Nafion-coated mercury thin film electrodes for batch-injection analysis with anodic stripping voltammetry

Batch-injection analysis exhibits the advantages of rapid and simple electroanalysis of microlitre samples. Nafion-coated mercury thin film electrodes have been evaluated for use in batch-injection analysis with anodic stripping voltammetry (BIA-ASV). The advantages of Nafion-coated electrodes in reducing electrode contamination by components of complex matrices are combined with the analysis of small microlitre sample volumes. The measurement of traces of lead and cadmium is used to illustrate the approach. An optimised procedure for formation of Nafion-coated mercury thin film electrodes is evolved. The relative sensitivity for BIA-ASV at electrodes with and without Nafion coatings is 0.9 and 0.8 for cadmium and lead respectively; detection limits are 2 x 10(-9) M and 4 x 10(-9) M. Studies were done concerning the influence of surfactants and their effect was found to be much less with the Nafion film coating. Applications to real environmental samples are demonstrated.     

The use of Nafion-coated thin mercury film electrodes for the determination of the dissolved copper speciation in estuarine water

Differential pulse anodic stripping voltammetry (DPASV) using a Nafion-coated thin mercury film electrode (NCTMFE) was implemented to determine the dissolved copper speciation in saline estuarine waters containing high concentrations of dissolved organic matter (DOM). The study used model ligands and estuarine water from San Francisco Bay, California, USA to demonstrate that the NCTMFE is more effective at distinguishing between electrochemically inert and labile copper species when compared to the conventional thin mercury film electrode (TMFE). Copper titration results verify that the NCTMFE better deals with high concentrations of DOM by creating a size-exclusion barrier that prevents DOM from interacting with the mercury electrode when performing copper speciation measurements. Pseudovoltammograms were used to illustrate that copper complexes found in natural waters were more apt to be electrochemically inert at the NCTMFE relative to the TMFE when subjected to high negative overpotentials. Copper speciation results using the NCTMFE from samples collected in San Francisco Bay estimated that >99.9% of all copper was bound to strong copper-binding ligands. These L₁-class ligands exceeded the concentration of total dissolved copper in all samples tested and control the equilibrium of ambient [Cu²⁺] in the San Francisco Bay estuary.     

Ion-exchange and permselectivity properties of poly(sodium 4-styrenesulfonate) coatings on glassy carbon: application in the modification of mercury film electrodes for the direct voltammetric analysis of trace metals in estuarine waters

The present work describes the optimisation and characterization of poly(sodium 4-styrenesulfonate)-coated thin mercury film electrodes (PSS-TMFE) for the direct analysis of trace metals in estuarine waters by square-wave anodic stripping voltammetry (SW-ASV). The morphology, thickness and ion exchange ability of the poly(sodium 4-styrenesulfonate) coatings onto glassy carbon were evaluated and these features particularly favoured the incorporation of cationic species, such as dopamine or lead cation. For the case of the heavy metal cations, a simple, sensitive and very reproducible methodology for their SW-ASV analysis could be developed. In fact, with the PSS-TMFE, a significant increase in the sensitivity of the ASV determination of lead was obtained compared both to the uncoated TMFE (ca. 82%) as well as to Nafion-coated electrodes of similar thickness (ca. 43-49%). Furthermore, the permselectivity of the poly(sodium 4-styrenesulfonate) coatings, based both on electrostatic interaction and molecular size, leads to an improved anti-fouling ability against surfactant species. The analytical usefulness of the poly(sodium 4-styrenesulfonate)-coated thin mercury film electrodes is demonstrated by application to the direct ASV determination of trace heavy metals at the low nanomolar level, in estuarine waters with moderate contents of dissolved organic matter, where the uncoated TMFE failed due to fouling.     

Perfluorinated anion-exchange polymer mercury film electrode for anodic stripping voltammetric determination of zinc(II): effect of model organic compounds

The determination of zinc ion (1-60 ng ml⁻¹) by anodic square-wave stripping voltammetry on an anion-exchange perfluorinated polymer Tosflex mercury film electrodes (TMFE) was evaluated. The detection limit was 0.1 ng ml⁻¹ Zn(II). The effect of various organic compounds (gelatin, albumin, starch, camphor, humic acid, Triton X-100, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB)) is explored. The results indicate that due to the size-exclusion and ion-exchange properties of Tosflex film, the TMFE is considerably more resistant to organic interference than the bare mercury film electrode.     

