Mercury Ultramicroelectrodes Research Articles

Double-Layer Inhibition of Peroxydisulfate Reduction at Mercury Ultramicroelectrodes. A Quantitative Analysis of the Frumkin Effect Including Molecular Transport and Long-Range Electron Transfer

The electrical double layer (EDL) can lead to unexpected voltammetric phenomena. A notable example is the Frumkin effect, observed for the reduction of peroxydisulfate (S2O82–). In the absence of excess supporting electrolyte, the rate of S2O82– reduction decreases as the cathodic overpotential is increased, contrary to expectations for faradaic electron-transfer processes. Here, we report a demonstration of the Frumkin effect for S2O82– reduction at Hg ultramicroelectrodes, revealing steady-state voltammetry behavior consistent with prior observations at rotating disk electrodes. A finite element model is used to simulate the effect of the EDL on S2O82– reduction in the presence and absence of excess supporting electrolyte (K2SO4). Semiquantitative agreement between experimental and simulated voltammograms is obtained, including capturing the decrease in current for S2O82– reduction with increasing overpotential. We show that S2O82– migration within the EDL, at electrode potentials negative of the potential of zero charge, is the primary process responsible for the observed decrease in current with increasing driving force. These combined experiments and finite element simulations represent the first quantitative description of the Frumkin effect with rigorous inclusion of the EDL, migrational and diffusional mass-transport, and long-range electron transfer kinetics.

Fabrication of carbon, gold, platinum, silver, and mercury ultramicroelectrodes with controlled geometry.

A simple, fast, and reproducible method for the fabrication of disk ultramicroelectrodes (UMEs) with controlled geometry is reported. The use of prepulled soda-lime glass capillaries allows one to bypass the irreproducible torch-sealing and experimentally challenging tip-sharpening steps used in conventional fabrication protocols. A micron-sized electroactive wire is sealed inside this capillary producing UMEs with a highly reproducible geometry. Total fabrication time (1 h) and experimental difficulty are significantly reduced. Disk UMEs with various diameters and cores were fabricated, including carbon fiber (7 and 11 μm), gold (10 and 25 μm), platinum (10 and 25 μm), silver (25 μm), and mercury (25 μm). The ratio of the insulating sheath to the electroactive core of the UMEs was 2.5-3.6. Silver UMEs were also used to produce a Ag/AgCl microreference electrode. This general fabrication method can readily be applied to other electroactive cores and could allow any research group to produce high quality disk UMEs, which are a prerequisite for quantitative scanning electrochemical microscopy.

Cyclic voltammetry studies of Cd 2+ and Zn 2+ complexation with hydroxyl-terminated polyamidoamine generation 2 dendrimer at a mercury microelectrode

Cyclic voltammetry was used to determine the stability constants for the complexation of cadmium and zinc ions with hydroxyl-terminated polyamidoamine generation 2 dendrimers with an ethylenediamine core (PAMAM G2-OH) at a mercury ultramicroelectrode. Cadmium and zinc ions were found to form poly-metal complexes through the interactions with interior amino groups rather than hydroxyl groups and the maximum number of metal ions per dendrimer or stoichiometry was 4. Complexation is accompanied by a significant decrease in the limiting current and by measuring the changes in the magnitude of cathodic current with the analytical dendrimer:metal ion ratio and fitting the resulting curves, the four cumulative consecutive complexation constants were determined: 1.3 × 10 5 M −1, 1.6 × 10 9 M −2, 3.3 × 10 12 M −3 and 4.6 × 10 14 M −4 for Cd 2+ and 1.0 × 10 6 M −1, 1.6 × 10 9 M −2, 4.6 × 10 14 M −3 and 2.6 × 10 17 M −4 for Zn 2+. The stronger interaction of zinc ions with PAMAM G2-OH compared to cadmium was accompanied with a sharper decrease of the cathodic current with the addition of dendrimer and a larger half-wave potential shift than cadmium. This was rationalized by evaluating the changes of the diffusion coefficients for these two different systems by electrochemical means.

