Raewyn M. Town

Fundamental features of metal ion determination by stripping chronopotentiometry

A study of the features of stripping chronopotentiometry (SCP) as a function of the stripping current magnitude is presented. Transformation of the E versus t traces into dt/dE versus E curves allows the charging current to be eliminated from the analytical signal (the transition time, τ). Manipulation of the applied stripping current allows the stripping time regime to be varied from conditions of semi-infinite linear diffusion (Iτ1/2 constant; τ∝c2) to conditions approaching complete depletion (Iτ constant; τ∝c). The sensitivity and resolution of SCP are greatest under conditions of semi-infinite diffusion, and are comparable with those of the more widely exploited DP-SV technique

Effects of adsorption in stripping chronopotentiometric metal speciation analysis

The effects of induced metal ion adsorption on speciation analysis by stripping chronopotentiometry (SCP) are examined and compared with those observed in stripping voltammetry (SV). SCP measurements performed under conditions approaching complete depletion (low stripping current) were found to have significant advantages: such measurements are essentially free from adsorption effects, and in addition, the requirement for excess ligand during the stripping step is minimised. In contrast, induced metal adsorption caused enhancement of SCP measurements performed under conditions of semi-infinite linear diffusion (incomplete depletion; high stripping current), DC-SV and DP-SV. We show that the adsorption effects for these shorter timescale techniques are in broad agreement with theoretical predictions, but rigorous analysis is precluded by the fact that adsorption is in some transient stage and thus varies both with time within the measurement period and with potential.

Stripping chronopotentiometry at scanned deposition potential (SSCP).: Part 1. Fundamental features

Abstract A rigorous theory for the shape and characteristics of (deposition potential) scanned stripping chronopotentiometry (SSCP) curves is presented. Their fundamental features are investigated and compared with those of dc polarograms. An equation is derived that fully describes the relationship between stripping time, τ , and deposition potential, E d , in the complete depletion regime at both a conventional HMDE and a microelectrode. This equation is explicit in the analytical signal, τ , and shows that for a reversible system the τ , E d waveform is basically different from the conventional polarogram. Specifically, it is distinctly steeper and does not conform to a linear log-plot. Still, the shift in half-wave potential in the presence of complexing agents, as well as the effect of a reduced diffusion coefficient for the metal complex species, can be interpreted straightforwardly, analogous to the DeFord–Hume expression for a polarographic wave.

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

Abstract 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.

Towards promising oxoanion extractants: azacages and open-chain counterparts

The efficiency of a series of amino-azacryptands for encapsulation and extraction of the oxoanions pertechnetate and perrhenate from aqueous solution is investigated and compared with that of their open-chain counterparts. The aqueous formation constants for oxoanion association with the cryptands were determined by pH potentiometry and/or NMR, and X-ray analysis of single crystals provides evidence for encapsulation. The extractabilities could not be explained solely on the basis of ligand lipophilicity; the level of protonation also plays an important role.

Stripping chronopotentiometry for metal ion speciation analysis at a microelectrode

The features of metal ion speciation determination by stripping chronopotentiometry (SCP) at a microelectrode are examined and compared with those of DP-SV. SCP measurements are essentially of a steady-state nature under experimentally achievable conditions and correspond to practically complete depletion of the microelectrode. SCP data for simple systems (pyridine-2,6-dicarboxylate complexes with Cd(II) and Pb(II)) and a heterogeneous system (Pb(II)–fulvic acid) were in general agreement with the recently developed theory that predicts reduced lability at microelectrodes due to enhanced diffusion. Furthermore, SCP measurements were not influenced by adsorption processes nor by the ligand-to-metal ratio at the electrode surface during stripping. In contrast, DP-SV data could not be interpreted in a straightforward manner due to (i) adsorption-induced enhancement of the stripping signals and (ii) severe peak distortion arising from saturation of the ligand at the electrode surface during stripping.

Relationship between Metal Speciation in Soil Solution and Metal Adsorption at the Root Surface of Ryegrass

The total metal content of the soil or total metal concentration in the soil solution is not always a good indicator for metal availability to plants. Therefore, several speciation techniques have been developed that measure a defined fraction of the total metal concentration in the soil solution. In this study the Donnan Membrane Technique (DMT) was used to measure free metal ion concentrations in CaCl(2) extractions (to mimic the soil solution, and to work under standardized conditions) of 10 different soils, whereas diffusive gradients in thin-films (DGT) and scanning chronopotentiometry (SCP) were used to measure the sum of free and labile metal concentrations in the CaCl(2) extracts. The DGT device was also exposed directly to the (wetted) soil (soil-DGT). The metal concentrations measured with the speciation techniques are related to the metal adsorption at the root surface of ryegrass (Lolium perenne L.), to be able to subsequently predict metal uptake. In most cases the metal adsorption related pH-dependently to the metal concentrations measured by DMT, SCP, and DGT in the CaCl(2) extract. However, the relationship between metal adsorption at the root surface and the metal concentrations measured by the soil-DGT was not-or only slightly-pH dependent. The correlations between metal adsorption at the root surface and metal speciation detected by different speciation techniques allow discussion about rate limiting steps in biouptake and the contribution of metal complexes to metal bioavailability.

Significance of wave form parameters in stripping chronopotentiometric metal speciation analysis

Abstract An analysis is presented of the significance of stripping chronopotentiometric (SCP) stripping peak parameters (peak potential, E p , and peak half-width, w 1/2 ) for determination of metal ion speciation. This study focuses on depletive SCP (low stripping current, Iτ constant), and considers the change in E p and w 1/2 in the presence of complexing and/or adsorbing ligands at both a conventional mercury drop electrode (HMDE) and a micro mercury droplet. We show that at a microelectrode, or at sufficiently long deposition time at a HMDE, the changes in E p and w 1/2 can be related to metal speciation in the bulk solution. Unlike transient stripping voltammetric methods such as DP-SV, depletive SCP is applicable even in the presence of adsorption of ligand and induced metal ion adsorption.

Dinegative tetrahedral oxoanion complexation; structural and solution phase observations

A structural and solution phase study of complexation of dinegative oxoanionic guests by protonated azacryptand hosts is presented. The interactions involve a complex balance of competing factors: anion free energy of hydration, host basicity and solvation, and complementarity of steric matching between host and guest. The formation constants of dinegatively charged anion cryptates are much larger than the analogous values for the mononegative analogues perchlorate or perrhenate.

Chemodynamics of soft nanoparticulate metal complexes in aqueous media: basic theory for spherical particles with homogeneous spatial distributions of sites and charges.

A theoretical discussion is presented to describe the formation and dissociation rate constants for metal ion binding by soft nanoparticulate complexants. The well-known framework of the Eigen mechanism for metal ion complexation by simple ligands in aqueous systems is the starting point. Expressions are derived for the rate constants for the intraparticulate individual outer-sphere and inner-sphere association and dissociation steps for the limiting cases of low and high charge densities. The charge density, binding site density, and size of the nanoparticle play crucial roles. The effects of the electrostatic potential and particle radius on the overall complexation reaction are compared with those for simple ligands. The limitations of the proposed approach for nanoparticulate ligands are discussed, and key issues for future developments are identified.