Publications

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.

Abstract Aqueous solutions of carboxylic acids and phenols at micromolar concentration levels were analysed by synchronous fluorescence (SyF) spectroscopy. A strategy, based on provoking controlled variations in SyF spectra by changing the pH and treating spectral data by evolving factor analysis with specific concentration constraints, was developed and evaluated to extract information from the SyF data to determine acid-base parameters for;simple acids and mixtures, The procedure was tested on an aqueous mixture of salicylic acid, caffeic acid and catechol as well as on pure solutions of these and other acids.

Abstract The interaction of Be(II) with two samples of fulvic acids (fua), extracted from materials present in a pinewood soil (fua2 from upper and fua3 from lower soil horizons), was followed by synchronous molecular fluorescence spectroscopy at pH = 6 and 11. The variations observed in the spectral data were analyzed by evolving factor analysis (EFA). It was found that Be(II) is a useful probe for detecting different ligand structures in complex mixtures of the fua type. The existence of common ligand structures in the two fua samples at both pH values studied was observed. These are probably of the salicylic acid type. Dihydroxylic structures of the catechol type were also detected. The logarithm of the mean conditional stability constants for coordination of the fua ligand structures of the salicylic acid type and Be(II) were obtained (at a pH of 6 and 11, respectively): fua2, 5.85(2) and 5.24(9); fua3, 6.5(1) and 5.08(4). This strong coordination indicates that fua can have an important function in the immobilization of Be(II) dispersed into the environment.

Abstract The following standard molar enthalpies of formation in the gaseous state at 298.15 K were determined from the enthalpies of combustion of the crystalline solids and the respective enthalpies of sublimation: 2,2'-bipyridine, 267.9 +/- 3.0 kJ mol(-1); 2,4'-bipyridine, 284.2 +/- 2.7 kJ mol(-1); 4,4'-bipyridine, 293.1 +/- 3.6 kJ mol(-1). Ab initio geometry optimizations of these molecules at the 3-21G level suggest that those with at least one ortho nitrogen atom assume near planar conformations preferentially, while the others have nonplanar most stable forms.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of combustion, at the temperature 298.15 K for crystalline N,N'-ethylenebis(4-aminopent-3-en-2-one), {CH3COCH:C(CH3)NHCH2 .}(2), H(2)ACACEN, and N,N'-ethylenebis(3-amino-1-phenylbut-2-en-1-one), H(2)BZACEN, {C6H5COCH:C(CH3)NHCH2 .}(2), were measured by static bomb-combustion calorimetry in oxygen. The standard molar enthalpies of sublimation of both compounds were measured by effusion techniques. These values were used to derive the standard molar enthalpies of formation of the compounds, in both the crystalline and the gaseous phases.

Abstract The standard (p(o) = 0.1 MPa) enthalpies of combustion at the temperature T = 298.15 K were measured by static-bomb calorimetry for crystalline 4-nitropyridine N-oxide and for pyridine-3-carboxylic acid N-oxide. The standard enthalpies of sublimation at T = 298.15 K were measured for 4-nitropyridine N-oxide by microcalorimetry and for pyridine-3-carboxylic acid N-oxide by a mass-loss effusion technique. -Delta(c)H(m)(o)(cr)/(kJ.mol(-1)) Delta(cr)(g)H(m)(o)(kJ.mol(-1)) 4-O2N-c-C-5 H4NO 2556.3 +/- 0.5 108.9 +/- 0.3 3-HO2C-c-C5H4NO 2698.2 +/- 0.7 152.3 +/- 1.9 From the standard molar enthalpies of formation of the gaseous compounds, the molar dissociation enthalpies of the (N-O) bonds were derived: D(N-O)/(kJ.mol(-1)) in 4-O2N-c-C5H4NO, 258.5 +/- 5.3; in 3-HO2C-c-C5H4NO, 252.8 +/- 2.7. The standard molar enthalpies of sublimation of 2-H3C-c-C5H4NO and 3-H3C-c-C5H4NO were measured by microcalorimetry: (78.2 +/- 2.2) kJ.mol(-1) and (82.2 +/- 2.4) kJ.mol(-1), respectively, permitting the comparison of D(N-O) values in several pyridine N-oxide derivatives.