Herman P. van Leeuwen

Guidelines for terms related to chemical speciation and fractionation of elements. Definitions, structural aspects, and methodological approaches (IUPAC Recommendations 2000)

This paper presents definitions of concepts related to speciation of elements, more particularly speciation analysis and chemical species. Fractionation is distinguished from speciation analysis, and a general outline of fractionation procedures is given. We propose a categorization of species according to isotopic composition of the element, its oxidation and electronic states, and its complex and molecular structure. Examples are given of methodological approaches used for speciation analysis. A synopsis of the methodology of dynamic speciation analysis is also presented.

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.

Effects of surface conduction on the electrokinetic properties of colloids

To quantify the effects of surface conduction behind the slip plane on the electrokinetic transport properties of colloids, we have extended the thin-double-layer theory of Fixman for dilute sols of spherical particles. The computations show that it causes the static conductivity and the low-frequency dielectric response of the sol to increase and the mobility of the particles to decrease. Hence, the occurrence of surface conduction significantly changes the relationships between the different electrokinetic properties. Therefore, for a rigorous interpretation of experimental results taking possible effects of surface conduction adequately into account, it is imperative to collect data with more than one electrokinetic technique. For polystyrene latices, a comparison between theory and experiment has been made. The mobility and static conductivity data can be well reconciled if surface conduction is accounted for. The corresponding estimates of surface charge densities agree very well with values obtained by independent measurements. However, the extreme dielectric behaviour of latex colloids cannot be attributed to surface conduction.

Double Layer of a Gold Electrode Probed by AFM Force Measurements

Colloidal probe atomic force microscopy was used to determine the electric double layer interactions between a gold electrode and a spherical silica probe. The double layer properties of the gold/solution interface were varied through the pH and salt concentration of the electrolyte, as well as by externally applying an electric potential. The double layer potentials ψ(d) of the gold surface were obtained by fitting the force-distance curves according to the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory, using earlier obtained values for the double layer potential of the silica probe as input parameter. It was found that the gold electrode combines the features of reversible and polarizable interfaces; i.e., its charge and potential are determined by both the solution pH and the external potential. The pH dependence is attributed to proton adsorption and desorption from oxidic groups on the gold surface. In the potential range studied, ψ(d) varies linearly with the applied potential; the variation in ψ(d) is roughly 10% of that in the applied potential. The potential of zero force (the external potential at which ψ(d) = 0) varies with pH. The various features of the gold/electrolyte interface are described well by an amphifunctional double layer model. The results of this study form the basis of the interpretation of adsorption studies on gold as a function of pH and externally applied 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.

Voltammetric titrations involving metal complexes: Effect of kinetics and diffusion coefficients.

Abstract This paper presents a theoretical consideration of the voltammetric titration of ligand with metal, under the condition that the diffusion coefficients of the complex and the free metal are unequal. The subject is closely connected with the (homogeneous) kinetic properties of the metal complex system. It is explained how, for the cases of labile, quasi-labile and non-labile complexes, an experimental titration curve must be interpreted. The known extrapolation procedures for finding an equivalence point can only be used in the extreme cases of non-labile and strong, labile complexes. In many situations the interpretation is more involved, if not impossible. It is strongly recommended that every voltammetric complex titration be accompanied by a proper characterization of the kinetic features (and, if necessary, the diffusion coefficient) of the complex.

Bipolar electrode behaviour of the aluminium surface in a lateral electric field

Abstract This paper reports on the electrochemical processes at the surface of conducting materials such as aluminium in a thin-layer cell usually employed for electrokinetic measurements. The cell contains one or more planar Al wafers in contact with an electrolyte solution, which is subjected to an external electric field parallel to the surfaces of the wafers. Beyond a certain threshold value of the magnitude of the field, the current through the cell increases more than proportionally with the field strength. This is due to faradaic processes occurring at the two ends of the conducting substrates, i.e. reduction at the positive side of the electric field in the solution and oxidation at the negative side. In the case of Al wafers, anodic dissolution of the metal takes place and the progression of the ‘corroding’ edge can be followed visually. The overall electrolytic process, corresponding with the distributed current along the surface of the wafer, could be explained and modeled on the basis of the conventionally measured Butler–Volmer characteristics of the monopolar Al electrode.

Faradaic depolarization in the electrokinetics of the metal-electrolyte solution interface.

Streaming potentials (E(str)) have been measured in a flat thin-layer cell with gold and aluminum surfaces. The conventional relation between E(str) and the zeta-potential is shown to be applicable only as long as charge transfer reactions at the metal-electrolyte solution interface are insignificant in terms of the ensuing contribution to the overall cell conductivity. Owing to the irreversibility of the reduction/oxidation of water at most metal surfaces, streaming potentials can be obtained over a very broad range of pressure gradients for metallic substrates in electrolytes such as KNO3. The situation changes drastically in the presence of a reversible redox couple like Fe(CN)(6)3-/Fe(CN)(6)4-. Even small streaming potentials are then greatly diminished due to the extensive conduction that results from the bipolar electrolysis at the metal surface. For gold and aluminum in the presence of various electroinactive and electroactive electrolytes, the measured values for E(str) are shown to be consistent with their conventional voltammetric characteristics.

Reactant adsorption in analytical pulse polarography: An outline in terms of the concentration profile

Abstract The consequences of adsorption of the electroactive species in analytical pulse polarography are discussed. For both normal pulse polarography (NPP) and differential pulse polarography (DPP) explanations are given in terms of the concentration profiles as they are developed upon adsorption before pulse application. An approximate interpretation of the depressed NPP limiting current is sometimes possible by taking its dependence on pulse duration into account. In a number of cases NPP offers the possibility of avoiding adsorption effects by choosing a suitable initial potential. In DPP reactant adsorption generally causes peak enlargement, but the theoretical approach is complex and practical means for checking or eliminating adsorption effects in DPP are limited. Analytical calibration curves start with a linear portion corresponding with the linear adsorption regime; at higher concentrations the DPP curve bends downwards, whereas the NPP curve bends slightly upwards, before reaching the linear region of diffusion control. Several analytical pitfalls connected with the adsorption phenomena are indicated.

The influence of Reactant Adsorption on Limiting Currents in Normal Pulse Polarography

Abstract Explicit expressions are derived for NPP limiting current densities ( i t ) influenced by reactant adsorption. Two cases are considered: (1) a linAbstract Explicit expressions are derived for NPP limiting current densities (it) influenced by reactant adsorption. Two cases are considered: (1) a lin