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.     

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.     

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.     

Stripping Potentiometry of Lead,Cadmium and Copper at a Nafion Coated Glassy Carbon Electrode with Encapsulated Mercury Acetate

Abstract

The stripping potentiometric determination of lead, cadmium and copper with mercury film glassy-carbon electrodes coated with a Nafion membrane was investigated. The mercury film was plated using either mercury(II) acetate encapsulated within the Nafion membrane or a mercury(II) solution. Dissolved dioxygen was used as the stripping agent. The electrodes showed promising properties, particularly robustness and response repeatability. A linear dependence of the stripping time on concentration was found in the μg l^?1 concentration range (s.d. of intercept ≤ 0.3 μg l^?1, r.s.d. of slope ≤ 1%, for both lead and cadmium).     

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.     

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.     

Nafion-coated bismuth film and nafion-coated mercury film electrodes for anodic stripping voltammetry combined on-line with ICP-mass spectrometry

Nafion-coated bismuth film electrodes (NCBFEs) and Nafion-coated mercury film electrodes (NCMFEs) were used to electrochemically preconcentrate metal analytes for subsequent analysis by inductively coupled plasma-mass spectrometry (ICP-MS). Either type of electrodes is part of a thin-layer electrochemical flow cell that is positioned upstream of a microconcentric nebulizer for the ICP-MS. Performances of these electrodes were compared in terms of the analytical "figures of merit" (e.g., dynamic ranges, reproducibility, hydrodynamic stability, and elimination of matrix effects detrimental to ICP-MS). The coupled technique (ASV-ICP-MS) is found to possess a wide dynamic range (at least 4 to 5 orders of magnitude) and to be reproducible. Both electrodes are much more stable than the thin mercury film electrode (TMFE) traditionally used for ASV-ICP-MS, with the lifetime of the NCBFE exceeding 8 h. Adopting these electrodes for ASV-ICP-MS overcomes the problems associated with a TMFE, the erosion of which decreases the sample throughput, affects the analysis precision, and contaminates conventional glass nebulizers and spray chambers of the spectrometer. The medium exchange procedure inherent in ASV is successfully implemented with a two-valve flow injection system for the accumulation of trace Cd2+ into the electrode from a certified seawater sample, followed by stripping Cd into a solution that is compatible to the ICP-MS operation.     

Electrochemical Impedance Characterization of Nafion‐Coated Carbon Film Resistor Electrodes for Electroanalysis

Carbon film disk electrodes with Nafion coatings have been characterized by electrochemical impedance spectroscopy (EIS) with a view to a better understanding of their advantages and limitations in electroanalysis, particularly in anodic stripping voltammetry of metal ions. After initial examination by cyclic voltammetry, spectra were recorded over the full potential range in acetate buffer solution at the bare electrodes, electrodes electrochemically pretreated in acid solution, and Nafion‐coated pretreated electrodes in the presence and absence of dissolved oxygen. EIS equivalent circuit analysis clearly demonstrated the changes between these electrode assemblies. In order to simulate anodic stripping voltammetry conditions, spectra were also obtained in the presence of cadmium and lead ions in solution at Nafion‐coated electrodes, both after metal ion deposition and following re‐oxidation. Permanent changes to the structure of the Nafion film occurred, which has implications for use of these electrode assemblies in anodic stripping voltammetry at relatively high trace metal ion concentrations.     

Anodic stripping voltammetry at a mercury film electrode: baseline concentrations of cadmium,lead, and copper in selected natural waters

A simple, rapid, and inexpensive anodic stripping voltammetric method with a mercury thin film electrode is reported for the establishment of baseline concentrations of cadmium, lead, and copper in natural waters. The procedure for routine surface preparation of wax-impregnated graphite mercury film electrodes requires about 30 min. Concentrations in the 0.006–6 μg l^-1 range are determined by linear d.c. voltage sweeps; the total time for a plating and stripping cycle is 6 min or less. The need for pressure-digesting samples for copper determinations is demonstrated. The a.s.v. results correlate well with corresponding analyses performed by graphite-furnace atomic absorption spectrometry.     

