Showing: 10 from total: 2477 publications
2341. Monitoring of molecular transformations in acid-base reactions by evolving factor analysis of Fourier transform infrared spectra data
daSilva, JCG ; Machado, AASC ; Oliveira, CJS
in TALANTA, 1996, ISSN: 0039-9140,  Volume: 43, 
Article,  Indexed in: crossref, scopus, wos 
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.

2342. Interaction of fulvic acids with Al(III) studied by self-modeling curve resolution of second-derivative synchronous fluorescence spectra
daSilva, JCGE ; Machado, AASC
in APPLIED SPECTROSCOPY, 1996, ISSN: 0003-7028,  Volume: 50, 
Article,  Indexed in: scopus, wos 
P-001-EFH
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).

2343. Simultaneous use of evolving factor analysis of fluorescence spectral data and analysis of pH titration data for comparison of the acid-base properties of fulvic acids
daSilva, JCGE ; Machado, AASC ; Silva, CSPCO
in ANALYTICA CHIMICA ACTA, 1996, ISSN: 0003-2670,  Volume: 318, 
Article,  Indexed in: scopus, wos 
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.

2344. Thermochemical and theoretical study of some methyldiazines
daSilva, MAVR ; Morais, VMF ; Matos, MAR ; Rio, CMA ; Piedade, CMGS
in STRUCTURAL CHEMISTRY, 1996, ISSN: 1040-0400,  Volume: 7, 
Article,  Indexed in: scopus, wos 
P-001-DBR
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.

2345. Temperature coefficient of the potential of zero charge and entropies of formation for the interface of stepped faces of gold in contact with aqueous perchloric acid solutions
Silva, F ; Sottomayor, MJ ; Martins, A
in JOURNAL OF THE CHEMICAL SOCIETY-FARADAY TRANSACTIONS, 1996, ISSN: 0956-5000,  Volume: 92, 
Article,  Indexed in: crossref, scopus, wos 
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.

2346. Electrochemical evidence of surfactant activity of the hepes pH buffer which may have implications on trace metal availability to cultures in vitro
Vasconcelos, MTSD ; Azenha, MAGO ; Lage, OM
in ANALYTICAL BIOCHEMISTRY, 1996, ISSN: 0003-2697,  Volume: 241, 
Article,  Indexed in: crossref, scopus, wos 
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.

2347. ORGANIC-LIGANDS REDUCE COPPER TOXICITY IN PSEUDOMONAS-SYRINGAE
AZENHA, M ; VASCONCELOS, MT ; CABRAL, JPS
in ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY, 1995, ISSN: 0730-7268,  Volume: 14, 
Article,  Indexed in: crossref, scopus, wos 
Abstract Pseudomonas syringae cells were exposed to 100 mu M copper alone, or to previously equilibrated copper sulfate-ligand solutions. Ligand concentrations were determined experimentally as those that reduced the free copper concentration to 5 mu M (determined with a Cu2+-selective electrode). These values were in agreement with those calculated by computational equilibrium simulation based on published stability constants. Exposure of P. syringae cells to copper sulfate, chloride, or nitrate resulted in similar high mortality, suggesting that copper was responsible for cell death. Acetate, succinate, proline, lysine, cysteine, and EDTA significantly reduced both the amount of copper bound to the cells and cell death, indicating that not only strong chelating agents but also weak and moderate copper ligands can effectively antagonize copper toxicity. However, cysteine and EDTA were considerably more effective than acetate, succinate, proline, and lysine, indicating that copper toxicity in not simply a function of free copper concentration but depends on the nature of the ligand. The results suggested that a significant fraction of copper bound to acetate, succinate, proline, or lysine was displaced to the bacteria or, alternatively, mixed copper-ligand-cell complexes could be formed. On the contrary, none of these phenomena occurred for the copper complexes with cysteine or EDTA.

2348. Organic ligands reduce copper toxicity inPseudomonas syringae
Azenha, M ; Vasconcelos, MT ; Cabral, JPS
in Environmental Toxicology and Chemistry, 1995, ISSN: 0730-7268,  Volume: 14, 
Article,  Indexed in: crossref 

2349. Thermochemical study of 2-, 4-, 6-, and 8-methylquinoline
;
in The Journal of Chemical Thermodynamics, 1995, ISSN: 0021-9614,  Volume: 27, 
Article,  Indexed in: crossref 

2350. THERMOCHEMICAL STUDY OF 2-METHYLQUINOLINE, 4-METHYLQUINOLINE, 6-METHYLQUINOLINE, AND 8-METHYLQUINOLINE
DASILVA, MAVR ; MATOS, MAR ; AMARAL, LMPF
in JOURNAL OF CHEMICAL THERMODYNAMICS, 1995, ISSN: 0021-9614,  Volume: 27, 
Article,  Indexed in: authenticus, crossref, scopus, wos 
Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation for the liquid 2-, 4-, 6-, and 8-methylquinolines: C9H6N(CH3) were derived from the standard molar enthalpies of combustion, in oxygen, at the temperature T = 298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of vaporization for the same compounds were determined by microcalorimetry. The calorimeter was tested by measuring the enthalpies of vaporization of undecane and hexadecane. [GRPAHICS]