Browsing by Author "Town, R. M."
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- Cd(II) speciation in alginate gelsPublication . Davis, T. A.; Kalis, E. J. J.; Pinheiro, J. P.; Town, R. M.; Van Leeuwen, H. P.Polysaccharides, such as those occurring in cell walls and biofilms, play an important role in metal speciation in natural aqueous systems. This work describes the speciation of Cd(II) in alginate gels chosen as a model system for biogels. The gels are formed by bridging calcium ions at junction zones present along adjacent homopolymeric guluronic acid chain sequences. The free Cd2+ concentration in the gel phase is measured by a novel in situ microelectrode voltammetric technique that monitors the electroactive probe cation Cd2+ by its reduction at a Au-amalgam microelectrode. In situ voltammetric measurement, coupled with total Ca(II) and Cd(II) determinations, as well as potentiometric titration, permits the full reconstruction of the charging environment and the cation binding for the gel phase. Three independent combinations of measuring and modeling the charged gel layer thereby permit accurate prediction of the Donnan potential, ΨD, and the Donnan enrichment coefficient, ΠD. At an ionic strength of 10 mM, Donnan potentials in the gel ranged from approximately −10 to −20 mV, corresponding to an enhancement in the level of free Cd2+ ions in the gel phase relative to the bulk solution by a factor of approximately 3. In contrast, the total level of Cd(II) was found to be enhanced by a factor of approximately 60, resulting predominantly from the specific binding of the Cd by the uronic acids of the alginate gel. These results emphasize that large differences in Cd(II) speciation can arise due to the combination of specific and electrostatic modes of binding. The results of this speciation analysis, for charged biological gels, have important consequences for mechanistic interpretation of metal biouptake processes involved in complex media.
- Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 6: features of irreversible complex systemsPublication . Town, R. M.; Pinheiro, J. P.; Domingos, R. F.; Van Leeuwen, H. P.The features of SSCP waves for complex species involving a nonreversible electron transfer process are described. For quasireversible systems, with electron transfer rate constants, k0, between O(10 4) to O(10 6) m s 1, the shape of the SSCP wave is dependent on the value of k0, 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, Ed,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., k0 remains the same. The experimental parameters, notably deposition time and electrode size, can be optimised to achieve this condition. For irreversible systems, k0 < O(10 7) m s 1, the SSCP wave shape is independent of k0, deposition time, and electrode size. Further decreasing k0 only leads to additional shift in the wave from E0. In such cases the contribution to the Ed,1/2 from the stability constant cannot be a priori deconvoluted from that due to k0 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.