Publications

Abstract The quenching of the fluorescence of three anthropogenic fulvic acids (FA) provoked by Cu(II) (pH 6.0), Fe(III) (pH 4.0) and UO(2)(2+) (pH 3.5), was analyzed by a non-linear method and by Stern-Volmer plots. The FA samples were extracted from composted sewage sludges (csFA), composted municipal wastes (mwFA) and composted livestock wastes (IsFA). Synchronous-scan fluorescence (SyF) spectra were collected as a function of metal ion concentration. Spectral data were treated by a self-modeling mixture analysis method (SIMPLISMA) to detect the SyF spectral band with the strongest quenching and to calculate the corresponding quenching profile. The analysis of these profiles by a non-linear method allowed the estimation of conditional stability constants (K) and of the percentage of non-complexing fluorophores. The same quantitative information was obtained by the modified Stern-Volmer equation taking into account the existence of fluorophores that do nor participate in the complexation. Good agreement was found between the results of the two procedures. The log K calculated by the non-linear method were (standard deviation in parenthesis): csFA, Cu(II), 4.22 (5); Fe(III), 5.0 (1); UO(2)(2+), 5.2 (2); mwFA, Cu(II), 4.21 (3); Fe(III), 5.6 (2); UO(2)(2+), 4.7 (3); 1sFA, Cu(II), 4.51 (8); Fe(III), 5.5 (2); UO(2)(2+), 3.6 (2). (C) 1998 Elsevier Science B.V.

Abstract The acid-base properties of four leaf litter extracts, LLE (eucalyptus, fern, oak and chestnut coverture), were characterized by synchronous fluorescence (SyF) spectroscopy and compared with those of two fulvic acids (FA) extracted from two different horizons (0-5 and 5-15 cm) of an oak forest soil. The characterization was based on the variation with the pH of the properties of the fluorescent structures present in the samples, which function as probes and provide a macroscopic image of their acid-base properties. These sets of SyF spectra collected as a function of the pH show fingerprinting potential. They were treated by a self-modeling curve resolution procedure, evolving factor analysis with a gradient concentration window (EFA-GCW), to reduce the raw spectral data to the number of components, their spectra, and SyF intensity profiles (pH distribution diagrams). Four acid-base systems were detected for all samples of LLE and FA with pKas ranging from 2.5 to 10.0 (pKas about 3, 5, 7 and 9, except for the fern LLE sample which showed pKas about 5, 7, 9 and 10). A similarity analysis of the spectra of the components, performed by cluster analysis, showed that the most acidic fluorescent structures of the LLE are not similar to those of the soil FA, but the less acidic are.

Abstract An extract of dead vine leaves (vitis viniferal) (VLE) was obtained by the extraction procedure for fulvic acids and its interaction (at a concentration of 25 mg/L in 0.1 M KNO3) with the Cu(II), Ni(II) and Fe(III) ions was studied in the pH range between 3 and 6. This interaction was monitored by synchronous molecular fluorescence, since bands due to the fluorescent ligands undergo quenching upon complex formation. After the chemometric isolation of the quenching profiles from the raw spectra by a self-modeling mixture analysis, SIMPLISMA, they were analyzed by two methods: (i) graphical procedures based on the Stem-Volmer model; and (ii) a non-linear least-squares procedure. For the Cu(II) and Ni(II) ions, the treatment by these two methods provided similar values both for the logarithm of the conditional stability constants (log Kc) and the percentage of fluorescent structures that do not participate in the complexation. The log Kc were (standard deviations in parenthesis): Cu(II) ion, 2.4(3), 3.37(3), 4.4(1) and 4.92(9), respectively, for pH = 3, 4, 5 and 6; Ni(II)ion, 2.9(1), 3.3(2), and 4.09(3), respectively, for pH = 4, 5 and 6. For Fe(III) an interaction with VLE was observed, but no values for Kc could be obtained.

Abstract A method for the rapid screening of five chlorophenols in the ppm concentration range (2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, 4-chloro-m-cresol, and pentachlorophenol) is described. It is based on the use of first derivative UV spectrophotometry with a reduced calibration set constituted by eleven standard solutions, selected according to a four level fractional factorial experimental design, and in the use of multivariate calibration: techniques (partial least squares and artificial neural networks) as calibration models. This methodology allows the development of calibration models with good prediction ability for the rapid simultaneous analysis of the five chlorophenols.

Abstract The standard molar enthalpies of formation at T = 298.15 K of some crystalline bis(8-hydroxy-2-methylquinolinate) metal complexes were determined from solution-reaction calorimetric measurements. 8-Hydroxy-2-methylquinoline; or 2-methyloxine, is written as Hmeox. The values of Delta(f)H(m)(0) (M(meox)(2), cr)/(kJ.mol(-1)) are: Mg(meox)(2), -609.0 +/- 4.5; Ca(meox)(2), -707.8 +/- 4.5; Sr(meox)(2), -700.0 +/- 4.8; Cu(meox)(2), -176.6 +/- 4.4; Zn(meox)(2) -369.6 +/- 4.5; Cd(meox)(2), -298.0 +/- 4.5; Pb(meox)(2), -239.9 +/- 4.6; Mn(meox)(2), -418.1 +/- 4.3; Co(meox)(2), -257.8 +/- 4.7; Ni(meox)(2), -243.0 +/- 6.6. Where feasible, the standard molar enthalpies of sublimation at T = 298.15 K were determined by an effusion method. The Delta(cr)(g)H(m)(0) (M(meox)(2))/(kJ.mol(-1)) values are: Mg(meox)(2), 224.3 +/- 6.5; Cu(meox)(2), 172.1 +/- 3.4; Zn(meox)(2), 179.4 +/- 5.0; Cd(meox)(2), 203.3 +/- 7.3; Pb(meox)(2), 188.6 +/- 5.9; Mn(meox)(2), 211.2 +/- 7.2; Co(meox)(2), 204.4 +/- 5.9; Ni(meox)(2), 180.9 +/- 3.7. From the standard molar enthalpies of formation in the gaseous state, the mean metal-ligand dissociation enthalpies were derived and compared with values for other complexes of these metals. (C) 1998 Academic Press Limited.

