Stripping Chronopotentiometry At Scanned Deposition Potential Research Articles

Investigating the Binding Heterogeneity of Trace Metal Cations With SiO2 Nanoparticles Using Full Wave Analysis of Stripping Chronopotentiometry at Scanned Deposition Potential

Silica oxides nano- and microparticles, as well as silica-based materials, are very abundant in nature and industrial processes. Trace metal cation binding with these bulk materials is generally not considered significant in speciation studies in environmental systems. Nonetheless, this might change for nanoparticulate systems as observed in a previous study of Pb(II) with a very small SiO2 particle (7.5 nm diameter). Besides, metal binding by those nanoparticles is surprisingly characterized by a heterogeneity that increases with the decrease of metal-to-particle ratio. Therefore, it is interesting to extend this study to investigate different trace metals and the influence of the nanoparticle size on the cation binding heterogeneity. Consequently, the Cd(II), Pb(II), and Zn(II) binding by two different sized SiO2 nanoparticles (Ludox LS30 and TM40) in aqueous dispersion was studied for a range of pH and ionic strength conditions, using the combination of the electroanalytical techniques Scanned Stripping ChronoPotentiometry and Absence of Gradients and Nernstian Equilibrium Stripping. The coupling of these techniques provides the free metal concentration in the bulk (AGNES) and information of the free and complex concentration at the electrode surface for each Stripping Chronopotentiometry at Scanned deposition Potential (SSCP). A recent mathematical treatment allows the reconstruction of a portion of the metal to ligand binding isotherm with the included heterogeneity information using the full SSCP wave analysis. In this work, we observed that the Zn(II) binding is homogeneous, Cd(II) is slightly heterogeneous, and Pb(II) is moderately heterogeneous, whereas the results obtained with the 7.5 nm diameter nanoparticle are slightly more heterogeneous than those obtained with the one of 17 nm. These findings suggest that the Zn(II) binding is electrostatic in nature, and for both Cd(II) and Pb(II), there should be a significant chemical binding contribution.

Full wave analysis of stripping chronopotentiometry at scanned deposition potential (SSCP): Obtaining binding curves in labile heterogeneous macromolecular systems for any metal-to-ligand ratio

Abstract The different deposition potentials applied in Scanned Stripping ChronoPotentiometry (SSCP) probe different values of the free metal and complex concentrations at the electrode surface. The knowledge of these concentrations gives access to a relevant window of the binding curve of a metal to a homogeneous or heterogeneous ligand. Here, a suitable mathematical treatment for the determination of these surface concentrations, when using a mercury thin film rotating disk electrode, is reported. The proposed procedure does not require ligand excess conditions and takes advantage of the knowledge of the free metal ion concentration in the bulk solution provided by the technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping). It is experimentally shown that the information derived from the SSCP points (i.e. at different deposition potentials in a unique solution) is consistent with the speciation results yielded by the technique AGNES in pertinent solutions prepared within a range of metal-to-ligand ratios. Successful examples with polystyrene sulfonate, Laurentian Fulvic acid and a peat humic acid are reported. It is concluded that, by running consecutively SSCP and AGNES in a single solution, speciation information corresponding to different compositions (i.e. those locally generated at the electrode surface by the end of the deposition step) can be retrieved, this enabling an easy determination of the binding curve.

Addressing temperature effects on metal chemodynamics studies using stripping electroanalytical techniques. Part 1: Lability of small complexes

Temperature effects on metal speciation dynamics were studied using Stripping Chronopotentiometry at Scanned deposition Potential (SSCP). The temporal and spatial scales of this study are respectively O(10−1s) and O(10−5M), characteristics of the thin mercury film rotating disk, used as working electrode. The lability degree and the association rate constant were evaluated in the temperature interval of 15–40°C for a significantly non-labile system, cadmium binding by nitrilotriacetic acid, and a quasi-labile system, lead binding by iminodiacetic acid. The results for both systems reveal that the lability of the metal complex significantly increases with temperature. This lability gain results from the thermal augmentation of the association rate constant and the broadening of the diffusion layer thickness.An evaluation of the metal calibration methodology for SSCP at different temperatures was conducted. It was found that although the variation of diffusion layer thickness can be correctly predicted, changes in standard reduction potential of the metals cannot, thus a calibration must be performed for each temperature studied.This work constitutes a first step toward the comprehension of the effect of temperature on metal chemodynamics.

