Potentiometric stripping determination of heavy metals using a graphite-reinforcement carbon vibrating electrode

Potentiometric stripping determination of Cd, Cu and Zn using a vibrating electrode (VE) is presented. A simple VE was constructed by using a piezoelectric bimorph oscillator and an inexpensive graphite-reinforcement carbon (GRC) rod (a mechanical pencil). Experimental results obtained with the simple VE follow the equation valid for quantitative application of potentiometric stripping analysis (PSA) in large solutions. It was found that the GRC vibrating electrode is suitable for multielement trace analysis of small samples of 0.05 mL. The relative standard deviations for divalent metals are better than 2%.     

Stripping voltammetry of copper and lead using gold electrodes modified with self-assembled monolayers

One problem associated with using bare solid metal electrodes, such as gold and platinum, in stripping analysis to determine heavy metal ions such as lead and copper ions in dilute solutions is that underpotential deposition (UPD) gives multiple stripping peaks in the analysis of mixtures. These peaks are often overlapped and cannot be conveniently used for analytical purposes. Bifunctional alkylthiols, such as 3-mercaptopropionic acid, with an ionizable group on the other terminal end of the thiol can form self-assembled monolayers (SAMs) on the surface of the gold electrode. It is shown that such an SAM-modified gold electrode minimizes the UPD effects for the stripping analysis of lead and copper. The anodic peak potential shifts and the peak shape changes, indicating that the SAM changes the deposition and stripping steps of these heavy metal ions. Thus, the sensitivity levels for both single species and mixtures can be significantly improved for the conventional solid electrodes. The mechanism of the deposition reaction at the SAM-modified gold electrodes is discussed. Received: 29 May 1997 / Accepted: 24 June 1997     

Anodic Stripping Voltammetry Using a Vibrating Electrode

This work proposes a vibrating microwire electrode as working electrode in stripping voltammetry. The vibration was found to maintain a constant and thin (1–2 μm) diffusion layer during the deposition step. The electrode vibration eliminated the need for external stirring of the solution, thus facilitating in situ detection in the environment. The vibration was effected by fixing a low‐voltage (3 V), asymmetric, electrical rotor to the working electrode (a gold microwire of either 5 or 25 μm). The sensitivity of the vibrated electrode was ca. 22×greater than stationary. Measurements of copper (4 nM) by anodic stripping voltammetry using the vibrating electrode had a low standard deviation (1% for n=6) indicating that the diffusion layer had only minor variability. The agitation mechanism was unaffected by water moving at >2 m s^?1 and by water pressure equivalent to a depth of >40 m, indicating its suitability for in situ measurements. The vibrating probe was used for in situ detection of copper by anodic stripping voltammetry to a depth of 6 m. Using a 5 min deposition time, the limit of detection for labile copper was 38 pM.     

Determination of the pesticide Chlorpyrifos by cathodic adsorptive stripping voltammetry

The adsorption behavior and differential pulse cathodic adsorptive stripping voltammetry of the pesticide Chlorpyrifos (CP) were investigated at the hanging mercury drop electrode (HMDE). The pesticide was accumulated at the HMDE and a well-defined stripping peak was obtained at –1.2 V vs Ag/AgCl electrode at pH 7.50. A voltammetric procedure was developed for the trace determination of Chlorpyrifos using differential pulse cathodic adsorptive stripping voltammetry (DP-CASV). The optimum working conditions for the determination of the compound were established. The peak current was linear over the concentration range 9.90 × 10^–8– 5.96 × 10^–7 mol/L of Chlorpyrifos. The influence of diverse ions and some other pesticides was investigated. The analysis of Chlorpyrifos in commercial formulations and treated waste water was carried out satisfactorily Received: 10 July 1997 / Revised: 1 April 1998 / Accepted: 6 April 1998     

