Mercury films on a glassy carbon support: attributes and problems

Optimization of the electrochemical pre-concentration of trivalent lanthanum from aqueous media

Electrochemical pre-concentration has been shown to effectively increase sample sensitivity and decrease processing time; however, the basic mechanism and optimal conditions of the technique remain unknown, specifically for lanthanides. To gain a better understanding of the mechanism of action, the aqueous solution conditions required to maximize the electrochemical pre-concentration of lanthanum (La) were studied. Parameters investigated included pH, applied potential, and ionic strength. To further optimize and elucidate the mechanism of lanthanide pre-concentration, specific interactions of lanthanum with the mercury film electrode were studied. Three possible mechanisms were proposed based on preliminary observations, including ligand bridging, hydroxide formation, and amalgamation.

Application of anodic stripping voltammetry for zinc, copper, and cadmium quantification in the aqueous humor: implications of pseudoexfoliation syndrome

Anodic stripping voltammetric (ASV) procedure, using mercury film electrode, was optimized and applied to determine the concentrations of zinc, cadmium, and copper in the aqueous humor. Concentration levels as low as 1 ppb of the test metals was possible to be detected using short electrolysis times (120 s) and microquantities of aqueous humor (up to 35 μL). As a first application of the voltammetric analysis of trace metals in the aqueous humor, the role of the three selected trace elements in the pseudoexfoliation (PEX) syndrome was examined. Samples from aqueous humor were collected during cataract extraction from patients with and without PEX. The zinc and copper concentration levels in the aqueous humor did not show statistically significant difference in the study and control group. Cadmium was detected in a small number of samples, without however statistical differences between the two groups. ASV proved to be a highly precise and sensitive tool for the quantification of heavy metal ions in aqueous humor. Further studies may lead to useful conclusions for the role of zinc, copper, or cadmium in PEX syndrome.

Laccase Biosensors Based on Mercury Thin Film Electrode

A biosensor was developed by immobilizing laccase onto mercury thin film electrode (MTFE) by means of gelatin that is then crosslinked with glutaraldehyde. Mercury thin film (MTF) was deposited onto glassy carbon electrode (GCE) and the obtained biosensor was utilized for the determination of phenolic compounds. The measurement was based on the amperometric detection of oxygen consumption in relation to analyte oxidation. The optimum experimental conditions for the biosensor were investigated and the system was calibrated for both catechol and phenol. A linear relationship between sensor responses and analyte concentrations was obtained in concentration range between 0.5 x 10(-6)-5.0 x 10(-6)M for catechol and 2.5 x 10(-6)-2.0 x 10(-6)M for phenol, respectively. Mercury thin film was also formed onto the surface of screen printed graphite electrodes and applied for the catechol detection. The linearity was observed in concentration range between 2.5 x 10(-6)-3.0 x 10(-5)M.

Multifunctional Polypeptide EQCN Sensors: Probing the Cysteamine-Glutathione Film Permeability with Hg(II) Ions

Multifunctional films are the basis of biosensors and play an important role in the emerging field of nanobioelectronics. In this work, films of a tripeptide glutathione (GSH) immobilized on a self-assembled monolayer of cysteamine (CA-SAM) on a quartz crystal Au piezosensor have been synthesized and characterized using electrochemical quartz crystal nanogravimetry (EQCN) with a Hg(II) ion probe. It has been found that in contrast to previously studied Au/GSH films, the Au/CA-GSH films strongly hinder the formation of Hg⁰ with bulk properties while still allowing for relatively easy permeation by Hg(II) ions. This results in complete disappearance of the sharp Hg⁰ electrodissolution peak which is observed on bare Au and Au/GSH piezosensors. The multiple-peak anodic behavior of Au/CA and bare Au is replaced by a single high-field anodic peak of mercury reoxidation in the case of Au/CA-GSH sensors. The mass-to-charge plots indicate predominant ingress/egress of Hg(II) to/from the film. The strong hindrance of CA-SAM to bulk-Hg⁰ formation is attributed to film-stabilizing formation of surface (CA)₂Hg²⁺ complexes with conformation evaluated by ab initio quantum mechanical calculations of electronic structure using Hartree-Fock methods. The associates CA-GSH provide an additional functionality of the side sulfhydryl group which is free for interactions, e.g. with heavy metals. It is proposed that in the film, the CA-GSH molecules can assume open (extended) conformation or bent hydrogen-bonded conformation with up to four possible internal hydrogen bonds.