Lead electrochemical speciation analysis in seawater media by using AGNES and SSCP techniques

Environmental context Metal contamination of seawater can present severe environmental problems owing to the high toxicity of metals and their persistence in the environment. This study explores the possibility of analysing lead in seawater media using two recently developed electrochemical methods. The methods are shown to be very useful tools to monitor the behaviour and fate of lead and other metals in seawater. Abstract The speciation of PbII in synthetic and real seawater is studied by absence of gradients and Nernstian equilibrium stripping (AGNES) and stripping chronopotentiometry at scanned deposition potential (SSCP). The usefulness of the combination of both techniques in the same electrochemical cell for trace metal speciation analysis is assessed at different pH values (2.7, 5.0, 6.0, 7.0 and 8.6). The AGNES (free metal ion concentrations) and SSCP (stability constants) results for synthetic seawater agree reasonably with each other and with the theoretical predictions of the software Visual MINTEQ 3.0. This is also true for real seawater media below pH 7.0. Because of the influence of natural organic matter (2.01mgL–1 total organic carbon) in the real seawater at pH 7.0 and 8.6 the SSCP signal showed that the PbII complexes became less labile and were formed by chemically heterogeneous ligands. At these pH values, free metal concentrations determined by AGNES agreed with concentrations predicted by Visual MINTEQ using a generic fulvic acid concentration.

Scanned stripping chronopotentiometry at bismuth film rotating disc electrodes: a method for quantitative dynamic metal speciation

Environmental context Electroanalytical methods have found wide application in trace metal speciation analysis in environmental systems. The need to find functional alternatives to mercury electrodes for in situ speciation studies has encouraged the use of bismuth as a solid-state electrode substrate. We demonstrate the utility of bismuth electrodes for quantitative dynamic speciation analysis. Abstract Bismuth film electrodes are employed for dynamic metal speciation analysis of PbII complexes by stripping chronopotentiometry at scanned deposition potential (SSCP). Their performance is found to be comparable to that of mercury-film electrodes. The quantitative SSCP expressions that describe the thermodynamic and kinetic complexation parameters are straightforwardly applicable to this solid electrode.

Analysis of metal speciation dynamics in clay minerals dispersion by stripping chronopotentiometry techniques

In aqueous systems, interactions of metal ions with colloidal interphases are considered as a key feature for analyzing trace metal speciation, mobility, lability and bioavailability. Among colloidal matters of interests, clay minerals play a major role in metal complexes formation and sorption. The aim of this study is to analyze the dynamic speciation of metal in the presence of clay as colloidal ligand. This work is achieved by means of two complementary electroanalytical techniques: Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) is used for the direct determination of the free metal in solution and stripping chronopotentiometry at scanned deposition potential (SSCP) is applied for the dynamic speciation analysis. The measured response of SSCP reflects the flux properties (limiting transport or kinetic fluxes) of metallic species and allows the determination of thermodynamic constants and analytical lability features of metal complexes. In addition to that, one of the main interests of using such a technique lies in the fact that analyses are achieved in metal concentrations of environmental relevance (from millimolar to nanomolar). For anisotropic and chemically heterogeneous particles such as clay particles, this study demonstrates the possibility to obtain useful information as the determination of diffusion coefficients of particles as well as the description of dynamic behavior of metal species as a function of physicochemical conditions of the suspension. In this regard, this study shows that the sorption of cadmium by clays can be described as a chemically homogeneous and labile system along a large range of pH whereas lead sorption shows some heterogeneity aspects, while remaining SSCP-labile.