Poly(ester sulphonic acid) coated mercury thin film electrodes: characterization and application in batch injection analysis stripping voltammetry of heavy metal ions

Mercury-thin film electrodes coated with a thin film of poly(ester sulphonic acid) (PESA) have been investigated for application in the analysis of trace heavy metals by square wave anodic stripping voltammetry using the batch injection analysis (BIA) technique. Different polymer dispersion concentrations in water/acetone mixed solvent are investigated and are characterised by electrochemical impedance measurements on glassy carbon and on mercury film electrodes. The influence of electrolyte anion, acetate or nitrate, on polymer film properties is demonstrated, acetate buffer being shown to be preferable for stripping voltammetry applications. Although stripping currents are between 30 and 70% less at the coated than at bare mercury thin film electrodes, the influence of model surfactants on stripping response is shown to be very small. The effect of the composition of the modifier film dispersion on calibration plots is shown; however, detection limits of around 5 nM are found for all modified electrodes tested. This coated electrode is an alternative to Nafion-coated mercury thin film electrodes for the analysis of trace metals in complex matrices, particularly useful when there is a high concentration of non-ionic detergents.     

Signal stability of Nafion-coated thin mercury film electrodes for stripping voltammetry

The signal stability of the Nafion-coated thin mercury film electrode (NCTMFE) was studied by using cadmium and lead as test analytes and differential pulse anodic stripping voltammetry as detection method. In particular, the effect of the casting solvent and the curing procedure employed in the preparation of the polymer film was examined. Best results were obtained with N,N-dimethylacetamide as casting solvent and a two-step curing procedure in which the polymer was evaporated to dryness at 55 degrees and cured at 105 degrees with a hot-air gun. Mercury plating was performed ex situ. An NCTMFE prepared in this manner has a better signal stability than ex situ-plated as well as in situ-plated conventional mercury film electrodes.     

Determination of Aluminum by Adsorptive Cathodic Stripping Voltammetry with 1,2‐Dihydroxyanthraquinone‐3‐Sulfonic Acid (DASA): Effect of Thin Mercury Film Electrode

An analytical technique for aluminum (Al) based upon the complexation reaction between Al and the ligand – DASA (1,2‐dihydroxy‐anthraquinone‐3‐sulfonic acid) has previously been implemented successfully at the hanging mercury drop electrode (HMDE). There are several advantages of using mercury film electrodes (TMFE) over the HMDE, particularly if disposal of mercury is of concern. The novelty of using TMFE for adsorptive cathodic stripping voltammetry (ACSV) of Al – DASA is demonstrated in this paper. The peak potential used for the detection of Al in this system was at ?1.15 V (vs. Ag/AgCl). The method produced a limit of detection (LOD) of 1 μM (n=5) and a linear working range of 1–20 μM Al. Atomic force microscopy methods were used to investigate the nature of the TMFE and its interaction with DASA. The mercury droplets on the TMFE have a limited volume and this may lead to overloading of the electrode at relatively low concentrations of DASA. Interferences from Fe, Ca, Zn and Mg were investigated with only Fe appearing to interfere with the Al‐DASA system. Two masking agents (o‐phenanthroline and 2,2′‐bipyridyl) were shown to be effective at preventing the Fe interference.     

Electrochemistry of Azocrown Ethers in Langmuir-Blodgett Monolayers

Abstract

Monomolecular films of amphiphilic derivatives of crown ethers bearing an azo group in the macrocycle were prepared on surfaces of pure water and transferred onto electrodes using the Langmuir-Blodgett technique. The azocompounds studied were separated into Z and E stereoisomers. Monolayers of both isomers of azocrowns were transferred onto the surfaces of hydrophilic (thin mercury film electrode, TMFE) and hydrophobic (indium-tin oxide, ITO) electrodes. The electrode processes showed more reversible cyclic voltammetry profiles when mercury was used instead of ITO as the electrode substrate. This difference was ascribed to the different orientation of the molecules on the electrode surface.     

