High-performance pulse and differential pulse polarography: Part I. Theoretical considerations

A theoretical approach is given for complete elimination of the charging current caused by drop growth in modified normal and differential pulse polarography (n.p.p. and d.p.p.). Maximum sensitivity can be obtained in modified n.p.p., but the resolution of waves is better in modified d.p.p. or can be increased in differential modified n.p.p. The best overall performance can be achieved with modified d.p.p., considering sensitivity and wave resolution together. Preliminary results for the determination of chloramphenicol are described.     

聚合物自由基锂二次电池正极材料的合成与电化学性能

合成了一种聚合物自由基聚 4 甲基丙烯酸 2 ,2 ,6 ,6 四甲基哌啶 1 氮氧自由基酯 (PTMA) ,并用红外光谱 (IR)、紫外光谱 (UV)、电子顺磁共振 (ESR)等证实了PTMA的结构 .PTMA的循环伏安曲线 (扫描速度为 5mV·s- 1)显示通过阳极的氧化电量和阴极的还原电量相等且氧化峰电流等于还原峰电流 ,表明PTMA的氧化还原反应可逆性很好 .PTMA的氧化峰电位 (Ea ,p=3 6 6VversusLi Li+ )与还原峰电位 (Ec,p=3 58V)之差为 80mV ,比其它锂二次电池的有机正极材料 (如有机二硫化物 )小得多 ,因此PTMA的氧化还原反应速度比较快 .PTMA的最大放电比容量为 78 4mAh·g- 1(以 0 2C充放电 ) ,是它理论比容量 ( 111mAh·g- 1)的 70 6 % ,它的充放电曲线分别在 3 6 5V和 3 56V处有一个很平稳的平台 ,经过 10 0次充放电循环后电池的放电比容量相对于最大放电比容量只衰减了 2 % ,表明PTMA 锂扣式电池具有优良的循环稳定性 .这些研究结果显示PTMA是一种非常有发展前景的有机聚合物自由基锂二次电池正极材料     

Determination of copper in sea water by cathodic stripping voltammetry of complexes with catechol

A reduction current is obtained when an aqueous solution of copper and catechol is subjected to differential-pulse cathodic stripping voltammetry (d.p.c.s.v.) because of the reduction of copper(II)—catechol complex ions which adsorb onto the hanging mercury drop electrode (HMDE). The most likely form of the adsorbed complex ions is CuL^2?₂ (L being catechol). A.c. polarographic measurements showed that these complex ions adsorb more strongly onto the drop than free catechol ions. Monolayer adsorption density is obtained at 2.1 × 10^?10 molecules/cm², equivalent to a surface area of 78 A² complex ion, which agrees well with the molecular surface area calculated from the bond lengths. Analytically useful currents are obtained at very low metal concentrations, such as in uncontaminated sea water. The possible interference by other trace metals, major cations, and organic complexing ligands is investigated. Competition for copper ions by natural organic complexing ligands is evident at low concentrations of catechol. Analysis of the dissolved copper concentration in sea water by d.p.c.s.v. at the HMDE (at neutral pH) compares favourably with the d.p.a.s.v. technique at a rotating disk electrode (at low pH) because of the shorter collection period and greater sensitivity.     

Characterization of mineral oxide charging in apolar media

This paper presents an investigation of the charging behavior of mineral oxide particles dispersed in apolar media. There are a growing number of applications that seek to use electrostatic effects in apolar media to control particle movement and improve aggregation stability. Progress is limited, however, by incomplete knowledge of the mechanism(s) of particle charging in these systems. It has been shown in a number of cases that the acid-base properties of both the particles and the surfactants used to stabilize charge play key roles. A mechanism for acid-base charging has previously been established for mineral oxides in aqueous systems, where the surface hydroxyl groups act as proton donors or receivers depending on the pH of the surrounding solution. In water, the pH at which the surface charge density is zero, i.e., the point of zero charge (PZC), can be used to characterize the acid-base nature of the mineral oxide particles. The current work explores the possible extension of this charging behavior to apolar systems, with the key difference that the surface hydroxyl groups of the mineral oxides react with the surfactant molecules instead of free ions in solution. The apolar charging behavior is explored by measuring the electrophoretic mobility of a series of mineral oxides dispersed in a solution of Isopar-L and AOT, a neutral surfactant in water. The electrophoretic mobility of the particles is found to scale quantitatively, with respect to both sign and magnitude, with their aqueous PZC value. This provides support for the theory of acid-base charging in apolar media and represents a method for predicting and controlling particle charge of mineral oxides dispersed in apolar media.     