Determination of trace metal speciation parameters by using screen-printed electrodes in stripping chronopotentiometry without deaerating

The objective of this study was to exploit the advantages of stripping chronopotentiometry (SCP) and stripping chronopotentiometry at scanned deposition potential (SSCP) for trace metal speciation analyses by using thin-film mercury screen-printed electrodes (TMF-SPE). At first, the SCP parameters were optimised for TMF-SPE, in order to reach the complete depletion regime. It has been shown that a stripping current higher than or equal to 10 μA allows this regime to be attained without removing oxygen from the solution. Then, these analytical conditions were used for the construction of SSCP curves for Cd-PDCA and Cd-NTA. When the concentration of free ligand in solution was known, the knowledge of the model describing the SSCP curves in absence and presence of a complex and the use of an effective fitting tool enabled estimating the stability constant and the rate constants for complexation. Further studies with complexes of restricted mobility are however necessary to assess the usefulness of this procedure to also estimate the diffusion coefficient of the complexes. Besides, this study showed that this approach was valid even when ligands were not in excess at the electrode during stripping.

Application of permeation liquid membrane and scanned stripping chronopotentiometry to metal speciation analysis of colloidal complexes

The potential of permeation liquid membrane (PLM) to obtain dynamic metal speciation information for colloidal complexes is evaluated by measurements of lead(II) and copper(II) complexation by carboxyl modified latex nanospheres of different radii (15, 35, 40 and 65 nm). The results are compared with those obtained by a well characterized technique: stripping chronopotentiometry at scanned deposition potential (SSCP). Under the PLM conditions employed, and for large particles or macromolecular ligands, membrane diffusion is the rate-limiting step. That is, the flux is proportional to the free metal ion concentration with only a small contribution from labile complexes. In the absence of ligand aggregation in the PLM channels, good agreement was obtained between the stability constants determined by PLM and SSCP for both metals.

Determination of diffusion coefficients of nanoparticles and humic substances using scanning stripping chronopotentiometry (SSCP)

A methodology, based on a labile metal ion probe using stripping chronopotentiometry at scanned deposition potential (SSCP), is presented for the determination of the diffusion coefficients of nanoparticles and humic matter. The novel methodology was successfully applied to the determination of diffusion coefficients (and thus hydrodynamic diameters) of eight standard nanoparticles with radii ranging from 5 to 129 nm and two samples of colloidal humic substances with hydrodynamic radii of ca. 1 nm. Good agreement was found between the SSCP determinations and results obtained by dynamic light scattering (DLS), transmission electron microscopy (TEM) and fluorescence correlation spectroscopy (FCS). The SSCP technique is critically analysed with respect to its use for the determination of diffusion coefficients of colloidal complexes.

Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 8. Metal speciation analysis in gels

Abstract In case of a finite negative Donnan potential difference, ΨD, metal speciation in a gel is significantly altered as compared to that in the surrounding aqueous medium. For anionic gels (e.g., typical biogels), ΨD leads to enhanced concentrations of positively charged species within the gel, and reduced concentrations of negatively charged ones. Thus both the absolute concentrations and the ratios of species are altered. Stripping chronopotentiometry at scanned deposition potential is shown to provide a reliable measure of the stability and lability of metal species in gels. Results are reported for labile, quasilabile and inert metal–ligand systems. Failure to take into account the Donnan partitioning leads to erroneous interpretations on the distribution and lability of species within the gel phase. Translation of the speciation measured within the gel to that in the surrounding medium is involved for chemically heterogeneous systems, e.g., natural waters. The results are of importance for interpretation of analytical signals from gel-integrated sensors and for understanding the behaviour of the different metal species in biogels.

Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 7. Kinetic currents for ML2 complexes

For several ML2 systems, the step ML → M is shown to be the rate-limiting one in the overall process of reduction of metal ions at macro- and microelectrodes. This is consistent with the more common situation in which the stepwise formation constant for ML is greater than that for ML2, meaning that ML2 and ML interconvert faster than ML and M. The experimental data are in convincing agreement with a recently developed analytical lability criterion for ML2 complexes based on the Koutecký–Koryta approximation for modelling lability in the diffusion layer/reaction layer. SSCP waves recorded as a function of ligand concentration verify the nature of the rate-limiting step and allow for determination of the complex stability constants from the shift in half-wave deposition potential. The approach is generally applicable to any multiligand system involving sequential complex formation.

Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 6: Features of irreversible complex systems

The features of SSCP waves for complex species involving a nonreversible electron transfer process are described. For quasi-reversible systems, with electron transfer rate constants, k 0, between O(10 −4) to O(10 −6) m s −1, the shape of the SSCP wave is dependent on the value of k 0, the deposition time and the electrode size. Under these conditions, the stability of a metal complex can be determined from the shift in half-wave deposition potential, E d,1/2, as compared to the metal-only case. This is true even when the system is not fully labile, so long as the shape of the SSCP wave is not altered in the presence of the complexant, i.e., k 0 remains the same. The experimental parameters, notably deposition time and electrode size, can be optimised to achieve this condition. For irreversible systems, k 0 < O(10 −7) m s −1, the SSCP wave shape is independent of k 0, deposition time, and electrode size. Further decreasing k 0 only leads to additional shift in the wave from E 0. In such cases the contribution to the E d,1/2 from the stability constant cannot be a priori deconvoluted from that due to k 0 if its value changes due to the presence of ligand. In irreversible systems, direct reduction of electroactive complexes is not uncommon, nor is its appearance at potentials more positive, i.e., more reversibly, than that for the metal itself.

Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 5. Features of multi-metal systems

Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 5. Features of multi-metal systems

Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 5. Features of multi-metal systems

This paper considers the mutual effects of metals on each other in scanned deposition potential stripping chronopotentiometry measurements (SSCP) on multi-metal systems. As compared to stripping voltammetry, depletive SSCP displays a certain resistance to the impact of intermetallic compounds, while ongoing deposition during the reoxidation step can be clearly identified and eliminated. Although depletive SSCP is susceptible to electroless reoxidation processes in the ongoing stripping step, this effect is minimised at a microelectrode, and would be of practical significance only under extreme metal concentration ratios. With respect to this phenomenon, SSCP and stripping voltammetry are, to a certain extent, complementary.

Depletive Stripping Chronopotentiometry: A Major Step Forward in Electrochemical Stripping Techniques for Metal Ion Speciation Analysis

AbstractA comparative evaluation of the utility of the various modes of stripping chronopotentiometry (SCP) for trace metal speciation analysis is presented in the broad context of stripping voltammetric techniques. The remarkable fundamental advantages of depletive SCP at scanned deposition potential (SSCP) are highlighted, and the rigorous underpinning theory is described. The distinctive features of SSCP include: i) an effective getting around part of the Nernstian extension of the reoxidation process, leading to ii) greater resolution than conventional stripping voltammetries; iii) a certain insensitivity to electrochemical irreversibility, especially at a microelectrode; iv) in principle, freedom from induced metal ion adsorption interferences; v) no requirement for excess ligand during stripping; and vi) ability to provide a certain measure of any chemical heterogeneity in the metal speciation that is easily distinguishable from effects of electrochemical irreversibility.

Dynamic Speciation Analysis of Heterogeneous Metal Complexes with Natural Ligands by Stripping Chronopotentiometry at Scanned Deposition Potential (SSCP)

Stripping chronopotentiometry at scanned deposition potential (SSCP) allows chemical heterogeneity in metal speciation to be unambiguously identified. In the labile regime, use of the Freundlich binding isotherm allows straightforward determination of parameters to describe the apparent stability and heterogeneity of metal complexes with humic substances. The extent of heterogeneity of metal binding by several humic substances follows the order Cu(ii) &gt;&gt; Pb(ii) &gt; Cd(ii). The lability of metal complexes decreases from the foot to the top of the wave, and the greater the degree of heterogeneity, the more readily lability is lost. In the kinetic current regime, the Koutecký–Koryta approximation allows an expression to be obtained for the SSCP wave that provides a good estimate of the experimental data for metal complexes with moderate degrees of heterogeneity.

Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 4. The kinetic current regime

The theory for depletive stripping chronopotentiometry at scanned deposition potential (SSCP) has been extended to include the case of kinetic currents. Invoking the Koutecký–Koryta approximation, i.e., assuming a spatially discontinuous transition from labile to non-labile behaviour, allows a rigorous expression to be obtained for the full SSCP wave. Theoretical predictions are supported by experimental data for the quasi-labile Cd–NTA system at a conventional HMDE and a mercury microelectrode. The characteristic features of the SSCP wave for systems influenced by homogeneous kinetics allow dynamic speciation characteristics to be unambiguously identified and discriminated from those due to irreversibility in the electrochemical reaction or heterogeneity in the chemical speciation.

Stripping chronopotentiometry at scanned deposition potential (SSCP): Part 3. Irreversible electrode reactions

A rigorous expression is derived for the scanned deposition potential stripping chronopotentiometry (SSCP) curve for the case of quasi-reversible and irreversible redox reactions. The equation is fully applicable to both a conventional (macro) electrode and a microelectrode. SSCP curves are shown to have a spectacular insensitivity to electrochemical irreversibility and this feature is even more pronounced at a microelectrode. These observations can be readily explained by considering the relative location of the curves on the potential axis. Modification of the experimental conditions (deposition time, electrode volume) allows the extent of resistance to irreversibility to be manipulated. These remarkable characteristics have far-reaching implications for in situ analysis, and allow the effects of irreversibility to be easily distinguished from e.g. those of chemical heterogeneity in the metal speciation.

Electrochemical metal speciation analysis of chemically heterogeneous samples: the outstanding features of stripping chronopotentiometry at scanned deposition potential.

The application of depletive stripping chronopotentiometry at scanned deposition potential (SSCP) to metal ion speciation analysis of chemically heterogeneous complex systems is described. In this electroanalytical stripping technique, metal which is accumulated in the electrode during the deposition step is quantitatively reoxidized during the detection step. SSCP is able to provide an unambiguous measure of heterogeneity that is not attainable by other nondepletive electroanalytical stripping techniques that measure only a proportion of the accumulated metal. SSCP data are not affected by the secondary interferences that plague conventional electrochemical techniques, that is, induced metal adsorption and insufficient ligand excess during stripping. Furthermore, the effect of heterogeneity, as manifested in flattening of the SSCP wave, can be distinguished from that of electrochemical irreversibility. Data are presented for complexation of Cu(II), Pb(II), and Cd(II) by several humic fractions: distinct heterogeneity differences are observed.

Stripping chronopotentiometry at scanned deposition potential (SSCP): Part 2. Determination of metal ion speciation parameters

Stripping chronopotentiometry at scanned deposition potential (SSCP) generates curves that are fundamentally different in form from classical polarographic waves. Still, despite their steeper slope and non-linear log plot, the shift in the SSCP half-wave deposition potential can be interpreted in a manner completely analogous to the DeFord–Hume treatment of polarographic waves. SSCP provides a reliable measure of metal speciation in the sample solution in the presence of simple ligands (i.e. those with non-distributed K and D values), even when there is relatively small ligand excess, and it is unaffected by induced metal adsorption. Data interpretation is not so straightforward for heterogeneous ligands: the presence of a range of metal complexes of slightly different stability and mobility flattens the SSCP wave as compared to the homogeneous case. The time dependent nature of the surface concentration of metal ion during deposition precludes straightforward rigorous interpretation of heterogeneous metal ion speciation.