Ion-exchange voltammetry with nafion/poly(sodium 4-styrenesulfonate) mixed coatings on mercury film electrodes: characterization studies and application to the determination of trace metals

This work aimed to produce improved polymer coatings for the modification of thin mercury film electrodes (TMFEs). The goal is to obtain sensitive, reproducible, mechanically stable and antifouling devices suitable for the determination of trace metal cations in complex media. Therefore, novel mixed coatings of two sulfonated cation-exchange polymers of dissimilar characteristics-Nafion (NA) and poly(sodium 4-styrenesulfonate) (PSS)-were produced by solvent evaporation onto glassy carbon electrodes. The effect of the mass ratio (NA:PSS) on the film morphology was studied by scanning electron microscopy, revealing the formation of biphasic polymer systems, where PSS bead-shaped clusters appeared randomly dispersed into a uniform and compact NA environment. The permselectivity/ion-exchange features of the mixed films onto glassy carbon were evaluated using cathecol, urate, and dopamine. To allow trace metal analysis, thin mercury films were plated through the NA/PSS coatings, being the reproducibility and ion-exchange features of the mixed coatings-TMFE evaluated using lead ions. The best NA/PSS coating was found for the mass ratio of 5.3. Analytical performance of the NA/PSS-TMFE yielded a detection limit of 5.5 nM (3sigma), and the application of this modified electrode to an untreated polluted estuarine water sample produced significant improvements in the quality of the signal compared with that for a bare TMFE.     

Validation of bismuth film electrode for determination of cobalt and cadmium in soil extracts using ICP-MS

A study is presented on the use of the bismuth film electrode (BiFE) operated in the anodic stripping and the cathodic adsorptive stripping voltammetry (ASV, CAdSV) modes, for the determination of two trace heavy metals (Cd and Co, respectively), in soil extract samples. Two types of BiFE were examined in this study: the in situ prepared BiFE, which was employed in ASV determination of Cd, and the ex situ prepared BiFE, which was used in CAdSV of Co with dimethylglyoxime (DMG) as complexing agent. A series of unpretreated soil extracts with varying Cd and Co concentrations were analyzed, and the results obtained compared to those determined using inductively coupled plasma-mass spectrometry (ICP-MS). The results revealed the suitability of stripping analysis at the BiFE for determination of μg l⁻¹ levels of heavy metals in soil extracts. The promising results obtained here, coupled with the non-toxic nature of bismuth (in comparison to commonly used mercury electrodes employed in stripping analysis), offer great promise in centralized and decentralized analysis of trace heavy metals in complex environmental matrices.     

Effect of model organic compounds on square-wave voltammetric stripping analysis at the Nafion/chelating agent mercury film electrodes

The effect of various organic compounds on the Nafion/chelating agent mercury film electrodes (NCAMFEs) in square-wave anodic-stripping voltammetry (SWASV) is explored. Two chelating agents used to prepare the NCAMFEs are dimethylglyoxime and 2,2'-bipyridyl. Triton X-100, sodium dodecyl sulfate, albumin, gelatin, starch, camphor, and humic acid are used as model organic compounds, while cadmium, lead, and copper are used as test metal ions. The NCAMFEs are considerably more resistant to organic interferences than the Nafion-coated mercury film electrode. The implications of these interferences for the reliability and feasibility of stripping measurements using the NCAMFEs in real samples are discussed. Results presented for untreated urine and natural water samples demonstrate the analytical utility of the NCAMFEs in SWASV.     

Anodic stripping voltammetric determination of bismuth(III) using a Tosflex-coated mercury film electrode

A Tosflex-mercury film electrode (TMFE) was prepared by spin-coating a solution of the perfluorinated anion exchange polymer Tosflex onto a glassy carbon electrode surface followed by electrodeposition of mercury film on this electrode. This electrode was used for the determination of trace bismuth(III) which was preconcentrated onto the TMFE as anionic bismuth(III) complexes with chloride in a chloride medium. The preconcentration was carried out at a potential of-0.2 V, and the preconcentration of the bismuth(III) was enhanced significantly by the anion-exchange feature of Tosflex. The accumulated bismuth(III) was then determined by anodic square-wave stripping voltammetry (SWSV). Various parameters influencing the determination of bismuth(III) were examined in detail. With 2 min accumulation, the analytical signal versus concentration dependence was linear up to 50 ppb, and the detection limit was 0.58 ppb. This modified electrode showed good resistance to the interferences from surface-active compounds and common ions.     