Electrochemical sample matrix elimination for trace-level potentiometric detection with polymeric membrane ion-selective electrodes

Potentiometric sensors are today sufficiently well understood and optimized to reach ultratrace level (subnanomolar) detection limits for numerous ions. In many cases of practical relevance, however, a high electrolyte background hampers the attainable detection limits. A particularly difficult sample matrix for potentiometric detection is seawater, where the high saline concentration forms a major interfering background and reduces the activity of most trace metals by complexation. This paper describes for the first time a hyphenated system for the online electrochemically modulated preconcentration and matrix elimination of trace metals, combined with a downstream potentiometric detection with solid contact polymeric membrane ion-selective microelectrodes. Following the preconcentration at the bismuth-coated electrode, the deposited metals are oxidized and released to a medium favorable to potentiometric detection, in this case calcium nitrate. Matrix interferences arising from the saline sample medium are thus circumvented. This concept is successfully evaluated with cadmium as a model trace element and offers potentiometric detection down to low parts per billion levels in samples containing 0.5 M NaCl background electrolyte.

Voltammetric studies on the electrochemical determination of methylmercury in chloride medium at carbon microelectrodes

Electroanalytical techniques have been used to determine methylmercury at low levels in environmental matrices. The electrochemical behaviour of methylmercury at carbon microelectrodes in a hydrochloric acid medium using cyclic, square wave and fast-scan linear-sweep voltammetric techniques has been investigated. The analytical utility of the methylmercury reoxidation peak has been explored, but the recorded peak currents were found to be poorly reproducible. This is ascribed to two factors: the adsorption of insoluble chloromercury compounds on the electrode surface, which appears to be an important contribution to hinder the voltammetric signal of methylmercury; and the competition between the reoxidation of the methylmercury radical and its dimerization reaction, which limits the reproducibility of the methylmercury peak. These problems were successfully overcome by adopting the appropriate experimental conditions. Fast-scan rates were employed and an efficient electrochemical regeneration procedure of the electrode surface was achieved, under potentiostatic conditions in a mercury-free solution containing potassium thiocyanate--a strong complexing agent. The influence of chloride ion concentration was analysed. Interference by metals, such as lead and cadmium, was considered. Calibration plots were obtained in the micromolar and submicromolar concentration ranges, allowing the electrochemical determination of methylmercury in trace amounts. An estuarine water sample was analysed using the new method with a glassy carbon microelectrode.

Measurement of Association and Dissociation Rate Constants for Lead(II)/18-Crown-6 Using Square Wave Voltammetry at a Glassy Carbon Mercury Film Electrode

Understanding the rate parameters of metal ion-ligand complexes is necessary for sensing, separations, and responsive materials. The complexation between 18-crown-6 and lead(II) is of particular interest due to the potential use of this chemistry in sensors and separations. We have applied square wave voltammetry at a glassy carbon mercury film electrode to this problem. Lead(II) in aqueous solution containing an excess of 18-crown-6, studied with different experimental time scales, yields stoichiometry, binding constants, and rate constants (25 degrees C). For pulse times longer than 10 ms, the glassy carbon mercury film electrode acts as a planar electrode. For shorter pulse times, a roughness correction factor must be used to calculate dimensionless current because of the increase in effective area due to the droplike nature of the adsorbed mercury. Lead(II) forms a 1:1 complex with 18-crown-6 in both nitrate and perchlorate media. Log K for the complex with the nitrate counterion is 4.13 +/- 0.09 (SEM); in the presence of perchlorate it is 4.35 +/- 0.09 (SEM). The formation rate constants, kf, for the nitrate and perchlorate systems are (3.82 +/- 0.89) x 107 and (5.92 +/- 1.97) x 106 M-1 s-1, respectively. The dissociation rate constants, kd, are (2.83 +/- 0.66) x 103 s-1 with nitrate as the counterion and (2.64 +/- 0.88) x 102 s-1 with perchlorate as the counterion. The significant difference in rate constants for the two anions is probably caused by the ion pairing that occurs with lead(II) nitrate.