Standard electrode reaction rates of bis(biphenyl) chromium(I) in N,N-dimethylformamide

Abstract The electrode kinetics for the one electron reduction of bis(biphenyl)chromium(I) tetraphenylborate were studied in N , N -dimethylformamide as a function of the concentrations of the supporting electrolyte tetrabutylammonium perchlorate. Cyclic voltammetry on a hemispherical mercury ultramicroelectrode as well as a.c. polarography on the dropping mercury electrode were employed to obtain k s app and α values. An increase in the standard electrode reaction rate constants with increasing concentrations of the supporting electrolyte was observed. Measurements in other supporting electrolytes (sodium perchloorate, tetrabutylammonium hexafluorophosphate and tetrabutylammonium tetraphenylborate) showed only minor influences of the nature of the supporting electrolyte on the kinetics of the electroreduction of bis(biphenyl)chromium(I). The observed changes in the apparent reaction rate constants are discussed, together with the influence of the solvents and electrolytes on the kinetic behaviour of other organometallic redox systems.

Use of High-Frequency AC Voltammetry To Study Kinetics of Physical Adsorption at Mercury Ultramicroelectrodes

Adsorption kinetics of small organic molecules was studied by ac voltammetry with mercury ultramicroelectrodes in the frequency range 0.22-6.25 MHz. It was found that the activation energy of the adsorption process can be expressed as a linear combination of the electrical component of the standard free energy of adsorption (a major contribution) and the energy of lateral interactions (a minor contribution). The results suggest that, during the adsorption process, work against an electrical field must be performed before excess energy (due to the change in the hydrophobic interactions) is released. The final state is reached by a rotational and translational motion of the surfactant molecule. At the zero charge potential the rate constant reaches the maximum value of (4.6 ± 0.3) x 10 9 s -1 . This value indicates that the viscosity of interfacial water at mercury near the zero charge potential is about 1.7 times higher than the bulk viscosity.

Kinetics and thermodynamics of thioglycol adsorption on mercury ultramicroelectrodes

A method for determination of the thermodynamic and kinetic parameters of electrosorption from fast sweep rate cyclic voltammograms was developed and tested using a simulation program. The method was then used for determination of the thioglycol adsorption kinetics on Hg in KOH and H 2SO 4 aqueous solutions. Cyclic voltammetric experiments were carried out with a hemispherical Hg ultramicroelectrode (5 μm radius) at sweep rates ranging from 1000 to 100 000 V s −1. The experimental data fit relatively well to a theoretical model based on the Frumkin adsorption isotherm, despite the fact that the current model assumes independence of the electrosorption valency and the interaction parameter of the potential and the electrode coverage, and that it neglects possible reorientation of adsorbed molecules on the surface with a change in electrode coverage and/or potential. It was found that the electrosorption valency for thioglycol in both solutions studied is similar and close to unity; the standard energies of adsorption are 2.93 and −46.6 kJ mol −1 in 1 M H 2SO 4 and 1 M KOH respectively; the interaction parameters are −3670 J mol −1 (in 1 M H 2SO 4) and 2450 J mol −1 (in 1 M KOH); the rates of adsorption and desorption steps extrapolated to the zero charge potential are k a 0 = 4.7 × 1O 5 s −1, k d 0 = 9.8 × 10 6 s −1 (in 1 M H 2SO 4) and k a 0 = 1.7 × 10 12 s −1, k d 0 = 2.12 × 10 4 s −1 (in 1 M KOH). The interaction parameter for the activated complex in both studied solutions is 3000 J mol −1 larger than the corresponding interaction parameter of the product. The results are discussed in terms of participation of protons in the adsorption-desorption process and the effect of potential on the orientation of adsorbed molecules.

Iridium-based ultramicroelectrode array fabricated by microlithography

A microlithographically fabricated iridium ultramicroelectrode array is used to provide a disk array for formation of mercury microhemispheres. The mercury ultramicroelectrode array (Hg UMEA) shows a response with chronoamperometry and square wave anodic stripping voltammetry equivalent to that expected for a single Hg UME. The array, made up of 19 interconnected 10-pm-diameter UMEs, provides a multiplication of the signal current proportional to the number of UMEs in the array, without any indications of overlap of the diffusion fields. Deposited mercury on the iridium disks is physically and chemically stable with no indication of degradation or dissolution of the iridium pads even after several days of use. This microlithographically fabricated device provides a stable, welldefined, and reproducible array of mercury ultramicroelectrodes. The analytical utility and performance of the Hg UMEA is demonstrated by the analysis of Cd(II), Pb(II), and Cu(I1) in neat spring water samples.

