pulp bleaching with chlorine dioxide

Chlorine dioxide is replacing chlorine as the active compound in pulp bleaching in order to reduce the amount of chlorine used in the process and hence also in the waste waters. In bleaching with chlorine dioxide part of the effective bleaching chemical is usually chlorite. The electrochemistry of chlorine dioxide and chlorite at solid electrodes was studied by cyclic voltammetry at different pH values. The observed voltammograms indicated that reduction of chlorine dioxide gives chlorite and oxidation of chlorite gives chlorine dioxide. Both voltammograms were well developed, indicating a reversible process. Both platinum and glassy carbon were used as the working electrode. The dependence of the limiting current of chlorine dioxide and chlorite on pH was studied at both electrodes. The method was tested in the chlorine dioxide bleaching stage D1 in a typical bleaching process. A good correlation was found between the concentrations of chlorine dioxide and chlorite measured by the in-line amperometric method and a standard titrimetric method.     

Picosecond laser photolysis study of the photocycloreversion of heterocoerdianthrone endoperoxide

The formation of heterocoerdianthrone (HCD) following laser pulse photolysis of its endoperoxide (HCDPO) was observed by laser-induced fluorescence. The rise time of HCD was determined to be τ_r = 40±10 ps. The lifetime of the S₃(ππ^*) state of HCDPO, from which cycloreversion occurs, was estimated by steady-state fluorescence measurements to be τ < 3 ps. These results indicate photocycloreversion of HCDPO as a two-step reaction.     

Electrochemical behavior of methyl viologen at graphite electrodes modified with Nafion sol–gel composite

Electrodes were prepared by spin-coating spectroscopic graphite rods with a Nafion doped sol. Coating solutions consisting of Nafion:TEOS (tetraethoxysilane) ratios of 3:1 and 4:1 gave smooth films on the electrode surface. These modified electrodes were evaluated and compared with Nafion modified and bare spectroscopic graphite electrodes using methyl viologen (MV²⁺) as a representative cationic electroactive probe. Substantial partitioning of MV²⁺ into the Nafion:sol–gel matrix to the electrode surface was observed by cyclic voltammetry and square wave voltammetry. Cyclic voltammograms of MV²⁺ in 0.1 M NaCl at Nafion:sol–gel 4:1 modified electrodes showed a reversible reduction to MV⁺ with E^0′=−0.695 V vs. Ag/AgCl. Results of scan rate variation showed the wave to be characterized by semi-infinite diffusion for scan rates in the range 50–500 mV/s. Slowing the scan rate below 50 mV/s resulted in a transition to thin-layer behavior. MV²⁺ partitioned much more quickly into the sol–gel-Nafion modified electrodes compared to pure Nafion modified electrodes. Reversibility of the MV²⁺-loaded modified Nafion-doped sol–gel coatings on electrodes was obtained by soaking in 1 M NaCl solution. Concentration calibration plots for MV²⁺ at the sol–gel-Nafion modified electrodes were nonlinear. Substantial enhancement of current signal at low concentrations was observed by square wave voltammetry.     

The i-E Equations of Steady-state Voltammograms at Microdisk Electrodes

Based on the theories of conventional electrodes, as well as the properties of microdisk electrode, the i-E equations for chronoamperometry at disk microelectrode for reversible, quasi-reversible and irreversible systems are derived. Steady-state voltammograms for the oxidation of [Fe(CN)6]4- , Fe2+ and ascorbic acid were measured at a series of microdisk electrodes with different radii. The conventional log-plot shows that oxidations of [Fe(CN)6]4- and ascorbic acid are reversible and totally irreversible, respectively, but the oxidation of Fe2+ is reversible at larger radius microdisk electrodes and quasi-reversible at smaller radius microdisk electrodes. The application of the log-plot to the voltammograms yielded a straight line, its slope allows us to evaluate the charge transfer coefficient and the intercept gives values of the electron transfer rate constant.     