Reaction pathway and potential barrier for the CaH product in the reaction of Ca(4s4p1P1) + H2 --> CaH(X2Sigma+) + H

The nascent CaH product in the reaction Ca(4s4p1P1) + H2 --> CaH(X2Sigma+) + H is obtained using a pump-probe technique. The CaH(v = 0,1) distributions, with a population ratio of CaH(v = 0)/CaH(v = 1) = 2.7+/-0.2, may be characterized by low Boltzmann rotational temperature. According to Arrhenius theory, the temperature dependence measurement yields a potential barrier of 3820+/-480 cm(-1) for the current reaction. As a result of the potential energy surfaces (PES) calculations, the reaction pathway favors a Ca insertion into the H2 bond along a (near) C2v geometric approach. As the H2 bond is elongated, the configurational mixing between the orbital components of the 4p and nearby low-lying 3d state with the same symmetry makes significant the nonadiabatic transition between the 5A' and 2A' surface in the repulsive limbs. Therefore, the collision species are anticipated to track along the 5A' surface, then undergo nonadiabatic transition to the inner limb of the 2A' surface, and finally cross to the reactive 1A' surface. The observed energy barrier probably accounts for the energy requirement to surmount the repulsive hill in the entrance. The findings of the nascent CaH product distributions may be reasonably interpreted from the nature of the intermediate structure and lifetime after the 2A'-1A' surface transition. The distinct product distributions between the Ca(4 1P1) and Mg(3 1P1) reactions with H2 may also be realized with the aid of the PES calculations.     

Lithium-lithium tetrachloroaluminate reference electrode in thionyl chloride

Three in situ techniques for charging current compensation (c.c.c.) in d.c. polarography are described. These methods envisage the superposition of an a.c. ripple and analogue processing of the response to yield a quantity proportional to the d.c. charging current. The theory of these procedures outlined in this paper, enables one to approach c.c.c. without any reliance on trial and error. Details of the appropriate circuitry and supporting experimental verification of the theory are also presented.     

Chemometrics assisted resolving of net faradaic current contribution from total current in potential step and staircase cyclic voltammetry

Total current in the electroanalytical data is assumed to be consisting of three main constituents: faradaic current, step charging current and induced charging current. Both charging currents can cause an interfering effect on precise determination of faradaic currents, and hence insert direct effects on sensitivity and detection limit of the electroanalytical techniques. Despite the widespread techniques introduced until now, the extraction of the net faradaic current from total current still remains a challenge. In this work, by using multivariate curve resolution-alternating least square (MCR-ALS) as a powerful curve resolution-based chemometrics method, a straightforward method has been introduced for resolving faradaic current from the two types of charging currents (step charging current and induced charging current) in single potential step and staircase cyclic voltammetric methods. By simultaneous analyses of the current data matrices for different electrochemical systems, the three sources of current were successfully identified and their contributions in the total signal were easily calculated. Also, in this manner, the cell time constant can be obtained easily. Contrary to the previously reported methods, the present method does not need any pre-determined mathematical method; particularly there is no need to know the cell time constant.     

