Inorganic arsenic speciation in water and seawater by anodic stripping voltammetry with a gold microelectrode

The determination of arsenic in sea and freshwater by anodic stripping voltammetry (ASV) was revisited because of problems related to unstable peaks and inconveniently strong acidic conditions used by existing methods. Contrary to previous work it was found, that As(III) can be determined by ASV using a gold microwire electrode at any pH including the neutral pH typical for natural waters. As(V) on the other hand, requires acidification to pH 1, but this is still a much milder condition than used previously. This is the basis of a new method for the chemical speciation of arsenic in natural waters. The limits of detection are 0.2 nM As(III) at pH 8 and 0.3 nM combined arsenic (III + V) at pH 1 with a 30 s deposition time. These limits are lowered by extending the deposition time. The detection step at pH 8 was stripping chronopotentiometry (SC) as this was found to give a lower detection limit than ASV. Copper is co-determined simultaneously with arsenic. The method was applied successfully to the determination of arsenic as well as copper in samples from the Irish Sea, mineral water and tap water.     

Hydrodynamic microelectrode voltammetry

This review provides a detailed overview of the progress made in the last 25 years in the field of hydrodynamic microelectrodes (HMEs) with particular application to the study of electrochemical kinetics. A survey is made of the various types of HMEs that have been reported in the literature, with summaries of theoretical and experimental details along with the results published by each methodology. Conclusions are drawn regarding the comparative advantages and disadvantages of these techniques, and the channel and microject electrodes found to be preferred. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 4, pp. 397–421. The text was submitted by the authors in English.     

Voltammetric characterisation of silicon-based microelectrode arrays and their application to mercury-free stripping voltammetry of copper ions

This paper describes the electrochemical characterisation of a range of gold and platinum microelectrode arrays (MEAs) fabricated by standard photolithographic methods. The inter-electrode spacing, geometry, numbers and dimensions of the electrodes in the arrays were found to influence the voltammetric behaviours obtained. Excellent correlation was found between experimental data and theoretical predictions employing published models of microelectrode behaviour. Gold MEAs were evaluated for their applicability to copper determination in a soil extract sample, where agreement was found between the standard analytical method and a method based on underpotential deposition—anodic stripping voltammetry (UPD-ASV) at the MEAs, offering a mercury-free alternative for copper sensing.     

Research on electrochemical properties of nonaqueous ionic liquid microemulsions

The nonaqueous ionic liquid (IL) microemulsions composed of 1-butyl-3-methylimidazolium tetrafluoroborate, Triton X-100, and toluene were prepared and the electrochemical properties of the nonaqueous IL microemulsions were investigated in this paper. It is shown that characteristics of the nonaqueous IL microemulsions such as electrical conductivity, electrochemical window, and solubility are good, which indicate that the nonaqueous IL microemulsions can be used as electrolyte for electrochemical research. The electrochemical properties of the nonaqueous IL microemulsions were researched by cyclic voltammetry (CV) and electrochemical impedance spectroscopy methods using potassium ferricyanide as electroactive probe. It was found that the reversibility was better and the peak current densities of CV were higher for the [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ electrode reaction in the nonaqueous IL microemulsions than those in IL. However, the electrochemical behavior of the probe in the nonaqueous IL microemulsions with different microenvironments (oil-in-IL, IL-in-oil, and bicontinuous) was different. The electrochemical property of the probe in the oil-in-IL microemulsion was the best, which was studied in detail.     

Comparison of fast scan voltammetry with microelectrode voltammetry of reduction of 1,4-benzoquinone

The reduction of 1,4-benzoquinone (BQ) in acetonitrile was blocked in steady-state microelectrode voltammetry although it has been believed to be a sluggish electron transfer reaction. Ferrocene was added to the BQ solution with equi-concentration in order to confirm the diffusion-control step and to evaluate accurately the kinetic effect from potential differences. Fast scan voltammograms showed the negative potential shifts both of the cathodic and the anodic peaks with an increase in the scan rate. In contrast, microelectrode voltammetry showed that the ratio of the steady-state limiting current for BQ to that for ferrocene decreased with a decrease in the electrode radii. The halfwave potential of the reduction of BQ did not vary with the radii within error. These features are largely deviated from the Butler–Volmer kinetic behavior. The electrode surface after the long term electro-reduction was coated with a precipitate. The proposed reaction mechanism is formation of films by follow-up chemical reactions of BQ associated with slight potential shift. The film blocks diffusion of BQ. This model was theoretically formulated and elucidated the experimental results.     