Pulse Voltammetric Determination of Thiazolidine-4-Carboxylic Acids

Abstract

The electrochemical behaviour of thiazolidine-4-carboxylic acid (thioproline, Thz) and its derivatives 2-propyl-thiazolidine-4-carboxylic acid (PrThz) and 2-tetrahydroxybutyl-thiazolidine-4-carboxylic acid (AThz) has been studied by cyclic, normal and reverse pulse, differential pulse and Osteryoung square wave voltam-metrics. Anodic waves, resulting from the formation of adsorbed mercury thiazolidates, were obtained for all the compounds at mercury electrodes in acetate and phosphate buffers at pH from 3.8 to 8. the initially formed mercury compounds rapidly transform into the final products accumulating on the electrode surface. the oxidation products inhibit the anodic processes and distorted voltammetric curves were obtained at the covered electrodes. Various effects of these processes in various pulse voltammetric techniques were evident. Normal pulse voltammograms were least affected by the product transformations. Osteryoung square wave voltammetry permitted very fast determinations of Thz derivatives.     

Screen‐Printed Disposable Sensors Modified with Bismuth Precursors for Rapid Voltammetric Determination of 3 Ecotoxic Nitrophenols

This work reports the application of screen‐printed electrodes bulk‐modified with bismuth precursors to the voltammetric determination of 2‐nitrophenol (2‐NP), 4‐nitrophenol (4‐NP) and 2,4‐dinitrophenol (2,4‐DNP) in water samples. A bismuth film was formed at the electrode surface via in situ reduction of the precursor compound contained in the electrode matrix by cathodic polarization at ?1.20 V. The formation of bismuth layer at the precursor‐modified electrodes was assessed by cyclic voltammetric (CV) at different pH values and by optical techniques. The target nitrophenols were voltammetrically determined by recording their reduction peaks in the differential pulse (DP) mode. The composition and content of the precursor compounds in the printed ink and the effect of the pH of the supporting electrolyte on the DP reduction currents of the 3 target nitrophenols were studied. The limits of quantification (LOQs) in three water matrices (distilled water, tap water and surface water) were in the range 1.1–2.2 µmol L^?1_. Using a simple solid‐phase extraction (SPE) procedure with Lichrolut EN cartridges and elution with methanol, a preconcentration factor of 100 was achieved; the LOQs were 0.021, 0.027 and 0.025 µmol L^?1 for 2‐NP, 4‐NP and 2,4‐DNP, respectively. The recoveries of samples spiked with the 3 target nitrophenols at two concentration levels (0.04 and 0.1 µmol L^?1) were always >87 %.     

2,2'-联吡啶In(Ⅲ)配合物与DNA作用的电化学研究

应用循环伏安法、微分脉冲伏安法和交流阻抗法研究了配合物In(bpy)Cl3.H2O与DNA在Tris-HCl缓冲溶液(pH=7.2)中的相互作用.结果表明:配合物中心In(Ⅲ)离子的循环伏安曲线上呈现一对准可逆的氧化还原波,DNA与配合物作用后,配位中心离子的氧化还原峰电流明显降低,扩散系数减小,电化学反应阻抗增大,式量电位负移,表明该配合物与DNA的作用方式为静电结合.     

Multiple square wave voltammetry of nanomolar and subnanomolar concentrations of europium (III) at polymer-coated electrodes

The ion-exchange voltammetric determination of europium(III) at polymer-coated electrodes is examined. The comparison between the ion-exchange cyclic voltammetric behaviour at glassy carbon electrodes, modified with the poly(estersulfonate) Kodak AQ55 and Nafion, indicates a better suitability of the latter. The use of multiple square wave voltammetry (MSWV) results in a dramatic improvement of the signal-to-noise ratios of the voltammograms recorded at the modified electrodes. Eu (III) determinations by MSWV at Nafion-coated electrodes are characterised by detection limits 2–3 orders of magnitude lower than those obtained with differential pulse voltammetry and the rare earth cation can be analysed at nano- and subnanomolar levels, depending on the experimental conditions adopted.     

Light-intensity dependence of the in vivo fluorescence lifetime of chlorophyll

Abstract— Phase-fluorometer measurements of the fluorescence lifetime, τ, from chlorophyll in Chlorella, Bishop's 8 and 11 Scenedesmus mutants, sugarbeet leaf and chloroplast fragments demonstrate that: τ is independent of modulation frequency at 27 and 14 mc. in the experimental-wavelength range from 650 to 735 nm (with blue or blue-green excitation); with Chlorelfa and chloroplast fragments τ rises hyperbolically with intensity to τ_max about 2 nsec and 0·7 nsec respectively; DCMU poisoned Chlorella and sugarbeet leaf as well as the mutants have τ values near 2 nsec; the lifetime-incident intensity relationship for Chlorella and chloroplast fragments is quantitatively similar to the incident-intensity dependence of fluorescence yield and oxygen evolution and thus supports the hypothesis that these three measuring variables are controlled by the concentration of ‘open’ trapping systems; τ is independent of emission wave-length to suggest that fluorescence is dominated by a single chlorophyll species. The reaction velocity-lifetime correlation indicates that fluorescence behavior is directly controlled by system II.     

