Publications

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of combustion at T = 298.15 K were measured by static bomb combustion calorimetry for liquid N,N-diethylaniline, N,N-dimethyl-m-toluidine, N,N-dimethyl-p-toluidine, and N-ethyl-m-toluidine. Vaporization enthalpies for N,N-dimethyl-m-toluidine and N-ethyl-m-toluidine were determined by correlation gas chromatography. Derived standard molar values of Delta(f)H(m) degrees(g) at 298.15 K for N,N-diethylaniline (62.1 +/- 7.6); N,N-dimethyl-m-toluidine (72.6 +/- 7.3), N,N-dimentyl-p-toluidine (68.9 +/- 7.4), N-ethyl-m-toluidine (30.5 +/- 3.8 kJ . mol(-1)) were obtained.

Abstract An evolving factor analysis procedure with concentration constraints (gradient concentration window) was applied to the analysis of data sets of aqueous Fourier transform infrared (FT-IR) spectra of carboxylic acids (acetic, malonic and succinic acids) collected in experiments with varying pH. Besides the calculation of the number of acid-base systems, this procedure allowed the calculation of the FT-IR spectra of the acid-base species present in equilibrium as well as the corresponding pK(a) values.

Abstract The interaction of AI(III) with two samples of fulvic acids (fua), extracted from materials present in a pinewood soil (fua2 from the upper soil horizon, or leaf litter, and fua3 from lower soil horizons), was followed by synchro nous fluorescence (SyF) spectroscopy at pH 4. The variations observed in the inverted second-derivative spectral data were analyzed by SIMPLISMA, a self-modeling curve resolution technique. Three: binding sites were detected for each fua sample, and their spectra and SyF intensity profiles were obtained. Conditional stability constants for coordination of one binding site of each fua and AI(III) were obtained (log values): fua2, 5.65(15); fua3, 4.64(2). The experimental and data analysis methodologies were validated by analysis of data obtained for salicylic acid under the same conditions. The log value found for its conditional stability constant for AI(III) coordination at pH 4 was 4.34(15).

Abstract Three fulvic acids (fua) extracted from a Portuguese pinewood soil (dry pine needles, fua1, leaf litter, fua2, and lower soil horizon, fua3) have been studied by synchronous molecular fluorescence spectroscopy (SyF) at variable pH and by potentiometric pH titrations. The number of macroscopic acid-base systems and their pK(a)s were calculated by analysis of the SyF-pH hyphenated data with a self-modeling curve resolution technique (evolving factor analysis, EFA). Four macroscopic acid-base systems were detected in the fulvic acid samples (with pK(a)s around 3.0, 4.5, 7.0 and 9.0). The concentrations of the groups responsible for the acid-base systems were calculated by a standard non-linear least squares adjustment of pH potentiometric titration curves using the pK(a) values obtained by EFA. The procedure detected differences between the three fua: the concentration of carboxylic groups increased from fua1 to fua3 but, in contrast, the concentration of the phenolic groups decreased. These results correlate well with the expected extent of humification of the samples.

Abstract The standard (p(0) = 0.1 MPa) molar enthalpies of formation for the liquid 2,3-dimethylpyrazine and trimethylpyrazine and the crystalline 2,3-dimethylquinoxaline and tetramethylpyrazine were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of vaporization or of sublimation for the same compounds were determined by Calvet microcalorimetry. Ab initio full geometry optimization at the 3-21G and 6-31G* levels were also performed for all the methylpyrazine isomers. MP2/RHF/3-21G//3-21G and DFT energies were also calculated for air the methylpyrazine isomers, thus allowing us to estimate their isodesmic resonance energies.

Abstract The behaviour of several high Miller index crystal surfaces of gold is examined by cyclic voltammetry in 0.01 mol dm(-3) perchloric acid solutions. The potentials of zero charge, E(sigma=0), and their temperature coefficients, dE(sigma=0)/dT for the stepped faces (332), (771), (320) and (410) are determined from a single minimum of the differential capacity curves. The results are compared with those existing for other stepped and low-index crystal faces in order to establish the relevance of the crystallographic structure in the double layer parameters. The influence of the step density and step orientation is discussed. The entropies of formation, S*, are calculated and discussed in terms of the gold-water interactions.

Abstract Hydrogen ion buffers are required for many different types of in vitro biological and chemical studies, but they may not be inert enough, thus interfering with the system under study. N-Hydroxyethylpiperazine-N'- ethanesulfonic acid (Hepes), a zwitterionic buffer in the 7.0-8.0 pH range, has been used in different biological studies. In this work, surfactant activity of Hepes and the buffer's capability to bind copper(II) were investigated through different electrochemical techniques: ion-selective electrode potentiometry, potentiometric stripping analysis, differential pulse anodic stripping voltammetry, normal pulse anodic stripping voltammetry, and alternating current polarography. Hepes concentrations between 0.1 (0.24 g liter(-1)) and 25 mmol liter(-1) (6.0 g liter(-1)) were studied at pH 8.0 (and at pH 6.5 for comparison). At pH 8.0 Hepes displayed surfactant activity, particularly when \Hepes\ > 10 mmol liter(-1) (2.4 g liter(-1)). The ability of Hepes to bind copper(II) was detected when Hepes was in large excess (\Hepes\/\Cu\ between 10,000 and 2000). Eventual implications in the bioavailability of copper(II) are also discussed. (C) 1996 Academic Press, Inc.