Publications

Abstract The variations observed in the synchronous fluorescence spectra of a soil fulvic acid, collected in the north of Portugal, with pH (between 2 and 7) and with Cu(II) concentration (at pH 3, 4, 5 and 6 and total copper concentrations between 0.005 and 4 mM depending on the pH) were studied by SIMPLISMA, a self-modelling mixture analysis technique. Three components were deeected when varying the pH, corresponding to three acid-base equilibria with pK(a) values around 3.0, 5.0 and 6.0, and their individual spectra were obtained. Two components were detected when varying the Cu(II) concentration, corresponding to two different ligand sites, and their spectra and concentration profiles (based on the two detected pure variables) were obtained. The spectrum of one binding site is similar to the component associated with the acid-base equilibrium with pK(a) = 3.0 and that of the other binding site is similar to the sum of the spectra of the components associated with the other two acid-base equilibria. Using a 1:1 stoichiometry for complex formation, complexation capacities (C-L) and stability constants (K) were calculated for the two binding sites at the four pH values. The calculated stability constants were log K-1 = 3.42(4), log K-2 = 3.05(5) at pH 3, log K-1 = 4.17(3), log K-2 = 3.71(1) at pH 4, log K-1 = 4.51(3), log K-2 = 3.95(2) at pH 5 and log K-1 = 4.57(3), log K-2 = 4.03(1) at pH 6.

Abstract Seven heat conduction calorimeters have been evaluated in terms of sensitivity and thermal response time. The use of a dynamic method to correct for the thermal inertia of the calorimeter is shown to reduce by one order of magnitude the time required to conduct a stepwise titration experiment involving a fast reaction. The ability to determine association constants of strong 1:1 complexes has been evaluated in terms of the precision of determining thermal energy.

Abstract Ion transfer reactions at single microhole interfaces were studied using only pure water as the aqueous phase electrolyte resulting in a significantly increased potential window. The organic phase supporting electrolyte concentrations were reduced to micromolar levels and the results obtained were analysed using the general theoretical relationships derived by Oldham (J. Electroanal. Chem., 250 (1988) 1) for steady state voltammetry with little added electrolyte. The voltammetric response of the single microhole interface was investigated with regard to size and shape and as a result a novel amperometric sensing system for ammonium ions is proposed.

Abstract The acid properties of a soil fulvic acid (sfua) were characterized by potentiometric titration with tetrabutylammonium hydroxide in two non-aqueous solvents with high acid-base resolution power, N,N-dimethylformamide (DMF) and acetonitrile. Synchronous fluorescence spectroscopy (SyF) was also used to monitor directly the sfua status during the potentiometric titration in DMF. The potentiometric titration curves showed no clear end-point and the analysis of the sets of spectra obtained at increasing neutralization degree, with a self-modeling curve resolution method (SIMPLISMA), revealed the existence of two components with featureless concentration profiles. Internal standards (maleic, salicylic and p-hydroxylbenzoic acids) were used to determine the amounts of acid groups with different acid strengths in the two non-aqueous solvents. It was shown that the variations observed in the SyF spectra sets of the internal standards are not correlated with those observed in the sfua data. The splitting of the sfua groups in the non-aqueous titration curves seems to be forced artificially depending on the standards used.

Abstract The variations observed in synchronous fluorescence spectra of aqueous solutions (pH = 4) of three fulvic acids (fua) upon variation of the concentration of Al3+ were analyzed by evolving factor analysis (EFA). 'rhe three fua samples were extracted from materials present in a pinewood soil: dry pine needles and the upper and lower soil horizons. The presence of Al3+ induces quenching and enhancement of bands in the synchronous fluorescence spectra, and concentration constraints are proposed for the EFA analysis that allow the calculation, on a self-modeling curve resolution basis, of the number of components that show linearly independent variations, their spectra, and corresponding concentration profiles. Simulated and experimental data sets of simple known compounds (catechol, caffeic acid, salicylic acid, and morin) were also studied as the first validation of the experimental and data treatment procedures.

Abstract The adsorption of mannitol and sorbitol on Au(111) has been followed by differential capacity measurements. Results obtained for the adsorption of these polyalcohols in dilute solutions of HClO4 and NaClO4 showed that the adsorption process could be described by a Frumkin isotherm, the parameters of which were determined and discussed. The values of the standard Gibbs energy of adsorption suggest physical adsorption of mannitol and sorbitol, without the existence of specific interactions of the adsorbed molecules with gold. The values of the interaction parameters are negative in both electrolytes, indicating a net attractive interaction in the adsorbed layer. There are noticeable differences between the values of the standard Gibbs energy of adsorption and of the interaction parameter for the two polyols, which have been interpreted in terms of the conformational properties of the two isomers in the adsorbed layer.