Optimisation of mercury film deposition on glassy carbon electrodes: evaluation of the combined effects of pH, thiocyanate ion and deposition potential

The combined effects of pH, thiocyanate ion and deposition potential in the characteristics of thin mercury film electrodes plated on glassy carbon surfaces are evaluated. Charges of deposited mercury are used as an experimental parameter for the estimation of the effectiveness of the mercury deposition procedure. The sensitivity of the anodic stripping voltammetry (ASV) method for the determination of lead at in situ and at ex situ formed thin mercury films are also examined. It was concluded that, in acidic solutions (pH 2.5-5.7) and fairly negative deposition potentials, e.g. −1.3 to −1.5 V, thiocyanate ion promotes the formation of the mercury film, in respect both to the amount of deposited mercury and to the mercury deposition rate. Also, the mercury coatings produced in thiocyanate solutions are more homogeneous, as depicted by microscopic examinations. In the presence of thiocyanate there is no obvious advantage of using high concentrations of mercury and/or high deposition times for the in situ and ex situ preparation of the mercury film electrodes. The optimised thin mercury film electrode ex situ prepared in a 5.0 mM thiocyanate solution of pH 3.4 was successfully applied to the ASV determination of lead and copper in acidified seawater (pH 2). The limit of detection (3σ) was 6×10⁻¹¹ M for lead and 2×10⁻¹⁰ M for copper for a deposition time of 5 min. Relative standard deviations (R.S.D.s) of <1.2% were obtained for determinations at the nanomolar of concentration level.     

Polymer-Mercury Coated Screen-Printed Sensors for Electrochemical Stripping Analysis of Heavy Metals

In the perspective of in-field stripping analysis of heavy metals, the use and disposal of toxic mercury solutions (necessary to plate a mercury film on a carbon electrode surface) presents a problem. The aim of this work was the development of mercury coated screen-printed electrodes previously prepared in the lab and ready to use in-field. Thus some commercially available polymers like Nafion®, Eastman Kodak AQ29®, and Methocel® were investigated as mercury entrapping systems for electrochemical stripping analysis of heavy metals. Screen-printed disposable cells with a silver pseudo-reference electrode, a graphite counter electrode, and a graphite working electrode were used. To modify the sensor, the polymer solution was cast onto the carbon working electrode surface. Detection limits of 0.8 and 1 μg/L were obtained for lead and cadmium respectively. Since Methocel® based electrodes showed the best performance, they were used for the analysis of real samples. The results were compared with those obtained using a classical thin mercury film electrode and ICP spectroscopy.

All the experiments reported here were performed in un-deareated solutions as required for in-field analysis.     

Anodic-stripping voltammetry of heavy metals in the presence of organic surfactants

The efficacy of fumed silica for removal of sorption interferences by organic surfactants in the anodic-stripping voltammetry of heavy metals is demonstrated. Appropriate addition of silica to the sample solution rapidly "purifies" it from interfering surfactants during the nitrogen purge step. Up to at least 6 ppm of gelatin, Triton X-100, albumin or Liqui-Nox then does not affect the stripping response of cadmium, lead and zinc at the hanging mercury drop electrode. A relative standard deviation of 5.5% is obtained for 20 successive measurements of 1 x 10(-7)M lead in the presence of 2 ppm Triton X-100. Analogous improvements are observed at the mercury film electrode (in the presence of up to 60 ppm of these surfactants). The use of silica thus possesses the advantages of speed, efficiency, simplicity and low cost compared to other schemes for dealing with surfactant interferences in anodic-stripping voltammetry.     

Electrochemistry of and Redox‐Induced Metal Release from Metallothioneins at a Nafion‐Coated Bismuth Film Electrode

Nafion‐coated bismuth film electrodes (NCBiFEs) were used to investigate the redox behavior of and metal release from rabbit liver metallothioneins (MTs) in an acetate buffer. Owing to the permselective exchange between positively charged MT molecules and cations in Nafion and the absence of detectable MT adsorption at bismuth surface, a diffusion‐controlled Nernstian redox wave of MTs (E_pc=?0.869 V) was observed for the first time. The Nernstian behavior of the diffusing MTs is in contrast to the voltammetric responses of MTs at thin mercury films or Nafion‐coated mercury film electrodes, which either result in the replacement of the metals originally present in MTs by mercury or lead to a noticeable MT adsorption. By avoiding these undesirable features, the NCBiFE provides an excellent milieu for voltammetric studies of different types of MTs and related isoforms, paving the way for studying the redox‐modulated metal transfer of MTs and understanding the biological role of MTs in heavy metal detoxification and essential metal regulations.     

