Publications

Abstract The standard molar enthalpies of formation of quinoxaline, quinazoline and phthalazine at 298.15 K in the gaseous state have been determined from the standard molar enthalpies of combustion of the crystalline solids and the respective enthalpies of sublimation: quinoxaline, 240.3 +/- 3.3 kJ mol(-1); quinazoline, 243.1 +/- 2.7 kJ mol(-1); phthalazine, 329.9 +/- 3.3 kJ mol(-1). Ab initio full geometry optimizations at the 3-21G and 6-31G* levels were also performed for these molecules. MP2/RHF/3-21G/3-21G energies were calculated for all isomers, and used to estimate their isodesmic resonance energies.

Abstract Vapour pressures as functions of temperature of seven crystalline copper(II) beta-diketonates, CuL(2), were measured using either the Knudsen mass-loss effusion technique (Oporto apparatus), the simultaneous torsion and mass-loss effusion technique (Utrecht apparatus), or both. From the temperature dependence of the vapour pressure, the standard molar enthalpies of sublimation, at the mean temperature of the experimental temperature range, Delta(cr)(g) H-m degrees ((T)), were derived, and the standard molar enthalpies of sublimation, at the temperature 298.15 K, were calculated: [GRAPHICS] where HACAC, pentane-2,4-dione; HPIPRM, 2,2-dimethylheptane-3,5-dione; HDIBM, 2,6-dimethylheptane-3,5-dione; HIBPM, 2,2, 6-trimethylheptane-3,5-dione; HDPM, 2,2,6,6-tetramethylheptane-3,5-dione; HBZAC, 1-phenylbutane-1,3-dione; and HTFAC, 1,1,1-trifluoropentane-2,4-dione. The consistency of the vapour-pressure measurements was tested by measuring the vapour pressures of benzoic acid, at various temperatures, by different experimental techniques: Knudsen mass-loss effusion, simultaneous torsion and mass-loss effusion technique, and spinning rotor. From the standard molar enthalpies of formation of the gaseous complexes, the mean molar (Cu-O) bond-dissociation enthalpies were derived.

Abstract A procedure for the analysis of the acid-base characteristics of humic substances based on a self-modeling analysis of synchronous fluorescence spectra, collected at varying pH, and on a non-linear least squares adjustment of potentiometic pH data, is described. The data analysis methodology consists of two steps: first, the number of acid-base systems and the corresponding spectra and distribution diagrams are calculated by evolving factor analysis (EFA) with concentration constraints of the spectroscopic data; second, the potentiometric data is analyzed by a standard non-linear least square procedure using as fixed parameters the number of components and their pK(a)s, determined in the first step of the analysis. As an example, for a sample of marine fulvic acids studied at pH between 2 and 11, four acid-base systems were found with average pK(a)s: 3.1, 4.8, 8.0 and 10.0. The concentrations of the corresponding systems were: 2.55(5), 1.95(7), 0.14(4) and 1.8(3) meq/g.

Abstract Synchronous fluorescence spectra of soil and marine fulvic acids and of a commercial humic acid collected by varying Cu(II) concentration at constant pH (quenching experiments) were analyzed by evolving factor analysis (EFA). The number of components in each data set were related to different binding sites on the molecules of the corresponding humic substances. Two binding sites were found for the marine fulvic acid (mfua) and humic acid (hua) samples and two or three binding sites, depending on the pH, for the soil fulvic acid (sfua) sample. Rules were developed for the EFA procedure to obtain, in a self-modeling base, spectra and fluorescence intensity profiles for the detected binding sites. From the calculated fluorescence intensity profiles, and using a 1 : 1 stoichiometry for the Cu(II)/binding site complex formation, the conditional stability constants (K) and complexing capacities for the detected stronger binding sites were calculated. The following values for log K (standard deviation) were obtained: mfua (pH=6), log K-1=5.2 (0.3); sfua (pH=6), log K-1=5.57 (0.06) and log K-2=4.92 (0.04); and hua (pH=7), log K-1=5.38 (0.07).

Abstract This paper reports the development of amperometric detection using a double channel FIA manifold for the control of vitamin C in pharmaceutical formulations. This method enables direct determination of aqueous solutions by simple solubilisation of pharmaceutical formultions. As sample filtration is not required, addition of the support electrolyte is carried out within the FIA manifold. Reliable ascorbic acid determinations in several pharmaceutical formulations were carried out in a double channel manifold with a sampling rate of about 120 samples/hour.

Abstract A procedure for the analysis of the acid-base characteristics of humic substances based on a self-modeling analysis of synchronous fluorescence spectra, collected at varying pH, and on a non-linear least squares adjustment of potentiometic pH data, is described. The data analysis methodology consists of two steps: First, the number of acid-base systems and the corresponding spectra and distribution diagrams are calculated by evolving factor analysis (EFA) with concentration constraints of the spectroscopic data; second, the potentiometric data is analyzed by a standard non-linear least square procedure using as fixed parameters the number of components and their pKas, determined in the first step of the analysis. As an example, for a sample of marine fulvic acids studied at pH between 2 and 11, four acid-base systems were found with average pKas: 3.1, 4.8, 8.0 and 10.0. The concentrations of the corresponding systems were: 2.55(5), 1.95(7), 0.14(4) and 1.8(3) meq/g.