Gold ultramicroelectrodes for on‐site monitoring of trace lead

AbstractGold and gold‐coated cylinder microelectrodes are used as stripping‐based microsensors for trace lead. These gold ultramicroelectrodes exhibit a favorable stripping performance, similar to that observed at common mercury ultramicroelectrodes. Such use of “mercury‐free” electrodes greatly facilitates the development of disposable sensors for lead blood screening programs. Potentiometric stripping analysis at these gold ultramicroelectrodes yields reproducible results for parts per billion (μg/L) concentrations of lead, following short preconcentration periods and using static, nondeaerated solutions. Numerous experimental variables have been characterized and optimized. Repetitive assays of 200‐μl sample drops yield a relative standard deviation of 2.2% (n = 20). The application of the gold ultramicroelectrodes for assays of various real samples is demonstrated, and prospects for their use in decentralized testings are discussed.

Voltammetric study of ammonium amalgam

Ammonium amalgam formation has been studied at mercury ultramicroelectrodes in various organic solvents and at standard size mercury electrodes in aqueous solution. Numerical simulations were carried out in order to establish the kinetics of the process. Ammonium amalgam is formed via a fast charge-transfer process, but it undergoes a decomposition according to the first-order rate law with a rate constant of about 0.6 s −1. In addition, some NH 4 + ions are irreversibly discharged at the electrode surface, presumably to form H 2 and NH 3. This competing process is affected strongly by the nature of the solvent. Matching of experimental and simulated curves suggests that the diffusion coefficient of NH 4 in mercury is about 1.8 × 10 −5 cm 2 s −1. The standard potential of NH 4 +(aq)/NH 4(Hg) was determined to be −1.723 V/SHE. The absolute chemical potential of ammonium ions in mercury was calculated and compared with that of alkali metal ions.

Medium effects in the electroreduction of benzophenone in aprotic solvents

The kinetics of electroreduction of benzophenone have been studied at a mercury ultramicroelectrode in seven aprotic solvents. In addition, the temperature dependence of the standard rate constant was determined in three solvents, and the effect of tetraalkylammonium cations on the kinetic parameters was assessed in acetonitrile. On the basis of an analysis of the solvent effects, it is shown that this fast electron-transfer reaction is adiabatic. A model for solvation of the activated complex based on three contiguous spheres corresponding to the two phenyl rings and the carbonyl group is described and used to assess the outer-sphere contribution to the Gibbs activation energy for electron transfer

Analytical utility of the iridium-based mercury ultramicroelectrode with square-wave anodic stripping voltammetry

The characterktics of the irldium-based hemkphwkai mercury uitramlcrodectrode (Ha-UME) in conjunction with squarewave anodic stripping voltammetry (SWASV) have been Investigated. Udng Hg-UMEs of 5-10-pn~ radii, samples were analyzed without any added electrolyte, deoxygenatlon, or forced convectlon. The peak stripping currents displayed a linear rdation8hip with d.porltion times varying from 1 to 1200 8. Peak potentlais and current remained substantlaily unchanged with electrolyte concentrations of 1-104 M. The reproduclblilty and useful lifetime of the Hg-UME was studied and found to be excellent over several days use. The application of SWASV theory for mercury film electrodes is shown to not entirely describe the rerpow at the Hg-UME, especially at low square-wave frequencies.

Adsorption voltammetry at a mercury ultramicroelectrode

Adsorptive voltammetric characteristics for reversible, totally irreversible and quasi-reversible interfacial reactions at a mercury ultramicroelectrode have been investigated. It was found that the dependences of the peak current on potential scan rates, preconcentration time in a quiescent solution, electrode radius and bulk concentration of adsorbed substance were in agreement with theoretical prediction. A procedure for determining trace nickel in microanalysis with 5 μl samples has been described.