A study of Nafion-coated bismuth-film electrodes for the determination of trace metals by anodic stripping voltammetry

This work reports on the fabrication, characterization and applications of Nafion-coated bismuth-film electrodes (NCBFE's) for the determination of trace metals by anodic stripping voltammetry (ASV). A NCBFE was typically prepared by first applying a 5 microl drop of a 1% Nafion solution onto the surface of a glassy-carbon rotating-disk electrode. After evaporation of the solvent, the Bi film was plated on the electrode in situ(i.e. by spiking the sample with 1000 microg l(-1) of Bi(iii) and simultaneous electrolytic deposition of the metal ions and bismuth film on the electrode surface at -1.4 V) or ex-situ(i.e. by electrolytic deposition of the bismuth film in a separate solution containing 1000 microg l(-1) of Bi(iii), followed by the ASV measurement step in the sample solution). Various fabrication and operational parameters were thoroughly investigated and discussed in terms of their effect on the ASV signals. It was found that this voltammetric sensor was suitable for the determination of metals at trace levels by square-wave ASV (SWASV) due to its multi-element detection potential, improved analytical sensitivity, high resistance to surfactants, low cost, ease of fabrication, robustness, speed of analysis and low toxicity (as compared to traditional mercury electrodes). In the presence of 4 mg l(-1) of Triton X-100, the NCBFE afforded a 10-fold peak height enhancement for the Pb peak and a 14-fold enhancement for the Cd peak over a bare BFE while the determination of Zn was feasible only on the NCBFE. The limits of detection (at a signal-to-noise ratio of 3) were 0.1 microg l(-1) for Cd and Pb and 0.4 microg l(-1) for Zn for a deposition time of 10 min. Finally, the electrode was applied to different real samples (tap-water, urine and wine) for the analysis of trace metals with satisfactory results.     

Comparison of physicochemical speciation procedures with metal toxicity to chlorella pyrenoidosa

Metal speciation as measured by anodic stripping voltammetry (ASV), adsorption on a resin with adsorbed hydrated aluminium oxide, and dialysis with receiving resins is compared with the toxicity of the metals to the freshwater green alga Chlorella pyrenoidosa Chick. In the presence of natural and synthetic ligands, similar labile fractions of metals were obtained for ASV at three electrodes (hanging mercury drop electrode, mercury film electrode and Nafion-coated mercury film electrode). The toxic fractions determined by ASV and bioassay were in reasonable agreement for zinc, cadmium and copper, and alteration of the analytical conditions provided some measure of agreement for lead. The resin with adsorbed hydrated aluminium oxide correlated well with bioassay for copper, but over-estimated lead toxicity and under-estimated cadmium toxicity. Dialysis with receiving resins under-estimated metal bioaccumulation in the presence of ligands. Metal speciation analysis for water samples gave higher toxic fractions for metals in samples of polluted river water and road runoff samples than those found in pristine river water.     

Voltammetry of osmium-modified DNA at a mercury film electrode: application in detecting DNA hybridization

Mercury film electrodes (MFE) have recently been used in nucleic acid electrochemical analysis as alternatives to the classical mercury drop ones. DNA modified with osmium tetroxide, 2,2'-bipyridine (Os,bipy) can be detected with a high sensitivity at mercury electrodes via measurements of a catalytic osmium signal. In this paper we show that mercury film on a glassy carbon electrode can be used in voltammetric analysis of Os,bipy-modified DNA. Application of the MFE as a detection electrode in double-surface electrochemical DNA hybridization assay involving osmium labeling of target DNA is demonstrated.     