Cathodic stripping voltammetry of NADH in presence of Cu2+

The adsorption and accumulation of NADH and the Cu²⁺-NADH system at the mercury electrode surface was examined using differential pulse cathodic stripping voltammetry (DPCSV). The method was developed for analytical trace determination of NADH. Experimental and operational parameters for the quantitative determination of NADH were optimized and the detection limit was found to be 9.960 × 10^–8 mol/L. The effect of some interferences (e.g. purine compounds, amino acids and some metal ions) was considered. Received: 3 January 1997 / Revised: 1 April 1997 / Accepted: 4 April 1997     

Cathodic stripping voltammetry of NADH in presence of Cu2+

The adsorption and accumulation of NADH and the Cu²⁺-NADH system at the mercury electrode surface was examined using differential pulse cathodic stripping voltammetry (DPCSV). The method was developed for analytical trace determination of NADH. Experimental and operational parameters for the quantitative determination of NADH were optimized and the detection limit was found to be 9.960 × 10^–8 mol/L. The effect of some interferences (e.g. purine compounds, amino acids and some metal ions) was considered. Received: 3 January 1997 / Revised: 1 April 1997 / Accepted: 4 April 1997     

Simultaneous determination of trace-levels of alloying zinc and copper by semi-mercury-free potentiometric stripping analysis with chemometric data treatment

Assays of copper and zinc in brass samples were performed by semi-mercury free potentiometric stripping analysis (S-MF PSA) using a thin-film mercury covered glassy-carbon working electrode and dissolved oxygen as oxidizing agent during the stripping step. The stripping peak transients were resolved by chemometrics, which enabled simultaneous determination of both the copper and the zinc concentrations, thereby eliminating the conventional necessary pretreatment of the sample solution, such as initial addition of Ga(III) or solvent extraction of copper. The brass samples were diluted by factors in the range 2 · 10⁴– 5 · 10⁵ which resulted in quantification of the copper and of zinc contents comparable to the specified values within 10%. On the basis of the chemometric treatment, an empirical expression is deduced relating the stripping time to the recorded potential. Received: 23 December 1997 / Revised: 25 February 1998 / Accepted: 28 February 1998     

Robotic heavy metal anodic stripping voltammetry: ease and efficacy for trace lead and cadmium electroanalysis

A novel strategy for the automation of trace lead (Pb²⁺) and cadmium (Cd²⁺) anodic stripping voltammetry (ASV) is described. This was achieved using an electrode assembly comprising a small standard reference electrode, a Pt wire counter electrode, and an in situ bismuth-plated pencil lead working electrode for ASV in a robotic device adapted for measurements in a 24-well microtiter plate format. The movement of the electrode assembly through individual wells was by computer-controlled micropositioning, and each microtiter plate run included a sequence of electrode pretreatment, water rinsing, and simultaneous Pb²⁺ and Cd²⁺ ASV measurements. Analyte concentrations down to 2 μg/L (Pb²⁺) and 20 μg/L (Cd²⁺) could be measured in drinking and tap water, a wastewater reference material and a soil sample, with an accuracy and standard deviation typical of stripping analysis. This robotic electrochemical strategy offers automated trace metal analysis with simple instrumentation and is suggested as an option for routine use in analytical laboratories such as those providing environmental heavy metal testing services.     

Sono-anodic stripping voltammetric determination of cadmium in the presence of surfactant

The effect of applying ultrasound in anodic stripping voltammetry analysis of Cd²⁺ in pH 4.6 buffer containing the surfactant Triton X-100 has been investigated. Manipulation of the horn intensity and the horn/electrode distance was found to reduce the deleterious influence of the surfactant on the height of the cadmium peak. The system has been optimised to take advantage of the in situ cleaning action offered by ultrasound. As a result facile detection of Cd²⁺ (0.05–4 μM) in solutions containing 10 μg/mL Triton X-100 can be achieved without recourse to sample pretreatments or electrode modifications. Received: 17 April 2000 / Revised: 2 June 2000 / /Accepted: 8 June 2000     

Sono-anodic stripping voltammetric determination of cadmium in the presence of surfactant