A detailed investigation for determination of tannic acid by anodic stripping voltammetry using porous electrochemical sensor

The electrochemical responses of tannic acid have been obtained at porous pseudo-carbon paste electrode (PPCPE), polypyrrole modified carbon paste electrode (PCPE), SBA-15 modified CPE (SBA-MCPE) and carbon paste electrode (CPE) under same conditions, respectively. The results show that the sensitivity of PPCPE is the highest among all the checked electrodes. The detection limit at PPCPE is 0.01 μM, which is about 10 times lower than that at CPE and is about 5 times lower than that at PCPE or SBA-MCPE. The developed electrode PPCPE possesses a few obvious advantages and no binding reagents are needed. The surface area of PPCPE is 59.26 m² g⁻¹ with pores ranging from 2 to 5 μm in diameter. The PPCPE is easy to preserve and has good reusability, which affords a nice electrochemical platform for detecting tannic acid.     

Label-free monitoring of site-specific DNA cleavage by EcoRI endonuclease using cyclic voltammetry and electrochemical impedance

Cyclic voltammetry (CV) combined with electrochemical impedance spectroscopy (EIS) were proposed to monitor the site-specific DNA cleavage by EcoRI endonuclease without using external label. The alteration of CV and EIS signal demonstrated that double-strands (dsDNA) contain recognition sequence was cleaved by EcoRI endonuclease. Real-time monitoring indicated that the dsDNA was cleaved by EcoRI more than 90% after 2 h of enzyme digestion time. Control experiment showed that the DNA cleavage by EcoRI endonuclease is site-specific for DNA sequence. Experimental results demonstrated that the efficiency of EcoRI cleavage was highly dependent on the concentration of EcoRI concentration in the range from 0.04 to 0.4 U μL⁻¹ with one almost linear relationship.     

Feasibility studies for the detection of volatile organic compounds in the gas phase using microelectrode sensors

The use of microelectrode sensors to detect volatile organic compounds (VOCs) in air is demonstrated. In general, VOCs that oxidize easily to form protons gave a larger electrochemical response. The use of voltammetry for speciation and the effect of electrode size on the electrochemical response are discussed. We demonstrate that surface enhanced Raman spectroscopy (SERS) can be used to monitor the electrochemical reactions in situ and discuss its applicability in identifying the electroactive species.     

Determination of the electrochemical diffusion coefficient of methylene blue in SDS/n-C5H11OH/H2O system by non-probe microelectrode voltammetry

The diffusion coefficient of methylene blue (MB) is determined by the method of non-probe microelectrode voltammetry in sodium dodecyl sulfate (SDS)/n-C₅H₁₁OH/H₂O lyotropic liquid crystal system. The results obtained show that the diffusion coefficient of MB increases with water and n-pentanol contents in the microemulsions and the lyotropic liquid crystal but decreases with SDS content. The diffusion coefficient of SDS droplet in the microemulsions and the diffusion coefficient of SDS molecule in the lyotropic liquid crystal with MB all are less than those without MB. The magnitude order of the diffusion coefficient of MB is as follows: the coefficient in the oil-in-water (O/W) microemulsion is greater than the coefficient in the water-in-oil (W/O) microemulsion which is greater than the coefficient in the lamellar liquid crystal (LLC), which is also greater than the coefficient in the Hex.     