Stripping chronopotentiometric measurements of lead(II) and cadmium(II) in soils extracts and wastewaters using a bismuth film screen-printed electrode assembly

The key to remediative processes is the ability to measure toxic contaminants on-site using simple and cheap sensing devices, which are field-portable and can facilitate more rapid decision-making. A three-electrode configuration system has been fabricated using low-cost screen-printing (thick-film) technology and this coupled with a portable electrochemical instrument has provided a a relatively inexpensive on-site detector for trace levels of toxic metals. The carbon surface of the screen-printed working electrode is used as a substrate for in situ deposition of a metallic film of bismuth, which allows the electrochemical preconcentration of metal ions. Lead and cadmium were simultaneously detected using stripping chronopotentiometry at the bismuth film electrode. Detection limits of 8 and 10 ppb were obtained for cadmium(II) and lead(II), respectively, for a deposition time of 120 s. The developed method was applied to the determination of lead and cadmium in soils extracts and wastewaters obtained from polluted sites. For comparison purposes, a mercury film electrode and ICP-MS were also used for validation.     

Determination of Cu, Pb, Cd, and Zn in river sediment extracts by sequential injection anodic stripping voltammetry with thin mercury film electrode

Determination of Cu, Pb, Cd and Zn was performed in sediment extracts obtained according to the three steps sequential extraction procedure proposed by the European Community Standards, Measurements and Testing Program. The metal content was determined by anodic stripping voltammetry with a thin mercury film electrode controlled by a sequential injection (SIA) system. The proposed method improved the reproducibility of conventional anodic stripping voltammetry, as well as the sample throughput, allowing analysis of 30 to 45 samples per hour. The influence of flow rate and sample volume was studied to achieve an adequate sensitivity for the leachate studied. No interferences due to adsorption of organic matter, colloids, or complexes with slow rate of dissociation were observed. The intermetallic formation of Cu-Zn was avoided by forming the mercury film in presence of Ga(III) ions in the SIA system, resulting in low consumption of reagent in comparison to flow injection or continuous flow systems. Results were in good agreement with those obtained by Induced Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES).     

Two-dimensional condensation of nucleic acid components at mercury film and gold electrodes

The adsorption of cytidine at the mercury film electrodes and at the Au (111) single crystal electrode has been investigated. Some kinetic aspects such as the influence of pH and temperature on the formation or dissolution of cytidine adlayer on the pyrolytic graphite electrode covered by a mercury film or on the Au (111) have been studied.     

An introduction to bismuth film electrode for use in cathodic electrochemical detection

A new electrode surface design, the bismuth film electrode (BiFE), is presented as a promising alternative to mercury and other solid electrodes for direct cathodic electrochemical detection of organic compounds. The preparation of the BiFE, involving an ex situ electroplating of metallic bismuth onto a glassy carbon (GC) substrate electrode, was optimised. The useful negative potential windows of the BiFE in the pH range 1 (−0.2 to −0.8 V vs Ag/AgCl) to 10 (−0.2 to −1.5 V) were determined. The reproducibility of measuring 2-nitrophenol as a model compound (relative standard deviation, r.s.d., n=10) was found to be 0.5% at the same BiFE, and 1.0% at successive newly prepared BiFEs. No polishing or any other pre-treatment of the substrate GC surface was required prior to re-plating of a new Bi film. The BiFE showed similar or even favourable voltammetric behaviour when compared to mercury and bare GC electrodes, and was successfully tested for amperometric detection under hydrodynamic conditions. The results revealed that BiFE is an attractive new non-mercury metallic electrode particularly suitable for cathodic electrochemical detection in flow analytical systems.     

General characteristics of the copper-based mercury film electrode

The optimum conditions for the preparation, storage, conditioning and renewal of copper-based mercury film electrodes (CBMFEs) are given. The voltammetric results obtained at these electrodes are compared with the predictions of the theory of cyclic and stripping voltammetry at the mercury film electrode, as well as with the results obtained at the silver-based and the platinum-based mercury film electrodes. The advantage of a CBMFE is prolonged life-time, whereas the disadvantages ar the decreased range of usable positive potentials and the possibility of interfering reactions of the electrodeposited metals with the copper substrate or copper dissolved in the mercury phase. The presence of copper has no essential influence on the behaviour of lead and thallium; it affects the behaviour of zinc markedly and that of cadmium and indium slightly. The conditions allowing the minimization of the harmful action of copper on the behaviour of cadmium and indium have been found.