The effect of applying ultrasound in anodic stripping voltammetry analysis of Cd²⁺ in pH 4.6 buffer containing the surfactant Triton X-100 has been investigated. Manipulation of the horn intensity and the horn/electrode distance was found to reduce the deleterious influence of the surfactant on the height of the cadmium peak. The system has been optimised to take advantage of the in situ cleaning action offered by ultrasound. As a result facile detection of Cd²⁺ (0.05–4 μM) in solutions containing 10 μg/mL Triton X-100 can be achieved without recourse to sample pretreatments or electrode modifications. Received: 17 April 2000 / Revised: 2 June 2000 / /Accepted: 8 June 2000     

Voltammetric Quantification of Zn and Cu,Together with Hg and Pb,Based on a Gold Microwire Electrode,in a Wider Spectrum of Surface Waters

The simultaneous determination of Zn and Cu by anodic stripping voltammetry (ASV) is prone to errors due to the formation of Cu‐Zn intermetallic compounds. The main aim of this work was to study the possibility of simultaneous determination of Zn and Cu, together with Hg and Pb, using a mercury‐free solid gold microwire electrode. The multi‐element detection was carried out by differential pulse anodic stripping voltammetry (DPASV), in a chloride medium (0.5 M NaCl) under moderate acid conditions (HCl 1.0 mM) in the presence of oxygen, where the gold microwire electrode was used as stationary or vibrating working electrode during the deposition step. Under these conditions, no formation of Cu‐Zn intermetallic compounds were found for concentrations usually determined in surface waters. In addition, quantification of Zn and Cu, together with Hg and Pb, can be performed in a wide range of concentrations (about two orders of magnitude) using the same sample, in a very short period of time. The detection limits for Cu, Hg, Pb and Zn, using a vibrating electrode and 30 s of deposition time, were 0.2 µg L^?1 for Hg, 0.3 µg L^?1 for Pb and 0.4 µg L^?1 for Zn and Cu, respectively. The proposed DPASV methods were successfully applied to the determination of Cu, Hg, Pb, and Zn in a certified reference fresh water, river, tap and coastal sea waters. These results proved the applicability and versatility of the proposed methods for the analysis of different water matrices and showed that a gold microwire electrode is a suitable choice to determine simultaneously Zn and Cu.     

Anodic stripping voltammetric determination of Zn, Pb and Cu traces in whisky samples

The simultaneous “in natura” determination of trace Zn, Pb and Cu in whisky samples by anodic stripping voltammetry (ASV), using a hanging mercury drop electrode, without previous treatment or addition of supporting electrolyte is described. The choice of an appropriate stripping voltammetric method and deposition potential minimizes the influence of the organic content and ensures a good reproducibility of the measurements. The reliability of the method was tested comparing the results with those of atomic absorption spectroscopy (AAS), with differences of about 10%. The method allows the determination of heavy metal ions in the μg L^–1 range. Received: 14 August 1997 / Revised: 10 December 1997 / Accepted: 11 December 1997     

Voltammetric stripping analysis of metal concentrates with polymers soluble in aqueous solutions

The conditions were worked out for the simultaneous stripping voltammetric determination of components of the ternary Pb(II)-Cu(II)-Cd(II) and binary Ag(I)-Cu(II) systems at a carbon-paste electrode in 2% aqueous solutions of the water soluble polymer polyethyleneimine (PEI) and its thiourea-containing derivative (TU-PEI). Water-soluble polymers are shown to reduce the mutual effects of the components of the binary and ternary system at the electrode surface. The different complex stability of Ag(I), Pb(II) or Cd(II) and Cu(II) with PEI and TU-PEI allows Ag(I), Pb(II) and Cd(II) to be determined in the presence of a large excess of Cu(II). Received: 17 July 1997 / Revised: 1 December 1997 / Accepted: 3 December 1997     

Chelating resin preconcentration and Hg(II) assisted elution followed by differential-pulse anodic stripping voltammetry for the determination of Cd, Cu, and Pb in seawater

