Comparison of Voltammetric Techniques for Ammonia Sensing in Ionic Liquids

In electroanalytical chemistry, it is often observed that square wave voltammetry (SWV) and differential pulse voltammetry (DPV) are more sensitive techniques compared to linear sweep voltammetry (LSV), due to their method of sampling which minimises the charging current (non‐faradaic processes). In this work, a comparison of the three techniques (LSV, DPV and SWV) is performed for ammonia (NH₃) gas oxidation (a chemically and electrochemically irreversible redox process) in an ionic liquid over a concentration range of 10–100 ppm. Four different platinum electrodes are employed: a screen‐printed electrode (SPE), a thin‐film electrode (TFE), a microarray thin‐film electrode (MATFE) and a Pt microdisk electrode (μ‐disk). Calibration plots (current vs concentration) for all three different electrochemical techniques on all four surfaces showed excellent linearity with increased concentrations of NH₃ gas and relatively low limits of detection (LODs). On the larger mm‐sized surfaces (SPE and TFE), the current responses for LSV and SWV were quite similar, but DPV gave the lowest currents. Whereas for the smaller micron sized electrodes (MATFE and μ‐disk), currents were of the order LSV>SWV>DPV, with LSV being far superior to the pulse techniques. These findings suggest that the pulse techniques of SWV and DPV may not be the optimum methods, particularly on microelectrodes, for the detection of analytes such as ammonia in RTILs.     

Use of computerized instrumentation for pseudo-derivative direct current and normal pulse polarography with correction for charging current

The application of computerized instrumentation to pseudo-derivative d.c. and normal pulse polarography is described. Calculation of background current by means of a linear least-squares fit of data obtained prior to the rising part of the sigmoidal d.c. or pulse curve and extrapolation to other potentials enables close to the faradaic response to be obtained over wide concentration ranges. After subtraction of background current, least-squares fitting of the sigmoidal curve enables data smoothing to be undertaken and other calculations to be performed. Pseudo-derivative curves obtained by plotting current differences obtained from corrected and smoothed data provides limits of detection comparable to those of more sophisticated polarographic methods. The reduction of cadmium in 1 M NaCl is considered because it occurs at potentials near the electrocapillary maximum where charging current correction is most difficult. The limits of detection are 10^-7 M for the pseudo-derivative d.c. method, and 10^-3 M for the pulse method.     

Phase Sensitive Alternating Current Polarography: A Chemometric Approach for the Selection of Phase Angles

A method based on the combined use of multivariate curve resolution by alternating least squares (MCR‐ALS) with phase sensitive alternating current polarography (ACP) is proposed to evaluate the phase angle where capacitive current is minimized in a much more accurate way than the visual inspection of ACP signals. The method allows, through the analysis of series of AC polarograms measured at different phase angles out the potential, to distinguish between faradaic and capacitive contributions. Then the angle at which the capacitive current is negligible can be shown and, in some cases, the influence of adsorption on measured currents minimized.     

Zeta potential determination by streaming current modelization and measurement in electrophoretic microfluidic systems

Electrophoresis in capillary and microfluidic systems, used in analytical chemistry to separate charged species, are quite sensitive to surface phenomena in terms of separation performances. In order to improve theses performances, new surface functionalization techniques are required. There is a need for methods to provide fast and accurate quantification about surface charges at liquid/solid interfaces. We present a fast, simple, and low-cost technique for the measurement of the zeta-potential, via the modelization and the measurement of streaming currents. Due to the small channel cross section in microfluidic devices, the streaming current modelization is easier than the streaming potential measurement. The modelization combines microfluidic simulations based on the Navier-Stokes equation and charge repartition simulations based on the Poisson-Boltzmann equation. This method has been validated with square and circular cross section shape fused-silica capillaries and can be easily transposed to any lab-on-chip microsystems.     

Staircase pulse voltammetry

The faradaic current of interest can often be obscured by some extraneous current. A new method called staircase pulse voltammetry (SPV), with which involving a potential pulse is superimposed on a staircase, is used for removing the double-layer charging current. This paper presents the theoretical consideration and the experimental evidence of this method for a simple reversible system and a catalytic process. The parameters affecting currents are also discussed. Experimental verifications are in agreement with the theories.     

Simultaneous determination of concentration,diffusion coefficient and number of electrons for electroactive species by combining suitable electroanalytical measurements

The electroanalytical method described for the simultaneous determination of concentration, the number of electrons involved in the redox process and diffusion coefficient is based on evaluation of the ratios between the currents recorded for the analyte and for an easily standardized reference species dissolved in the same medium. Three different electroanalytical techniques are used in which the currents exhibit, for two techniques at least, different dependences on both the diffusion coefficient and electron number. The approach is applicable to diffusion-controlled processes, regardless of the degree of reversibility involved. Reliability tests with electroactive organic compounds dissolved in dimethyl sulphoxide show that both accuracy and precision are within 10% depending on the chosen combination of techniques.     