Ultrafast decay of superexcited c 4Σu- nlσg v=0,1 states of O2 probed with femtosecond photoelectron spectroscopy

Neutral superexcited states in molecular oxygen converging to the O(2)(+) c (4)Σ(u)(-) ion state are excited and probed with femtosecond time-resolved photoelectron spectroscopy to investigate predissociation and autoionization relaxation channels as the superexcited states decay. The c (4)Σ(u)(-) 4sσ(g) v=0, c (4)Σ(u)(-) 4sσ(g) v=1, and c (4)Σ(u)(-) 3dσ(g) v=1 superexcited states are prepared with pulsed high-harmonic radiation centered at 23.10 eV. A time-delayed 805 nm laser pulse is used to probe the excited molecular states and neutral atomic fragments by ionization; the ejected photoelectrons from these states are spectrally resolved with a velocity map imaging spectrometer. Three excited neutral O* atom products are identified in the photoelectron spectrum as 4d(1)?(3)D(J)°, 4p(1) (5)P(J)° and 3d(1) (3)D(J)° fragments. Additionally, several features in the photoelectron spectrum are assigned to photoionization of the transiently populated superexcited states. Using principles of the ion core dissociation model, the atomic fragments measured are correlated with the molecular superexcited states from which they originate. The 4d(1) (3)D(J)° fragment is observed to be formed on a timescale of 65 ± 5 fs and is likely a photoproduct of the 4sσ(g) v = 1 state. The 4p(1) (5)P(J)° fragment is formed on a timescale of 427 ± 75 fs and correlated with the neutral predissociation of the 4sσ(g) v = 0 state. The timescales represent the sum of predissociation and autoionization decay rates for the respective superexcited state. The production of the 3d(1) (3)D(J)° fragment is not unambiguously resolved in time due to an overlapping decay of a v = 1 superexcited state photoelectron signal. The observed 65 fs timescale is in good agreement with previous experiments and theory on the predissociation lifetimes of the v = 1 ion state, suggesting that predissociation may dominate the decay dynamics from the v = 1 superexcited states. An unidentified molecular state is inferred by the detection of a long-lived depletion signal (reduction in autoionization) associated with the B (2)Σ(g)(-) ion state that persists up to time delays of 105 ps.     

Theory and application of cyclic voltammetry for measurement of fast electrode kinetics at microdisk electrode

An analytical expression describing voltammetric behaviors responses at microdisk electrode for various diffusion conditions and degrees of reversibility is reported in this paper. Results of theoretical calculation made it possible to use cyclic voltammetry to evaluate heterogeneous rate constants under intermediate diffusion conditions. At relatively low scan rate, the distortion of current-voltage can be reduced due to small iR drops and charging current. The effects of transfer coefficient, a, kinetic parameter, (=k0r/4D), and switching potential, s, on potential peak separation are discussed in detail. The relationship obtained in this paper between potential peak separation and \ is in good agreement with that in Ref. 14, whose authors have got their results by using digital simulation technique. After the experiment of Fe(CN)64- oxidation, k0 and a were obtained by the theory of this paper. The result agrees with that in Ref. 19.     

电流反馈示波极谱滴定法

本文提出了电流反馈示波极谱滴定法的原理、仪器装置及操作技术。为了提高去极剂切口的灵敏度, 可将dE/dt信号转换为电流信号后再反馈回电解池以补偿充电电流,一般可将切口灵敏度提高1至5倍, 图形分辨率也较经典方法高, 从而使滴定终点的变化更为敏锐, 扩大了示波极谱滴定的应用范围, 推导了电流反馈的交流电流极化理论公式, 分析了电流反馈对示波极谱图形、去极剂切口灵敏度及切口电位的影响。     

Instrumentation for charging current compensation in d.c. polarography

It is shown that the non-linear charging current in d.c. polarography can be fairly well compensated by a method (suggested by Barker), which consists in applying a small a.c. voltage to the polarographic cell, rectifying the a.c. response, integrating it and subtracting a suitable fraction from the total d.c. response. An instrument incorporating such an automatic compensation for the nonlinear charging current is described. Results obtained with this instrument are presented to show that the limit of measureable concentrations is about 10^?7M for reversible waves and the precision is ±2.5 to ±3.0%. It is possible to obtain, with the instrument described here, well defined polarograms even when the supporting electrolyte concentration is low.     

