Voltammetric investigation of the complexation equilibria in the presence of a low level of supporting electrolyte Part 1: Steady-state current-potential curves for inert complexes (original) (raw)
Related papers
Linear regression models in the study of charge-transfer complexation
The Journal of Physical Chemistry, 1970
Linear regression models are analyzed in the determination of equilibrium constants, K , and extinction coefficients, E, of charge-transfer complexes. Criteria for separating K and e from the Ke product in weak complexes are refined. For strong complexes where linear approximations lead to large error, equations are developed which permit reasonable corrections to be made from a knowledge of the experimental conditions.
Voltammetric lability of multiligand complexes: the case of ML2
Journal of Electroanalytical Chemistry, 2004
The voltammetric lability of a complex system where a metal ion M and a ligand L form the species ML and ML 2 is examined. Together with the rigorous numerical simulation of the problem, two limiting cases are analysed for the overall process ML 2 → M: (i) the most common case for aqueous complexes where ML → M is the kinetically limiting step and (ii) the case where ML 2 → ML is limiting. In both cases, analytical expressions for the lability criteria are provided which show good agreement with the results obtained from the rigorous numerical simulation of the problem. reprints also to galceran@quimica.udl.cat 3 [3]. This arises logically from the definition of J kin : this parameter represents the increase of metal flux appearing in the system as compared to that for a system in the absence of complexed species, but with the same free metal concentration (i. e. J kin is the contribution of the complex to the metal supply). The actual J kin is compared with the maximum kinetic flux that could arise in the system (i.e. when the complex is fully labile) which coincides with the maximum purely diffusive flux of the complex, J dif . For the practical application of this comparison, J dif is estimated by means of the diffusion layer approximation while J kin is obtained by means of the classical reaction layer approximation. Based on bulk concentrations, this approach is a simplification that overestimates J kin : the resulting J kin is the maximum kinetic flux attainable because it assumes an unlimited supply of complex, i.e. that no depletion occurs in the diffusion layer. Using this formalism based on bulk solution concentrations, J kin largely outweighs the maximum J dif of ML for fully labile complexes, whilst in the non-labile case the maximum J dif of ML is much larger than J kin . Drastic changes in lability can occur with time, and with spatial scale. The well-known dimensionless lability criterion parameter, L, (= J kin /J dif ) is used to quantify lability for a given system at a given timescale, with L >> 1 for the fully labile case. An inert (static) complex represents the trivial case in which ML does not contribute to the flux.
Journal of Electroanalytical Chemistry, 2000
Computer simulations of cyclic voltammetry experiments at liquid liquid interfaces are presented for the transfer of ions (M z + ) assisted by z charged ligands (L − ). The main difference, when compared with neutral ligands, is that in the case presented here, all the complexes formed have a different charge and the flux of the ligands has to be taken into account in the definition of the current, in addition to that of the metal ion and those of the (z−1) charged complexes (the complex with the highest stoichiometry being neutral). The particular cases of M + /L − and M 2 + /L − systems are presented. The effect, on the cyclic voltammetry response, of various parameters such as association constants, metal and ligand concentration, is analysed and results are compared to the case of neutral ionophores.
Analytical Chemistry, 1996
An exact treatment is developed to predict the steady-state limiting voltammetric current, I L , for a system in which the reaction O + + A-/ N occurs reversibly in solution, in the presence of supporting electrolyte C + A-. Either or both O + and N undergo a one-electron reduction at the hemispherical microelectrode. The dependence of I L on the formation constant of the equilibration reaction, the electrolyte concentration, and the support ratio is derived for any trio of values of these parameters and for any combination of diffusivities of the O + , N, and Aspecies.
Portugaliae Electrochimica Acta, 2009
Voltammetric reduction of Zn (II) using L-lysine, L-ornithine, L-threonine, L-serine, Lphenylglycine, L-phenylalanine, L-glutamic acid, L-aspartic acid and vitamin-PP (nicotinamide, niacinamide) at pH = 7.30 ± 0.01, and µ = 1.0 M NaClO 4 was reported at 25 and 35 ºC. The nature of current voltage curves was quasireversible and diffusion controlled. Zn (II) formed 1:1:1, 1:1:2 and 1:2:1 complexes with these drugs as confirmed by Schaap and McMaster method. The sequence of stability constant of complexes L-lysine < L-ornithine < L-threonine < L-serine < L-phenylglycine < Lphenylalanine < L-glutamic acid < L-aspartic acid can be explained on the basis of size, basicity and steric hindrance of ligands. The thermodynamic parameters such as enthalpy (∆H), free energy (∆G) and entropy change (∆S) have also been reported. The kinetic parameters viz. transfer coefficient (α), degree of irreversibility (λ), diffusion coefficient (D) and standard rate constant (k) were calculated. The values of 'α' confirmed the symmetric nature of 'activated complex' between oxidants and reductants response to applied potential between dropping mercury electrode and solution interface.
Journal of Chemical Education, 1988
The modification of one or more of the properties of transition metal ions (e.g., solubility, optical density, redox potential, electric charge, stability) is frequently required in order to use them for specific applications, such as medicinal and personal hygiene products (I), food, cleaning, and photography (2), photoelectrochemical cells (3-5), redox flow cells (6, 7), electrochemical reaction initiators (8,9), micro- biological fuel cells (lo), electron acceptors for hydrogen production (ll), electrocatalysis (12,13), etc. Such modification of properties can he achieved in most cases by complex formation, where the transition metal ion reacts with an organic or inorganic electron donor ligand. In the following experiments, students taking analytical, inorganic, or electrochemistry courses will he able to prepare in situ soluble complexes of Fe(II1) with different ligands and to observe and estimate the change in the formal potential E0' (14) that the Fe3+ undergoes upon complexation. In addition, they will he able to form soluble complexes of two different metal ions, Fe(II1) and Co(I1) with the same ligand (ortho-phenanthroline), and likewise observe the effect produced upon the formal potentials of the two ions by COG- plexation with the same ligand. The variations of such potentials can be estimated by using the technique of cyclic voltammetry (CV), which has been widely described and used in this Journal (15-21). Even though CV is not generally used to determine exact Eo' values, it allows to perform a fast and reasonably good determination by doing a single scan in just one solution, whereas, if equilibrium techniques were used (such as potentiometry), several measurements with several solutions would be required for each metal ion or complex.
Electroanalysis, 2001
A multivariate curve resolution method using alternating least squares (MCR-ALS) has been applied to the study of the Zn-glycine system by polarographic techniques. The electroanalytical behavior of this system does not allow the direct application of hard modelling methods, and makes it an ideal model system to be studied by soft modelling methods. Although the condition of linearity of the current data with respect to the concentrations is not totally fulfilled, the obtained results suggest that the system is reasonably well explained considering the formation of three successive complexes, the two first ones being electrochemically labile and the third one being inert. A satisfactory resolution was achieved if during the ALS optimization the unimodality constraint was only applied to some signals (those corresponding to the free metal ion and to the first two complexes), based on the observed morphology of the experimental voltammograms. Stability constants of these complexes have been estimated from the concentration profiles obtained in the ALS optimization. They have been satisfactorily compared with stability constants from the literature, suggesting that soft modelling provides good estimations of the complexation parameters, and that it is a very useful tool for cases where hard modelling can not be applied.