Minimization of Electrode Adsorption Effects: The Cadmium–Humic Acid System Studied by Phase Sensitive Alternating Current Polarography

The use of phase‐sensitive Alternating Current Polarography (ACP) is investigated for the minimization of adsorption effects in metal‐ligand systems with induced metal adsorption onto the electrode. When ACP is applied to obtain information on metal complexation in bulk, some problems arise from the faradaic contribution of adsorbed species. This effect can be corrected by using the capacitive current measured in the potential region of the faradaic peak. Using this correction, ACP produces similar results to those from Reverse Pulse Polarography (RPP), a technique that minimizes electrode adsorption effects on the measured currents. The method proposed is applied to the study of the Cd–humic acid system that has been investigated by ACP, RPP and Differential Pulse Polarography (DPP).     

Nanotribology-based novel characterization techniques for the dielectric charging failure mechanism in electrostatically actuated NEMS/MEMS devices using force-distance curve measurements

The work presents a comprehensive package of novel nanoscale characterization techniques to study dielectric charging in electrostatic nano- and microelectromechanical systems (NEMS and MEMS). The proposed assessment methodologies are based on the force-distance curve (FDC) measurements performed using an atomic force microscope (AFM) to measure, for the first time, the induced surface potential and adhesive force over charged dielectric films. They were employed to study plasma enhanced chemical vapor deposition (PECVD) silicon nitride films for application in electrostatic capacitive RF MEMS switches. Three different techniques were introduced including the application of FDC measurements to study charging in bare SiN(x) films, metal-insulator-metal (MIM) capacitors, and MEMS switches. The results from the three methods were correlated and compared with the published data from other characterization techniques, mainly charge/discharge current transient (C/DCT) and Kelvin probe force microscopy (KPFM). The unique advantages of the proposed FDC-based characterization techniques are twofold. First, they can measure the multiphysics coupling between the dielectric charging phenomenon and tribological issues at the interface between the switch bridge and the dielectric surface. Second, the FDC-based techniques can measure larger levels of induced surface potential over charged dielectric films which results from the high electric field normally used to actuate MEMS switches. Based on the proposed FDC techniques, the influence of several parameters on dielectric charging/discharging processes was investigated: the dielectric film thickness, deposition conditions, substrate, and electrical stress conditions.     

傅里叶变换用于快扫伏安法的电流分离

根据电化学池的电学模型，推导出相应的数学模型，和傅里变变换相结合，实现了扫描伏安数据的充电电流和法拉第电流的分离。模拟数据和实验数据的处理结果表明，所提出的方法均能实现充电电流和法拉第电流的分离，得到的结果与理论模型一致。     

An introduction to thiol redox proteins in the endoplasmic reticulum and a review of current electrochemical methods of detection of thiols

This aim of this paper is to expound the complexity of thiol redox systems in the endoplasmic reticulum of eukaryotic cells to the electroanalytical community. A summary of the state of the art in electrochemical methods for detection of thiols gives an insight into the challenges that need to be addressed to bridge the disparity between current analytical techniques and applications in a 'real' biological scenario.     

The electrochemistry of electrode edges and its relevance to partially blocked voltammetric electrodes

Analytical mathematics and digital simulation are used to predict the response, to a potential jump, of the junction between insulating and conducting regions of an electrode. The simulation is carried out differentially and employs other novel features. Concentrations in the vicinity of edges of positive and negative curvatures, as well as straight edges, are analyzed by the model and thereby the faradaic current densities and currents are predicted. It is shown that, in addition to the well-understood cottrellian current arising from the surface of the conducting electrode, currents are generated that are proportional to the length of the edge and to its curvature. These results are then applied to inlaid disks and to partially blocked electrodes. The possibility is explored of using the response to a potential step to gain information on the geometry of a partially blocked electrode.     

The acquisition and processing of current-time data in controlled-potential electrolyses

An on-line system is described for acquiring and processing data on the variation of current integral with time during a controlled-potential electrolysis with a stirred mercury-pool electrode. With simple systems it yields coulometric results that are comparable to the best obtained in traditional ways. It should be possible to effect substantial further improvement by controlling sources of random error so small that they have always been neglected in controlled-potential coulometry. Current-time data would not serve the purpose because fluctuations of charging and faradaic currents at stirred mercury pools make it impossible to secure the desired accuracy and precision.     

The induced current strengths and aromatic pathways of heteroporphyrins and their antiaromatic derivatives

The magnetically induced current strengths as well as nucleus independent chemical shifts of aromatic heteroporphyrins and antiaromatic 22,24‐dideazaheteroporphyrins have been studied using the density functional theory method. The induced current strengths and pathways are obtained by numerical integration of the induced current densities following the specific chemical bonds. The total induced current strengths of antiaromatic 22,24‐dideazaheteroporphyrins is about 6 nA/T weaker than the one for the heteroporphyrins in absolute value. The substitution of pyrrole NH groups by O and S atoms does not change the total induced current strengths. The induced currents around the molecular macroring split at the heterocycles (pyrrole, furan, and thiophene) into the inner and outer routes. The heteroatoms (N, O, and S) have high resistance and consequently lead to a weaker induced current strength than the one passing the outer route in aromatic heteroporphyrins. For antiaromatic 22,24‐dideazaheteroporphyrins, the heteroatoms enhance the current strength and change the main current pathway into the inner route. The induced current strength following the NH moiety is stronger than the one passing the oxygen moiety of furan ring and the sulfur moiety of the thiophene ring in both heteroporphyrins and 22,24‐dideazaheteroporphyrins. © 2015 Wiley Periodicals, Inc.     