Properties of glucose sensors prepared by the electropolymerization of a positively charged pyrrole derivative

Amperometric glucose sensors were prepared by electropolymerization of a pyrrole derivative having the positively charged group, 3-(1-pyrrolyl)propyltrimethylammonium bromide, in the presence of glucose oxidase on bare and Nafion-coated platinum electrodes. Linear relationships between the glucose concentration and the response current for the electrode with and without Nafion inner film were up to 10.0 and 6.0 mmol dm⁻³, respectively. The introduction of Nafion inner film lowered the influence of electroactive compounds, such as ascorbic acid, uric acid, and acetoaminophen, on the sensor response, but was not able to eliminate the influence of these compounds sufficiently. However, Nafion inner film was effective in increasing the electrode stability. The response current of the electrode with Nafion film remained stable for more than 50 days, while that without Nafion film was significantly reduced after 20 days of use.     

Nanostructure Restoration of Thermally Reduced Graphene Oxide Electrode upon Incorporation of Nafion for Detection of Trace Heavy Metals in Aqueous Solution

High performance reduced graphene oxide (RGO)‐Nafion (N) thin film electrodes coated on silicon (Si) substrates (RGO‐N/Si) were successfully developed through thermal reduction of GO‐N without delamination from the substrates. The restoration of the RGO‐N nanostructure upon the addition of Nafion was proven by Raman spectroscopy (RS) and field emission scanning electron microscopy, and the restoration mechanism of the RGO‐N nanostructure was proposed. Through the investigation using x‐ray photoelectron spectroscopy (XPS), the polyfluorocarbon from Nafion possessed a function that could prevent the delamination of the RGO sheets from the substrates during the thermal reduction. The RGO‐N/Si samples were later used for the determination of trace heavy metals, such as divalent lead, cadmium and copper ions (Pb²⁺, Cd²⁺ and Cu²⁺, respectively) using square wave anodic stripping voltammetry in a 0.1 M acetate buffer solution (pH 5). Based on the electroanalytical measurements, the RGO‐N/Si samples exhibited a highly linear behavior in the detection of Cd²⁺, Pb²⁺ and Cu²⁺ over the concentration range of 50 nM to 300 nM with detection limits at nM levels. In addition, the RGO‐N/Si samples presented good recoveries of target metals in tap water samples.     

Disposable Bismuth Oxide Screen Printed Electrodes for the High Throughput Screening of Heavy Metals

We present a simplified approach for the trace screening of toxic heavy metals utilizing bismuth oxide screen printed electrodes. The use of bismuth oxide instead of toxic mercury films facilitates the reliable sensing of lead(II), cadmium(II) and zinc(II). A linear range over 5 to 150 μg L^?1 with detection limits of 2.5 and 5 μg L^?1 are readily observed for cadmium and lead in 0.1 M HCl, respectively. Conducting a simultaneous multi‐elemental voltammetric detection of zinc, cadmium and lead in a higher pH medium (0.1 M sodium acetate solution) exhibited a linear range between 10 and 150 μg L^?1 with detection limits of 5, 10 and 30 μg L^?1 for cadmium, lead and zinc respectively. The sensor is greatly simplified over those recently reported such as bismuth nanoparticle modified electrodes and bismuth film coated screen printed electrodes. The scope of applications of this sensor with the inherent advances in electroanalysis coupled with the negliable toxicity of bismuth is extensive allowing high throughput electroanalysis.     

Temperature dependence of oxygen reduction activity at Nafion-coated bulk Pt and Pt/carbon black catalysts

Oxygen reduction reaction (ORR) activity and H(2)O(2) formation at Nafion-coated film electrodes of bulk-Pt and Pt nanoparticles dispersed on carbon black (Pt/CB) were investigated in 0.1 M HClO(4) solution at 30 to 110 degrees C by using a channel flow double electrode method. We have found that the apparent rate constants k(app) (per real Pt active surface area) for the ORR at bulk-Pt (with and without Nafion-coating) and Nafion-coated Pt/CB (19.3 and 46.7 wt % Pt, d(Pt) = 2.6 to 2.7 nm) thin-film electrodes were in beautiful agreement with each other in the operation conditions of polymer electrolyte fuel cells (PEFCs), i.e., 30-110 degrees C and ca. 0.7 to 0.8 V vs RHE. The H(2)O(2) yield was 0.6-1.0% at 0.7-0.8 V on all Nafion-coated Pt/CB and bulk-Pt and irrespective of Pt-loading level and temperature. Nafion coating was pointed out to be a major factor for the H(2)O(2) formation on Pt catalysts modifying the surface property, because H(2)O(2) production was not detected at the bulk-Pt electrode without Nafion coating.     