Discrete electrostatic charge transfer by the electrophoresis of a charged droplet in a dielectric liquid

We have experimentally investigated the electrostatic charging of a water droplet on an electrified electrode surface to explain the detailed inductive charging processes and use them for the detection of droplet position in a lab-on-a-chip system. The periodic bouncing motion of a droplet between two planar electrodes has been examined by using a high-resolution electrometer and an image analysis method. We have found that this charging process consists of three steps. The first step is inductive charge accumulation on the opposite electrode by the charge of a droplet. This induction process occurs while the droplet approaches the electrode, and it produces an induction current signal at the electrometer. The second step is the discharging of the droplet by the accumulated induced charge at the moment of contact. For this second step, there is no charge-transfer detection at the electrometer. The third step is the charging of the neutralized droplet to a certain charged state while the droplet is in contact with the electrode. The charge transfer of the third step is detected as the pulse-type signal of an electrometer. The second and third steps occur simultaneously and rapidly. We have found that the induction current by the movement of a charged droplet can be accurately used to measure the charge of the droplet and can also be used to monitor the position of a droplet under actuation. The implications of the current findings for understanding and measuring the charging process are discussed.     

Flow-injection methods for the determination of uracil derivatives with voltammetric detection

Flow-injection methods are described for the determination of 18 uracil derivatives and related compounds, by means of differential-pulse amperometry (d.p.a.) or differential-pulse cathodic stripping voltammetry (d.p.c.s.v.). The carrier stream is a borax/KNO₃/HNO₃ (of NaOH) solution containing 0.001% (v/v) Triton X-100. This surfactant displaces the oxygen reduction peak to such negative potentials that deaeration is unnecessary for detection of compounds having peak potentials in the range 180–70 mV (vs. Ag/AgCI) at pH 7.6. At the hanging mercury drop electrode, the uracil derivative is deposited from the flowing sample at a fixed potential more positive than the relevant peak potential and stripped under stopped-flow or slow-flow conditions. In the amperometric mode, a constant potential also more positive than the relevant peak potential is applied to the dropping mercury electrode and the resulting peak is measured under flow conditions. Linear calibration graphs were found for most of the compounds at 10^?6–10^?7 M by d.p.a, and about one order of magnitude lower by d.p.c.s.v.. The limit of determination for 5-iodouracil was 5×10^?9 M (ca. 1.2 ng ml^?1). Separation is needed for applications to blood or urine. Simple deproteination followed by high-performance liquid chromatography with a reversed-phase column proved satisfactory. Separations of various uracil derivatives, and of 5-fluorouracil, uric acid and 5-fluorodeoxyuridine, are described; spectrophotometric and amperometric detectors were used sequentially to check performance.     

The charging of a liquid dielectric upon its flow past a flat-plate

The charging of a liquid dielectric upon its flow past a flat-plate is considered. Analytical expressions for the density distribution of electric charge and charging current are derived. The dependences of the current and charge density distribution on the system parameters are investigated. The effect of the electric field that emerges owing to charge separation on the charging process is taken into account. Consideration of the electric field is shown to lead to lower values of the electric charge density and charging current. As the Debye number decreases, the charge density also decreases. The charging current increases with an increase in the Debye number.     

Application of convolution procedures to chronopotentiometry

Convolution procedures are used to extract the faradaic information from chronopotentiometric data, in conditions where significant distortion by double layer charging occurs. The faradaic component of the imposed current is obtained, after measurement of the double layer capacitance, by differentiation of the initial chronopotentiogram. Convolution of this current with the function (πt)^?1/2 leads to a potential-convoluted current relationship freed from the effect of double layer charging. The kinetic characterization of the system using a combined analysis of this relationship and that relating the faradaic current to the electrode potential is discussed for the various types of reaction mechanism. The efficiency of the proposed procedure is tested on the galvanostatic reduction of fluorenone in DMF.     

A direct differential pulse anodic stripping voltammetric method for the determination of thallium in natural waters

A dual direct method for the ultratrace determination of thallium in natural waters by differential pulse anodic stripping voltamrnetry (d.p.a.s.v.) is presented. D.p.a.s.v. at the hanging mercury drop electrode and at the mercury film electrode is used in the concentration ranges 0.5–100 μg Tl l^-1, and 0.01–10 μg Tl l^-1, respectively. Quantification is aided by the technique of standard additions. The response of the method is optimized for typical natural surface water matrices. An intercomparison of thalium determinations performed by the two anodic stripping methods and electrothermal-atomization atomic absorption spectrometry on normal and thallium-spiked surface water samples demonstrates equivalent accuracy within the range where atomic absorption is applicable. The method appears free from serious interferences.     