Competitive Metal Ion Adsorption in Goethite Systems Using In Situ Voltammetric Methods and Potentiometry

Competitive complexation with respect to the binary-metal ion combinations Cu(II)-Zn(II), Cu(II)-Pb(II), and Pb(II)-Zn(II) were studied at the goethite (alpha-FeOOH)-water interface (25 degrees C, I = 0.1 M NaNO(3)). In addition to potentiometric titrations and batch adsorption experiments, an in situ voltammetric technique was utilized. Sorption studies were performed within the ranges 3.5 </= pH </= 8.5, 10(-6) </= [M(II)](tot) </= 10(-3 )M, and with solid concentrations between 0.09 and 9 g dm(-3). The results obtained (metal uptake and proton release curves) were compared with corresponding predicted values, based upon previously determined thermodynamic surface complexation models for the different single-metal ion goethite systems. The surface complexation modeling was based upon the constant capacitance concept. The results showed that the Cu(II) and Zn(II) surface complexation submodels were combinatory in all two-metal systems studied, both at 10(-3) M and at 10(-6 )M concentrations of the metals. For Pb(II) a somewhat stronger adsorption to goethite than predicted by the previously determined submodel was observed in all two-metal systems studied. These deviations could be corrected for by a moderate adjustment of the equilibrium constant for the most acidic surface complex &tbond;FeOHPb(2+). It can also be concluded that the in situ voltammetric technique can be applied to measure trace concentrations of at least two metal ions simultaneously occurring in goethite suspensions under varying experimental conditions with good accuracy. Copyright 1999 Academic Press.

Preconcentration and voltammetry of mercury on a functionalized sol-gel thin film modified glassy carbon electrode

A functionalized sol-gel thin film modified glassy carbon electrode has been prepared for the determination of mercury. The tetrasulfide in the sol-gel matrix has a high affinity for mercury species. The chemically integrated functional groups in the sol-gel precursor ensure a high chemical and mechanical stability of the modified electrodes, and therefore a stable and reproducible analytical performance. By medium exchange anodic stripping voltammetry, this modified electrode allows reliable, low cost and interference-free determination of mercury.

In Situ Voltammetric Determinations of Metal Ions in Goethite Suspensions: Single Metal Ion Systems

In the present paper voltammetric techniques have been used to study the sorption of Pb(II), Cd(II), Zn(II), and Cu(II) at low concentrations ( approximately 10(-6) M) in goethite (alpha-FeOOH) suspensions with varying solid concentrations as a function of pH. Metal ion adsorption on the cell walls was minimized by using a Teflon cell. Measurements in the absence of goethite showed the applicability of the voltammetric techniques for trace analysis in a wide pH range. The analysis seems to be limited mainly by the onset of metal ion hydrolysis at high pH values, where neutral and negatively charged species are formed. The results obtained in the presence of goethite were compared with previously determined surface complexation models, evaluated from relatively high metal ion ( approximately 10(-3) M) and solid concentrations ( approximately 10 g/L). Application of these models to more dilute conditions could thus be questioned. The results obtained indicated that no "high-affinity" sites are present on the surface of alpha-FeOOH. The voltammetric measurements showed good agreement for the Zn(II) and Cu(II) goethite systems. In the case of Pb(II), deviations between calculated and experimentally determined metal ion concentrations were detected at pH >7 and a molar ratio of Pb(II) to the surface concentration of goethite of 0.17. These deviations may be explained by assuming either formation of polymeric species or surface precipitates at high total concentrations (instead of the previously suggested surface complex identical withFeOPbOH) or slow adsorption kinetics at low concentrations. Also in the Cd(II) goethite system, a corresponding weaker adsorption than predicted was found. These observations strongly emphasize the importance of evaluating surface complexation models from data within concentration ranges as wide as possible. This holds for all components including the solid concentration. Copyright 1997 Academic Press.