Determination of trace iodide in iodised table salt on silver sulfate-modified carbon paste electrode by differential pulse voltammetry with electrochemical solid phase nano-extraction

Electrochemical solid phase nano-extraction, a novel sample preparation technique, was used for the determination of trace iodide in iodised table salt based on the silver sulfate nanoparticle-modified carbon paste electrode. Electrochemical solid phase nano-extraction was realized in the exchange between the sulfate anion in nanoparticles and an iodide anion from aqueous solution. The released silver cation serves as the electrochemical probe for the determination of iodide. The extraction follows a Freundlich adsorption isotherm, and can be used in the detection of iodide in the concentration range 5.0 × 10⁻¹²-4.0 × 10⁻⁹ M. The amount of iodide in iodised table salt was determined as 0.875 ± 0.002 μg/g, which is about 2.5% of the addition amount of iodate with a relative deviation of 5.92% and a standard addition recovery of 90-110%. The large amounts of chloride and iodate did not interfere with the detection.     

Electrochemical solid phase micro-extraction and determination of salicylic acid from blood samples by cyclic voltammetry and differential pulse voltammetry

The electrochemical solid phase micro-extraction of salicylic acid (SA) at graphite-epoxy-composed solid electrode surface was studied by cyclic voltammetry. SA was oxidized electrochemically in pH 12.0 aqueous solution at 0.70 V (vs. saturated calomel electrode) for 7 s. The oxidized product shows two surface-controlled reversible redox couples with two proton transferred in the pH range of 1.0∼6.0 and one proton transferred in the pH range of 10.0∼13.0 and is extracted on the electrode surface with a kinetic Boltzman function of i _p = 3.473–4.499/[1 + e^{(t − 7.332)/6.123}] (χ ² = 0.00285 μA). The anodic peak current of the extracted specie in differential pulse voltammograms is proportional to the concentration of SA with regression equation of i _p = −5.913 + 0.4843 c (R = 0.995, SD = 1.6 μA) in the range of 5.00∼200 μM. The detection limit is 5.00 μM with RSD of 1.59% at 60 μM. The method is sensitive and convenient and was applied to the detection of SA in mouse blood samples with satisfactory results.     

Advantages of using a mercury coated, micro-wire, electrode in adsorptive cathodic stripping voltammetry

A mercury coated, gold, micro-wire electrode is used here for the determination of iron in seawater by catalytic cathodic stripping voltammetry (CSV) with a limit of detection of 0.1 nM Fe at a 60 s adsorption time. It was found that the electrode surface is stable for extended periods of analyses (at least five days) and that it is reactivated by briefly (2 s) applying a negative potential prior to each scan. Advantages of this electrode over mercury drop electrodes are that metallic mercury use is eliminated and that it can be readily used for flow analysis. This is demonstrated here by the determination of iron in seawater by continuous flow analysis. It is likely that this method can be extended to other elements. Experiments using bismuth coated, carbon fibre, electrodes showed that the bismuth catalyses the oxidation of the important oxidants bromate and hydrogen peroxide, which makes it impossible to use bismuth based electrodes for catalytic CSV involving these oxidants. For this reason mercury coated electrodes retain a major advantage for catalytic voltammetric analyses.     

Investigation of the reactivity of powder surfaces by abrasive voltammetry

The ability of abrasive voltammetry to reflect the reactivity of iron powder immobilized on paraffin-impregnated graphite electrodes was investigated. Normal pulse voltammetry was used as the preferred method. The peak shape and position on the potential/time axis reflected both the reactivity of iron powder particles as well as their size fraction. “Relative peak height” defined as peak height to area ratio, i.e., current to charge ratio, correlated well with the rate of the electrochemical dissolution process. It showed not only a decreasing dissolution rate with an increase in particle size, but also the differencies in the dissolution rate of iron powder of various reactive surfaces as a consequence of various pretreatment methods. It was found that chemical pretreatment of the powder always resulted in a faster dissolution process in comparison with other non-chemical pretreatment methods. Received: 30 June 1998 / Accepted: 30 September 1998     

High-Speed voltammetry of Mn-doped LiCoO2 using a microelectrode technique

The microelectrode technique was applied to investigate the electrochemical properties of LiCo_{1- x} Mn _x O₂ (x=0, 0.01, 0.05, 0.2, or 0.5) synthesized using the citrate process. From the X-ray diffraction measurements, an expansion of the c-axis and a decrease in the crystal size of the materials were observed on substitution of Mn into LiCoO₂. In the electrochemical measurements, the high-speed cyclic voltammogram for the Mn-substituted materials gave one set of peaks at 3.9 V vs. Li/Li⁺. The apparent chemical diffusion constant (D _app) of LiCo_0.8Mn_0.2O₂ obtained from the potential step experiment was 6.4 × 10⁻⁸ cm²/s, which is larger than that of LiCoO₂. The increase in D _app is attributable to the expansion of the c-axis and/or the decrease in the crystal size. In addition, the increase in Mn substitution up to 20% lead to an improvement in the kinetic reversibility and the cycle stability of LiCoO₂. Received: 7 June 1999 / Accepted: 25 June 1999     

