Assessing the formation of weak sodium complexes with negatively charged ligands.

The stability of sodium complexes with poly-carboxylic and polyamino-carboxylic acids is investigated with ion-selective electrode-Na(+) potentiometry, working at strictly constant ionic strength. It is observed that the formation constants of the Na(+) complexes with monoligand stoichiometry (ML) increase with the number of charges on the ligand. For example, in poly-carboxylic acids this dependency is linear and is well captured by an experimental equation. A different behaviour is observed for the poly-amino carboxylic acids, which show higher complexation capabilities reaching a plateau of the binding energy past a specific ligand size. The experimental results are discussed qualitatively using ab initio calculations based on DFT B3LYP, and the principal electronic characteristics of the ligands under investigation are identified. As a result of the flexibility imparted by the long chains of polyamino-carboxylic ligands, both experimental and theoretical models demonstrate that nitrogen atoms in proximity of Na(+) ions can participate in the metal coordination, thus providing further stabilization for the complexes. Moreover, by increasing the ligand size the stabilization gained in terms of ΔG reached a plateau for EDTA, in agreement with experimental observations.

Combining synchrotron radiation techniques for the analysis of gold coins from the Roman Empire.

Four gold coins minted in the V century have been studied with non-destructive synchrotron radiation techniques, namely X-Ray Fluorescence (XRF) and X-ray Absorption Near Edge Spectroscopy (XANES). XRF data analyzed coupling standard and statistical methods were used to distinguish the composition of the alloy constituting the coins from that of successive deposits processes. Our analysis presents a quantification of the trace elements present in the metallic alloy providing interesting details for historical insight. Furthermore, on the basis of the XRF maps, some regions of interest were selected for XANES at the K-edge of Fe. Our analysis of the Fe spectra points out two main phases which can be related to Fe oxides naturally present in soil. From the relative abundance of these oxides, information on the site where the coins were found can be obtained, providing additional information on their fate across the centuries.

Optimization of the separation of mono- and dichloroanilines in ion interaction high-performance liquid chromatography.

To optimize the ion-interaction chromatographic separation of nine chloroaniline isomers, the effect on retention of six experimental parameters is investigated by means of multivariate analysis. The factors considered are the organic modifier concentration in the mobile phase; the length of the alkyl chain of the alkylammoniumion salts used as the ion-interaction reagents (IIRs); the concentration of IIRs; the pH of the mobile phase, the flow-rate and the ionic strength. The use of fractional factorial and star designs allowed one to draw out useful information on the retention mechanism involved and to build a model characterized by both descriptive and predictive ability. Concerning descriptions, the results suggest a retention mechanism mainly based on reversed-phase partition, while the main role of the alkylamine (used as IIR) seems to mask the activity of the residual silanol groups on the stationary phase. As a result efficiency is improved. For prediction purposes, the regression models allow the optimization of the chromatographic separation, as regards both resolution and total analysis time. The study allowed one to develop a method able to separate the nine mono- and dichloroanilines in a total analysis time within 66 min and with detection limits ranging from 4.0 to 21.0 microg/l.

Copper(II) complexes of N-(phosphonomethyl)glycine in aqueous solution: a thermodynamic and spectrophotometric study.

Copper(II) complexes with the herbicide N-(phosphonomethyl)glycine (glyphosate) have been investigated in aqueous solution by means of pH-metric measurements at different temperatures, 5

Speciation and structure of copper(II) complexes with histidine-containing peptides in aqueous medium: a combined potentiometric and spectroscopic study.

The complex formation between copper(II) and some histidine-containing peptides has been investigated by means of potentiometric and spectroscopic measurements. Due to the interesting co-ordination mode towards copper(II), peptides with histidyl residue located in second position from the N-terminal amino group have been chosen. The stability constants evaluation has been performed by both pH-metric and ESR (room temperature) measurements; in this context a suitable computer program for the calculation of both stability constant values and the ESR spectrum for each complex has been written. Visible spectrophotometric and circular dichroism spectra, together with the isotropic ESR parameters, were used in order to propose a structure for each complex having a significant percentage of formation in solution.

Characterization of ligand sites on natural sediment particles.

Organic and inorganic ligand sites on sediment particles were alkalimetrically titrated using a glass electrode as indicating device. Data obtained for suspensions containing known masses of sediment were used to calculate the concentration of surface ligand sites and their stability constants for complex formation with proton and copper(II) ion. The relationship between the concentration of ligand sites and the concentrations of metals (Cd, Cr, Cu, Fe, Mn, Pb, and Zn) and of C, N, and S was used to try to discriminate between the contributions of organic and inorganic components to the total ligand capacity of the sediment. The reliability of the chemical model deduced from potentiometric data was checked by comparing calculated values for aqueous copper(II) as a function of pH with values experimentally determined via atomic absorption spectrometry. The procedure proposed might contribute to the modeling of sediment-contaminant interaction, providing information on the nature of the ligands involved.

Interaction of l-malic acid with alkaline metals and open chain polyammonium cations in aqueous solution.

This work reports a potentiometric, calorimetric and spectropolarimetric ultraviolet circular dichroism (UV/CD) study of the interaction of l-malic acid with alkaline metals or (poly)ammonium (methylamine, ethylenediamine, diethylenetriamine, triethylenetetramine, spermine, tetraethylenepentamine and pentaethylenehexamine) cations. Stability data (logK, DeltaG(0)) were obtained potentiometrically for the l-malic acid with (poly)ammonium cations systems; calorimetric measurements (25 degrees C) made it possible to obtain DeltaH(0) and TDeltaS(0) values for the complexes formed in the systems under examination. logK values calculated (for the reaction: H(i)A(i+)+H(j)L((j-z))=ALH(r)((i+j-z)), with r=i+j) range between 0.8 and 4.6, i.e., the interactions are from weak to fairly strong while maximum stability for each system is given by the species with the highest z(anion)xz(cation) (z=charge) value. Enthalpy changes associated with reactions H(n)A(n+)+L(2-)=ALH(n)((n-2)) and H(n)A(n+)+HL(-)=ALH(n+1)((n-1)) are always positive and increase progressively with n. The same is valid for T DeltaS(0) values, which indicate that these species are entropically stabilized, as expected for electrostatic interactions. It was verified that the UV/CD signal depends on both ionic medium and ionic strength value; for comparison, we used the l-malic acid signal recorded in tetramethylammonium chloride as baseline background salt (as in potentiometry). UV/CD spectra were recorded for solutions containing both cationic and anionic species. When the cation was a protonated polyamine, CD spectra calculations were performed for most stable ion pairs: the results show remarkable differences in Deltaepsilon (dm(3) mol(-1) cm(-1)) values at 205 nm (which is the l-malate UV/CD lambda(max)) between the chiral ligand and its complex with a polyamine.