Abstract A novel separation method, i.e. electro-assisted solvent extraction, based on the principles of two-phase electrochemistry, has been demonstrated for the successful separation of Ni2+, Cu2+ and Cd2+ ions. 2-Heptanone has been used as the organic solvent for the transfer of Ni2+, Cu2+ and Cd2+ facilitated by 2,2';6',2 "-terpyridine. It is shown that extraction efficiency depends on the concentration ratio of ligand to metal ion and importantly, on the applied interfacial potential difference. Tt was Found that Ni2+ (not Cd2+ or Cu2+) extraction is suppressed in the presence of Mg2+ ion due to competitive complexation.

Abstract Electrochemical evidence of the surface reconstruction, induced thermally or potentially, was sought for ten gold single crystals with high Miller index faces, in order to assess the influence of the nature and density of steps. Occurrence of thermally and electrochemically induced surface reconstruction was deduced for the following stepped faces: Au(755), Au(332), Au(533), Au(410), Au(771) and Au(331). The structure of the reconstructed surfaces seems to be-dictated by the geometry of the terraces, while the extension of the phenomena is constrained by the density and orientation of steps. This study was conducted in the presence of the following oxoanions: perchlorate, nitrate, sulphate and phosphate, in order to rationalize the influence of adsorption on the surface structure transitions. Results were interpreted in the light of the known reconstruction mechanisms for the gold low index faces.

Abstract The assisted transfer of Cu(II) by interfacial complexation with 6,7-dimethyl-2,3-di(2-pyridyl)quinoxaline (DMDPQ) is studied at the polarised water\1,2-dichloroethane junction. Thermodynamic and kinetic aspects were addressed by cyclic voltammetry and in situ spectrophotometric measurements. The dependence of the half wave transfer potential on the ligand concentration suggests that the equilibrium is effectively displaced towards a 1:3 (metal:ligand) stoichiometry, with an association constant of log beta(3)degrees = 26.0. Visible spectra recorded by reflection from the aqueous phase showed the presence of two absorption bands at 400 and 500 nm associated with the Cu(II) complex. Chronoabsorptometric studies in the presence of a ligand excess suggest that the assisted transfer kinetics are effectively controlled by the diffusion of the metal ion to the interface.

Abstract Disadvantages of the some zwitterionic pH buffers are (i) that they can interact with metal ions as well as protons, and (ii) that they may have a surfactant effect in chemical or in vitro biological or biochemical studies. This has to be taken into account when a buffer is selected. Here, the copper-complexing capacity and the surfactant activity of three compounds, 3-[N,N-bis (2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO), N-(2-hydroxyethyl)piperazine-N'-(2-hydroxypropanesulfonic acid) (HEPPSO), and piperazine-Ni,N'-bis(2-ethanesulfonic acid) (Pipes), were investigated. Global stability constants (log beta(abc)) of copper(II)-buffer complexes were determined, at 25 degrees C, 0.5 M ionic strength, and at 0.8 mM buffer concentration, by pH and pCu ion-selective electrode measurements. Here, beta(abc) corresponds to the equilibrium: aCu(2+) + bL(-) + cH(+) <-> (CUaLbHc)((2a-b+c)); HL = DIPSO or HEPPSO; c: +1 = proton; -1 = hydroxide. Using SUPERQUAD constants were calculated, giving: DIPSO: log beta(110) = 5.02, log beta(120) 8.99, log beta(130) = 13.0, log beta(140) = 16.3, log beta(14-1) = 9.26, log beta(14-2) = 0.645, log beta(150) = 20.5, log beta(160) = 24.3, log beta(16-1) 16.1, log beta(16-2) = 8.98; HEPPSO: log beta(110) = 4.29, log beta(120) = 8.35, log beta(130) = 12.1, log beta(140) = 15.9, log beta(150) = 19.6, log beta(160) = 23.4, log beta(16-1) = 14.9. The pK(a) values were determined at higher buffer concentrations, giving a value 7.33 for DIPSO and 7.84 for HEPPSO at 2.0 mM buffer concentration. Effects of buffer concentration on stability and acidity constants were investigated and compared with measurements using voltammetric and potentiometric stripping analysis, confirming no copper(II) complexation by Pipes. Surfactant activities were determined using alternating current polarography, confirming marked surface activity of 10 mM of DIPSO or HEPPSO. (C) 1998 Academic Press.

Abstract Ion association at the interface between two immiscible electrolyte solutions may manifest itself as a marked increase in the interfacial capacity, This effect is illustrated by experimental investigations in which the ions in the organic phase are varied, while the composition of the aqueous phase is kept constant. The increase in the capacity is explained by a simple lattice-gas model. The magnitude of this effect depends both on the association constant and the width of the interface. (C) 1997 Elsevier Science S.A.