Voltammetric behavior and determination of tocopherols with partial least squares calibration: analysis in vegetable oil samples

The voltammetric behavior of Vitamin E in the presence of olive oil is studied at the glassy carbon electrode, in a hexane-ethanol medium, with diverse techniques: sampled DC, differential pulse, and square-wave voltammetry. The influence of such variables as the hexane-ethanol proportion, sulfuric acid concentration, and instrumental parameters is studied. Separate voltammetric peaks are obtained for α-tocopherol and δ-tocopherol, but the peaks for β-tocopherol and γ-tocopherol overlap. For the simultaneous determination of α-, β+γ-, and δ-tocopherols in vegetable oils by the PLS-1 multivariate calibration method, the results using sampled DC and DPV voltammograms are compared. The DPV voltammograms are found to be the best data set. The proposed method is applied to the determination of the tocopherols in different vegetable oil samples. The olive oil samples needed a prior cleaning stage by solid-phase extraction on silica cartridges. The results are very acceptable.     

Voltammetry at microcylinder electrodes: Part III. Chronopotentiometry

Expressions for chronopotentiometry at stationary microcylinder electrodes are presented. The transition time, τ, is expressed as a function of a single parameter λ ( = nFc^*_RD / ia, where a is the radius of the electrode, D is the diffusion coefficient, i is the current density, n is the number of transferred electrons, F is the Faraday constant and c^*_R is the bulk concentration. When the values of λ are small, the transition-time constant, i√τ / c^*_R, depends linearly on λ, with the intercept predicted from the Sand equation. Conversely, when values of λ increase, the transition time also increases exponentially. Therefore transition necessarily occurs no matter how small a current flows. An approximate equation for the transition time is presented, from which one can evaluate the diffusion coefficient. Equations for the potential-time curve and the quarter-wave potential are also obtained. The equations were tested experimentally using carbon fiber electrodes (a = 4.1 μ m) and platinum wire electrodes (a = 10-100μ m). The transition times obtained experimentally were in good agreement with those predicted theoretically for various values of the applied current, for several different radii of the electrodes.     

Electrocatalytic Reduction of Chromium(VI) by Thionin: Electrochemical Properties and Mechanistic Study

The electrochemical properties of aqueous thionin (an electroactive water soluble dye) of pH 1–12 were investigated by cyclic voltammetry at a boron doped diamond(BDD) electrode. A well defined reversible redox couple was observed in acidic, neutral and alkaline solutions. The standard potential and kinetic parameters for thionin were obtained by fitting experimental cyclic voltammograms to those generated by the DigiSim program. The electrogenerated reduced form of thionin has been used as an efficient organic catalyst for the reduction of Cr(VI) at a BDD electrode immersed in aqueous media. The cyclic voltammetry measurements indicate that an electrocatalytic process occurs, where electrochemically generated thionin reduced species (Leucothionin) is oxidized by Cr(VI) back to the parent thionin species via a EC' reaction mechanism. The determination of catalytic rate constant (K_cat) was accomplished again by fitting experimental cyclic voltammograms with simulated ones.     

铜(Ⅱ)邻菲咯啉蛋氨酸配合物与DNA相互作用的研究

在pH=6．86磷酸盐缓冲溶液中，采用电化学(循环伏安法、微分脉冲伏安法和 交流阻抗及数据拟合技术)、粘度测定、电子吸收光谱和溴化乙锭(EB)荧光分析法 研究了[Cu(phen)(H2O)(L—Met)]^+(phen＝1，10-邻菲咯琳，L—Met=L-蛋氨酸)与 小牛胸腺DNA的相互作用。结果发现中心铜离子在循环伏安图上呈现1对明显的准可 逆氧化还原波。当加入一定量的DNA时，配合物的氧化还原峰电流明显降低，扩散 系数减小，电化学反应电阻增大，电子光谱的最大吸收峰明显红移，产生明显的减 色效应，同时，配合物也能较大程度地猝灭EB-DNA体系的荧光，说明[Cu(phen) (H2O)(L—Met)]^+与DNA的作用较强，作用模式为部分插入作用。