Trace analysis by polarography and voltammetry in pharmaceutical and environmental chemistry

Differential pulse polarography (d.p.p.), anodic and cathodic stripping voltammetry (a.s.v. and c.s.v.) and on-line electrochemical detection (e.g., HPLC-e.d.) have been extensively exploited in recent years for the determination of trace concentrations of many organic and organometallic molecules of biological significance. Such analyses can be carried out in complex matrices such as body fluids, foods, agricultural materials and aquatic matrices^1,2,3.     

氟代吡啶阳离子的理论研究

Cations of fluorinated pyridines（pentafluoropyridine,2,6-difluoropyridine,and 2-fluoropyridine）have been studied by using density functional B3LYP method in conjunction with 6-31G（d,p）,6-311G（d,p）,6-31+G（d,p）,and 6-311+G（d,p）basis sets. B3LYP geometry optimization and frequency analysis calculations indicate that the pentafluoropyridine cation,2,6-difluoropyridine cation,and 2-fluoropyridine cation have C2v,C2v,and Cs structures in the 2A2,2A2,and 2A" ground states,respectively. The calculated geometries of the cations and the parent molecules were compared. The natural population analysis calculations at the B3LYP level with different basis sets were performed on the three cations and the three parent molecules. The isotropic hyperfine coupling constants in the three cations（radicals）were calculated. The vertical and adiabatic ionization potential（VIP and AIP）values of the pentafluoropyridine,2,6-difluoropyridine,and 2-fluoropyridine molecules were calculated by using the B3LYP method,and the calculated VIP values are in excellent agreement with experiment.     

Dielectric properties under corona charging of thermoplastics for holographic optical switching

Photothermoplastics are materials which have been proposed for optical switching devices: a grating is recorded by surface deformation of the thermoplastic layer, coated on an organic photoconductive layer used, like in xerography, for an electrostatic image recording material. But first of all an electrical field is applied through a corona charging sequence and obviously dielectric properties of the abietic ester thermoplastic material are of importance. This paper discusses the behaviour of such a layered thermoplastic material in a corona set-up, by measuring the corona current and the surface voltage following thickness and temperature of the sample and charging or discharging times.     

Determination of uranium(VI) in sea water by cathodic stripping voltammetry of complexes with catechol

Polarographic (d.c.) measurements showed that complex ions of uranium(VI) with catechol adsorb on the dropping mercury electrode. This effect is used to determine uranium(VI) directly in sea water. Optimal conditions include pH 6.8, 2 × 10^?3 M catechol, and a collection potential between ?0.1 and ?0.4 V (vs. Ag/AgCl) at a hanging mercury drop electrode. The cathodic scan is made with the linear-scan or differential-pulse mode (d.p.c.s.v.). The detection limit with the d.p.c.s.v, mode is 3 × 10^?10 M after a collection period of 2.5 min. Between pH 6 and 8, the peak height increases with pH and with catechol concentration up to 5 × 10^?3 M. There is linear relationship between the collection time and the measured peak height until the drop surface becomes saturated. With a collection period of 3 min, the reduction current increases linearly with the metal concentration up to about 5 × 10^?3 M U(VI). The maximum adsorption capacity of the mercury drop is 4.4 × 10^?10 mol cm²; each complex ion then occupies 0.38 nm², equivalent to the size of about one catechol molecule. Interference by high concentrations of Fe(III) is overcome by selectively adsorbing U(VI) at a collection potential near the reduction potential of Fe(III). Organic surfactants reduce the peak height for uranium by up to 75% at unnaturally high concentrations only (4 mg l^?1 Triton X-100). Competition by high concentrations of Cu(II) for space on the surface of the drop is eliminated by addition of EDTA.