A new and efficient Hg(II) back-elution method for the desorption of Cd, Cu, and Pb from Chelex-100 chelating resin was developed. A smaller eluent volume and shorter elution time can be achieved using an Hg(II) containing eluent rather than pure nitric acid. Owing to the remaining Hg(II) ion in the effluent, a mercury thin-film electrode is formed in-situ during the anodic stripping voltammetric determination without any further addition of Hg(II). The results indicate that all the analytes in seawater matrix can be completely adsorbed on Chelex-100 resin from the sample at pH 6.5, and subsequently eluted from the resin with an acid solution of 5 × 10^–4 mol/L Hg²⁺ + 1 mol/L HClO₄. The detection limits obtained from the differential-pulse anodic (μg L^–1 to ng L^–1) stripping voltammetry are at sub-ppb to ppt (μg L^–1 to ng L^–1) levels permitting to determine Cd, Cu and Pb traces in seawater. The analytical reliability was confirmed by the analysis of the certified reference material CASS-II (open ocean seawater). Received: 22 April 1997 / Revised: 5 August 1997 / Accepted: 7 August 1997     

Simultaneous electrochemical determination of arsenic, copper, lead and mercury in unpolluted fresh waters using a vibrating gold microwire electrode

In this work, a simple, rapid, reliable and low cost method for simultaneous electrochemical determination of As, Cu, Hg and Pb ions, on a vibrating gold microwire electrode combined with stripping voltammetry, is described for the first time.The multi-element detection was performed in the presence of oxygen by differential pulse anodic stripping voltammetry (DPASV) in HCl 0.1 M with NaCl 0.5 M. This media was found optimum in terms of peak resolution, peak shape and sensitivities, and has a composition similar to seawater to which the method could potentially be applied. The gold microwire electrode presented well defined, undistorted, sharp and reproducible peaks for trace concentrations of Cu, Hg and Pb and As presented a reproducible peak with a small shoulder. Using a gold vibrating microwire electrode of 25 μm diameter and 30 s deposition time, the detection limits of As, Cu, Hg and Pb were 0.07, 0.4, 0.07 and 0.2 μg L⁻¹, respectively. Possible effects of Al, Cd, Cr, Fe, Mn, Ni, Sb and Zn were investigated but did not cause any significant interferences.Finally, the method was applied for the simultaneous determination of these four metals in unpolluted river water samples and the results were validated by Atomic Absorption Spectroscopy with Electrothermal Atomization (AAS-EA) or by Inductively Coupled Plasma Mass Spectrometry (ICP-MS).     

Palladium Particles‐Impregnated Natural Phosphate Electrodes for Electrochemical Determination of Mercury in Ambient Water Samples

We present here a simple procedure for the determination of mercury(II) using differential pulse anodic stripping voltammetry (DPASV) at palladium particles‐impregnated natural phosphate modified carbon paste electrodes (Pd‐NP‐CPE). The surface of modified electrode was characterized using SEM, infrared spectroscopy, X‐ray diffraction and electrochemical analysis. All experimental variables involved in the voltammetric stripping method were optimized. The detection limit was found to be 4.99×10^?8 mol L^?1 (S/N=3) that is not different to the permitted value for Hg(II) in water reported by the Environmental Protection Agency (EPA). The proposed electrode exhibits good applicability for monitoring Hg(II) in tap and wastewater.     

Use of disposable GRC electrodes for the detection of phenol and chlorophenols in liquid chromatography.

In this study, a wall-jet flow cell with a GRC (graphite reinforced by carbon) electrode was designed for the amperometric detection of phenol and chlorophenols in liquid chromatography. The voltammetric responses of these analytes at the GRC electrodes are very similar to those at conventional glassy carbon electrodes. As the GRC electrodes were made of the same materials as commercially available mechanical pencil leads, they exhibit the advantages of low cost, simple surface renewability, lower residual current, and good electrode-to-electrode reproducibility, and thus can be used as disposable-type electrodes. Chromatographic separations of phenol, o-chlorophenol (o-CP), 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), and pentachlorophenol (PCP) were achieved with an ODS column using a mobile phase containing a mixture of CH3CN and H20 (40:60) containing 25 mM L-(+)tartaric acid (pH = 4.5). Amperometric detections were based on the electrochemical oxidation of these compounds around +0.9 V vs. Ag/AgCl. Under the optimized conditions, linear calibrations were obtained in a range up to 100 microM for phenol, o-CP, 2,4-DCP, 2,4,6-TCP, and 200 microM for PCP, with the correlation coefficients r2 of 0.9992, 0.9997, 0.9986, 0.9992, and 0.9968, respectively. The chromatographic detection limits for the tested analytes were obtained at pmol levels.     