Photophysical and electrochemical properties of heteroleptic tris-cyclometallated Ir(III) complexes

Some new heteroleptic tris-cyclometallated iridium(III) complexes have been synthesized and fully characterized. Among these iridium(III) complexes, bis(1-phenylpyrazolato-N,C^2′)iridium(III)[5-(2′-pyridyl)tetrazolate] (3) and bis(3-methyl-1-phenylpyrazolato-N,C^2′)iridium(III)[5-(2′-pyridyl)tetrazolate] (4) show excellent quantum yields at room temperature, the electron density being perturbed by introducing the pyridyltetrazole ligand, making k_r > k_nr. This destroys the concept of phenylpyrazole based iridium complexes.     

Development in synthesis and electrochemical properties of thienyl derivatives of carbazole

A convenient and higher yielding synthetic route to N-alkyl-bis(thiophene)- and N-alkyl-bis(ethylenedioxythiophene) carbazole derivatives is reported and their aggregation, and electrochemical properties are characterized. The key step in the synthesis of this group of compounds has been the Stille-type coupling reaction between the N-alkyldibromocarbazole and tin derivatives of thiophene or ethylenedioxythiophene, as the best way for preparation of conjugated N-alkylcarbazole derivatives. For this group of compounds we also present an electrochemical polymerization effect.     

Study on surface acid-base property of carboxylic acid-terminated self-assembled monolayers by cyclic voltammetry and electrochemical impedance spectroscopy

Cyclic voltammetry and electrochemical impedance spec-troscopy were used to study the surface acid-base property of carboxylic acid-terminated self-assembled monolayers(SAMs).A carboxylic acid-terminated thiol,such as thioctic acid(1,2-dithiolane-3-pentanoic acid),was self-assembled on gold electrodes.Electron transfer between the bulk solution and the SAM modified electrode was studied at different pH using Fe(CN)63-as a probe.The surface pka of thioctic acid was determined by cyclic voltammetry and electrochemical impedance spectroscopy to be 5.6 ±0.1 and 5.8±0.1,respectively.The method is compared with other methods of monolayer pKa measurement.     

维生素B_6在微电极上的电化学行为及测定

用伏安法研究了维生素B6(VB6)在碳纤维微电极(CFME)上的电化学行为及其测定。以0.1 mol/L的邻苯二甲酸氢钾(pH 5)的缓冲液为底液,在最佳实验条件下,VB6在0.94 V(vs,SCE)的氧化峰峰电流与VB6在5.0×10-6~8.0×10-4mol/L范围内呈良好的线性关系,检出限为1.0×10-6mol/L(S/N=3)。方法用于药剂中的VB6的分析。     

Preparation and electrochemical properties of catalysts based on macrobicyclic d-metal tris-dioximate complexes

The use of macrobicyclic tris-dioximate complexes of cobalt, K₃[Co(Dmg)₃] (where DmgH₂=C₄H₈N₂O₂), and iron, (BPh)₂[Fe(Nx)₃] (where NxH₂=C₆H₁₀N₂O₂), and the carbon-supported complexes and their pyrolyzates, as catalysts for oxygen reduction was examined utilizing the combination of thermogravimetry, infrared spectroscopy, mass spectrometry and electrochemical techniques. The results have shown that macrobicyclic cobalt complexes adsorbed on the surface of carbon- based synthetic porous polymers have a pronounced catalytic effect on the reduction of molecular oxygen. A reduction in the oxygen overpotential was observed with heat treatment of the catalysts prepared in an inert atmosphere. The optimum pyrolysis temperature for the electrochemically most active catalysts for oxygen reduction was estimated. Electronic Publication