Pseudopolarography at the mercury hemisphere ultramicroelectrode: theory and experiment

Trace heavy metals play a central role in many biogeochemical processes. Their toxicity and bioavailability in aqueous systems has been shown to be directly related to the chemical forms in which they exist, i.e., their chemical speciation. Especially promising in speciation studies has been an electrochemical technique referred to as pseudopolarography or stripping polarography. The theoretical expressions for pseudopolarography at a hemispherical mercury ultramicroelectrode are derived and compared with experimental data at varying deposition times and low and high electrolyte concentrations and with previous theory for other electrodes. It is shown that, even though several of the physical conditions are different, the behavior of both the pseudopolarographic half-wave potential and the limiting peak stripping current is similar to that for the HMDE. The fit between the theory and the experimental data was excellent. A correction term for the increase in the limiting peak stripping current with increasing negative deposition potentials seen in this and previous works is discussed and attributed to adsorption of trace organics on the mercury electrode surface. 21 refs., 4 figs., 1 tab.

Anodic stripping voltammetric determination of trace lead in micro-samples with a mercury ultramicroelectrode

Anodic stripping voltammetric determination of trace lead in micro-samples with a mercury ultramicroelectrode

Anodic stripping voltammetric determination of trace lead in micro-samples with a mercury ultramicroelectrode

The anodic stripping voltammetric characteristics of lead at a mercury ultramicroelectrode were investigated. A procedure for the digestion of 1 mg human hair with nitric and perchloric acids and an anodic stripping voltammetric technique for determining trace lead in 5 μl of solutionn are described. The relative standard deviation of the method is 4.1% for 1 × 10 −7 mol 1 −1 Pb 2+ at a mercury ultramicroelectrode with a radius of 5 μm.

An indium based mercury ultramicroelectrode: Fabrication and characterization

A mercury ultramicroelectrode (Hg-UME) has been prepared by electrochemical deposition of mercury on an iridium substrate. The iridium UMEs can be prepared with radii of 1–20 μm using a molten salt etching solution of NaCl and NaNO 3. The Ir and Hg UMEs have been characterized by cyclic voltammetry and chronoamperometry, and microscopically, and are shown to exhibit diffusional behavior in accord with microelectrode theory and geometry. The mercury is well adhering and stable for long periods of time, with no indication of amalgam formation. The iridium based Hg-UME appears to be a stable and rugged mercury electrode with potential uses for in situ and in vivo electrochemical analysis.

Fast surface reaction study using ultramicroelectrodes

Cyclic voltammetry is a powerful tool for the study of surface reaction kinetics. On the other hand, the upper limit of measurability of the rate constant is often restricted by the shortest experimental time scale which can be achieved. When ultramicroelectrodes are used, the scan rate of cyclic voltammetry can be increased by several orders of magnitude; the examination of kinetic parameters for very fast surface reactions is thus possible. The electron transfer process of several adsorptive species at mercury ultramicroelectrodes has been investigated. The surface reaction rate constant above 500,000 s −1 can be measured readily while the scan rate is at 100 kV s −1 or higher. The ratio of the faradaic current to the charging current is usually high. However, both charging current and faradaic current contribute a sizable iR drop and the data could be distorted, especially at very high scan rates. To evaluate the kinetic parameters reliably, the solution resistance has to be determined and corrected. The approaches for ohmic drop correction are discussed.

ChemInform Abstract: In situ Measurements of Pb2+ Concentration in the Lead‐Acid Battery Using Mercury Ultramicroelectrodes.

Mercury ultramicroelectrodes have been prepared by the electrodeposition of mercury on 0.6 μ platinum microdiscs. The electrodes have been used for the determination of soluble Pb(II) in sulphuric acid solutions and for in-situ measurements in the lead acid battery. The results show that the ultra-microelectrodes offer a simple and reliable method of probing the solution composition under conditions where large area electrodes cannot be used.

In-situ measurements of Pb 2+ concentration in the lead-acid battery using mercury ultramicroelectrodes

Mercury ultramicroelectrodes have been prepared by the electrodeposition of mercury on 0.6 μ platinum microdiscs. The electrodes have been used for the determination of soluble Pb(II) in sulphuric acid solutions and for in-situ measurements in the lead acid battery. The results show that the ultra-microelectrodes offer a simple and reliable method of probing the solution composition under conditions where large area electrodes cannot be used.