Voltammetric determination of heavy metals leached from ceramics

Anodic stripping voltammetry using a mercury film electrode (MFE) and hanging mercury drop electrode (HMDE) was applied for the determination of trace amounts of lead, cadmium, zinc and copper leached from plates by 4% acetic acid standard solution. The use of a MFE, the fast scan rate (1000 mV/s) by DC voltammetry with subtraction of background current allowed to determine low concentrations of heavy metals. The amounts of metals leached from ware were Pb: 0.1 to 25, Cd: 0.015 to 0.44, Zn: 0.07 to 1.06, Cu: 0.14 to 0.40 μg/mL, depending on the kind of plate and manufacture. Determinations of copper and zinc were carried out using the HMDE electrode in the leachate, determination of tin after medium exchange, and thallium after complexing of lead by EDTA. A significant decrease of metals leached during the second leaching and after washing with detergent solution was found for plates with overglaze decoration. Received: 5 November 1997 / Revised: 24 February 1998 / Accepted: 26 February 1998     

Development of cysteine-modified screen-printed electrode for the chronopotentiometric stripping analysis of cadmium(II) in wastewater and soil extracts

Screen-printed carbon electrodes were fabricated with amino acid functionality by using in situ co-deposition of mercury and cysteine. The three-electrode configuration (graphite carbon working electrode, carbon counter electrode and silver/silver chloride reference electrode) incorporating a cysteine-modified working electrode exhibited good sensitivity towards cadmium(II). Several experimental variables affecting the sensor stripping response were characterised and optimised. These include cysteine and mercury concentrations, deposition time, deposition potential and stripping current. Surface analysis was also conducted using scanning electron microscopy (SEM) in order to characterize the electrode surface during cadmium analysis. The stripping chronopotentiometric response for cadmium(II) was linear in the concentration range 0.4–800 g L^–1 when a deposition time of 2 min was used. A detection limit of 0.4 g L^–1 was obtained using 0.025 M Tris–HCl buffer containing 0.1 M KCl (pH 7.4) as the supporting electrolyte. The analytical utility of the cysteine-modified sensor was demonstrated by applying it to cadmium analysis in various wastewater and soil samples collected from a contaminated site and extracted using acetic acid. The results obtained using the developed electrodes agreed satisfactorily with the values achieved using atomic absorption spectrometry and inductively coupled plasma mass spectrometry analysis. These results demonstrate the feasibility of using this type of sensor for cadmium analysis.     

Preconcentration and voltammetric determination of trace myoglobin at a 6-mercaptopurine modified silver electrode

6-Mercaptopurine(6-MP) is covalently modified onto a silver electrode to construct a chemically modified electrode (CME). It exhibits the capability of selectively complexing myoglobin and can be applied as a selective biosensor for this compound. Myoglobin is accumulated onto the CME at 0.32 V (vs. SCE); after exchanging the medium it is determined by differential pulse stripping voltammetry. A cathodic stripping peak is obtained at 0.15 V (vs. SCE) by scanning the potential from 0.35 to –0.1 V. The peak currents are linearly proportional to the concentration of myoglobin in the range of 0.2 ∼ 4 μg/mL with a relative standard deviation of 7.8%. The detection limit is about 0.1 μg/mL. The mechanism of the complexation is also discussed. Received: 30 January 1997 / Revised: 7 May 1996 / Accepted: 10 May 1996