Showing: 10 from total: 2477 publications
2361. Evolving factor analysis of synchronous fluorescence spectra of humic substances in the presence of Cu(II)
Esteves da Silva, JC ; Machado, AA
in Chemometrics and Intelligent Laboratory Systems, 1995, ISSN: 0169-7439,  Volume: 27, 
Article,  Indexed in: crossref 

2362. Self-modelling curve resolution analysis of synchronous fluorescence spectroscopy data for characterization of acid mixtures and study of acid–base equilibria
Esteves da Silva, JCG ; Machado, AASC
in The Analyst, 1995, ISSN: 0003-2654,  Volume: 120, 
Article,  Indexed in: crossref 

2363. Characterization of the acid-base properties of humic substances by chemometric analysis of synchronous fluorescence and ph potentiometric data
Esteves da Silva, JCG ; Machado, AASC
in Analytical Letters, 1995, ISSN: 0003-2719,  Volume: 28, 
Article,  Indexed in: crossref, scopus 
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.

2364. Beryllium(II) as a Probe for Study of the Interactions of Metals and Fulvic Acids by Synchronous Fluorescence Spectroscopy
Silva, JCGED ; Machado, AASC ; Garcia, TMO
in Applied Spectroscopy, 1995, ISSN: 0003-7028,  Volume: 49, 
Article,  Indexed in: crossref 

2365. SELF-MODELING CURVE RESOLUTION ANALYSIS OF SYNCHRONOUS FLUORESCENCE SPECTROSCOPY DATA FOR CHARACTERIZATION OF ACID MIXTURES AND STUDY OF ACID-BASE EQUILIBRIA
DESILVA, JCGE ; MACHADO, AASC
in ANALYST, 1995, ISSN: 0003-2654,  Volume: 120, 
Article,  Indexed in: scopus, wos 
P-001-G5X
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.

2366. BERYLLIUM(II) AS A PROBE FOR STUDY OF THE INTERACTIONS OF METALS AND FULVIC-ACIDS BY SYNCHRONOUS FLUORESCENCE SPECTROSCOPY
DASILVA, JCGE ; MACHADO, AASC ; GARCIA, TMO
in APPLIED SPECTROSCOPY, 1995, ISSN: 0003-7028,  Volume: 49, 
Article,  Indexed in: wos 
P-001-G61
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.

2367. THERMOCHEMICAL AND THEORETICAL-STUDIES OF SOME BIPYRIDINES
DASILVA, MAVR ; MORAIS, VMF ; MATOS, MAR ; RIO, CMA
in JOURNAL OF ORGANIC CHEMISTRY, 1995, ISSN: 0022-3263,  Volume: 60, 
Article,  Indexed in: scopus, wos 
P-001-GFT
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.

2368. STANDARD ENTHALPIES OF FORMATION OF N,N'-ETHYLENEBIS(4-AMINOPENT-3-EN-2-ONE) AND N,N'-ETHYLENEBIS(3-AMINO-1-PHENYLBUT-2-EN-1-ONE)
DASILVA, MAVR ; DASILVA, MDMCR ; MONTE, MJS ; GONCALVES, JM ; FERNANDES, EMR
in JOURNAL OF CHEMICAL THERMODYNAMICS, 1995, ISSN: 0021-9614,  Volume: 27, 
Article,  Indexed in: crossref, scopus, wos 
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.

2369. ENTHALPIES OF COMBUSTION OF 4-NITROPYRIDINE N-OXIDE AND PYRIDINE-3-CARBOXYLIC ACID N-OXIDE - THE DISSOCIATION ENTHALPIES OF THE N-O BONDS IN PYRIDINE N-OXIDE DERIVATIVES
ACREE, WE ; TUCKER, SA ; DASILVA, MDMCR ; MATOS, MAR ; GONCALVES, JM ; DASILVA, MAVR ; PILCHER, G
in JOURNAL OF CHEMICAL THERMODYNAMICS, 1995, ISSN: 0021-9614,  Volume: 27, 
Article,  Indexed in: crossref, scopus, wos 
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.

2370. DIFFERENTIAL CAPACITANCE OF LIQUID/LIQUID INTERFACES - EFFECT OF ELECTROLYTES PRESENT IN EACH PHASE
PEREIRA, CM ; MARTINS, A ; ROCHA, M ; SILVA, CJ ; SILVA, F
in JOURNAL OF THE CHEMICAL SOCIETY-FARADAY TRANSACTIONS, 1994, ISSN: 0956-5000,  Volume: 90, 
Article,  Indexed in: crossref, scopus, wos 
Abstract The effect of electrolyte type and concentration on the differential capacitance of the 1,2-dichloroethane/water interface has been measured. The results show that the differential capacitance is controlled by the pair ion (1)-counter-ion (2), where 1 and 2 denote the different phases. At constant organic electrolyte composition the capacity increases in the order, F-<SO42-<HO-<ClO4-,Li+<H+<K+<Mg2+<Ba2+<Sr(2+)approximate to Ca2+. By contrast, at positive potentials and constant aqueous electrolyte composition, the capacitance decreases in the order, TPA(+)>MTMA(+)>TBA(+)>TOA(+),TPB->TCPB-. In very dilute solutions, the effects are not noticeable.