Silver based mercury film electrode: Part IV. Comparison of theoretical and experimental voltammetric results obtained for cadmium

Using the silver based mercury film electrode (SBMFE) and cyclic and stripping voltammetric techniques the behaviour of cadmium has been compared with the behaviour of lead as well as with the predictions of the theory of de Vries and van Dalen. At the SBMFE with film thickness between 0.1 and 2 μm lead behaves with good agreement with theoretical predictions and only at thicknesses higher than 2 μm some deviations occur due to collecting of the excess mercury at the bottom of the wire electrode. On the other hand even at thin film electrodes the behaviour of cadmium deviates significantly from the predictions of the theory. The height of the anodic peak decreases and its width increases; also the displacement of the potentials of both cathodic and anodic peaks is smaller than the values predicted theoretically. The deviations are caused neither by the formation of intermetallic compounds in the bulk of mercury phase nor by the formation of heterogenous cadmium amalgam; they reflect the interaction between cadmium dissolved in mercury and the solid silver amalgam which is the substrate of the mercury film. Owing to the effect discussed the stripping determination of cadmium at SBMFE is characterized by a lower sensitivity and reproducibility.     

A Cu/Nafion/Bi electrode for on-site monitoring of trace heavy metals in natural waters using anodic stripping voltammetry: An alternative to mercury-based electrodes

Stripping analysis has been widely recognised as a powerful tool for trace metal analysis. Its remarkable sensitivity is attributed to the combination of a preconcentration step coupled with differential measurements that generate an extremely favourable signal-to-noise ratio. Mercury electrodes have been traditionally employed for achieving high reproducibility and sensitivity of the stripping technique. However, because of the toxicity of mercury, new alternative electrode materials are highly desired, particularly for on-site environmental monitoring of trace pollutants. Bismuth is an electrode material characterized by its low toxicity and its ability to form alloys with some metals of interest like cadmium, lead or zinc, allowing their preconcentration at the electrode surface. We present here the preparation of Cu/Nafion/Bi electrodes and their application to heavy metal analysis by anodic stripping voltammetry. First, the main limitations of the basic Cu/Bi electrode for on-site monitoring in natural waters are highlighted. Then the modification of the Cu/Bi electrode by a Nafion membrane is presented. The analytical performances of this new electrode for trace cadmium and lead analysis were evaluated in non-deaerated solutions. Linear calibration curves were obtained in synthetic solutions for concentrations ranging from 2 to 12 and 2 to 18 μg L⁻¹ for cadmium and lead, respectively, with relative standard deviations lower than 5% (n = 15). The analytical methodology was then successfully applied to monitor the Cd²⁺ and Pb²⁺ content in real samples such as ground water and aquatic plant extracts. The results favourably compared to those obtained using a mercury drop electrode and were validated by ICP-MS.     

Voltammetric analysis of ceftazidime after preconcentration at various mercury and carbon electrodes: application to sub-ppb level determination in urine samples

The electrochemical behavior of ceftazidime (CFZ) at four different kinds of electrodes viz. static mercury drop electrode (SMDE), controlled growth mercury electrode (CGME), glassy carbon electrode (GCE) and carbon paste electrode (CPE) has been presented. Optimal operational parameters have been selected for the drug preconcentration and determination in aqueous medium. Down to 2x10(-10) M CFZ is achieved as detection limit at the CGME. Modification of the CPE with polyvinyl alcohol (PVA) enhances both the sensitivity and selectivity for the drug accumulation and, therefore, its determination at very low levels. Application of the proposed method for CFZ analysis in spiked urine samples or those taken after metabolism has been easily assessed. Down to 1x10(-9) M CFZ (0.695 ng ml(-1)) could be easily achieved in such samples.