Bipolar pulse conductometric monitoring of ion-selective electrodes: Part 1. Method development with a calcium-selective electrode and elucidation of the basic principles involved

The potential generated by a plastic-membrane calcium ion-selective electrode (i.s.e.) is shown to be indirectly measurable by a non-zero current method based on bipolar pulse conductance. Linear current—voltage curves are obtained using 0–5-V pulses; the current axis intercept is related to the i.s.e. potential. A simple electrical contact (e.g., platinum or stainless steel) can be used instead of a poised reference electrode as the counter electrode in this two-electrode system. Long-term exposure of the i.s.e. to calcium solutions causes an upward drift in the measured current. This drift is minimized by avoiding long exposure times to solution, rinsing the electrode between measurements, and constructing current—voltage curves for determination of the current axis intercepts. Voltage pulses lasting 100 μs are optimum for this method. Shorter pulses are subject to error from capacitive charging currents, and longer pulses yield poorer precision, and degrade the electrode through faradaic reactions. The measured signal is dependent upon Ca²⁺ concentration (rather than activity), making ionic strength adjustment unnecessary. The concentration dependence is induced by application of voltage pulses greater than ~ 15 mV in amplitude. Selectivities of the potentiometric and conductometric methods are shown to be comparable for a variety of interfering monovalent and divalent cations. The conductometric method yields a fast i.s.e. response because of induced migration of Ca²⁺ into the membrane. Response time decreases as the pulse height increases. Pulses greater than 2 V in magnitude yield response times limited by the solution mixing time rather than by the electrode.     

Application of potentiometric stripping analysis with constant inverse current for determining soluble lead in human teeth

A method for the determination of soluble lead in human teeth by potentiometric stripping analysis with constant inverse current in the analytic step (PSA-i_R), is described. The metal ions were concentrated as their amalgams on the glassy carbon surface of a working electrode that was previously coated with a thin mercury film and then stripped by a suitable oxidant. This paper examined effect of various factors on the PSA-i_R results including the electrolysis potential, the solution stirring rate, and the constant inverse current. Quantitative analysis was carried out by both standard addition and calibration curve methods; a good linearity was obtained in the concentration range from 5 to 25 μg/dm³. A detection limit of 0.64 μg/dm³ was obtained, with a 5.21% coefficient of variation. Results obtained for teeth were not significantly different from these obtained by flameless atomic absorption spectrophotometry (FAAS).     

Integrated pulsed amperometry for the analysis of organic compounds

Integrated pulsed amperometric detection (IPAD) was applied for the detection of organic compounds for flow injection analysis. The pulse waveform used in the integrated pulsed amperometry consisted of three steps: detection potential, oxidation potential, and adsorption potential. The pulse waveform was applied to the working electrode as the analyte flowed through the electrochemical cell. Unlike ordinary pulsed amperometry, a faradaic current was integrated over the duration period of the detection potential in the IPAD. Therefore, the total charge was measured by integrating the current after the detection potential was applied. The current for the initial 10 ms, after applying the detection potential, was excluded from the integration due to a large charging current at the initial period. Compared with pulsed amperometry, integrated pulsed amperometry provides a better signal-to-noise ratio and a lower detection limit. This method was applied to the quantitative analysis of thiourea as a representative analyte of organosulfur compounds in a flow injection analysis.     

Square-wave amperometry

A new electroanalytical technique involving the use of two indicator electrodes polarised by a square-wave potential is described. The output signal is a de faradaic current proportional to the concentration of the electroactive species. The technique is very sensitive, shows rapid response to concentration change and the observed current is independent of stirring.     

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.     

电化学方法研究X120管线钢氢致裂纹行为

采用Davanathan-Stachursky双电池渗氢装置研究了X120试验管线钢中电化学氢渗透动力学行为,运用电化学充氢方法对X120管线钢进行预制微裂纹,采用SEM观察钢中夹杂物及其与氢致裂纹（HIC）形成的关系。 实验结果表明,随着充氢电流密度的增大,饱和阳极电流I∞、氢扩散通量J∞、氢有效扩散系数Deff均逐渐增大;HIC萌生与钢中夹杂物化学成分有关,X120管线钢中主要含有Al、Mn的硫化物、氧化物夹杂,氮化钛、铌的复合夹杂以及Ca-Al-O-S的混合物夹杂,HIC更易于在含Al、Mn的夹杂物处萌生及形成,且裂纹形核后一般沿轧制方向生长。