E. Casassas

Simultaneous analysis of several spectroscopic titrations with self-modelling curve resolution

Abstract Self-modelling curve resolution of individual spectroscopic titrations of a multiequilibria system based on evolving factor analysis does not always provide the right qualitative and quantitative solution, even when the constraints of non-negativity, unimodality and closure are applied. The ambiguities still present in that treatment can be removed if several spectroscopic titrations at different starting conditions (different initial concentrations of the constituents) are analyzed simultaneously and if the spectra of the common species in these different spectroscopic titrations are equal.

Self-modelling curve resolution in studies of spectrometric titrations of multi-equilibria systems by factor analysis

Abstract Recently developed techniques of factor analysis are incorporated in a revised algorithm for resolving the spectra and concentration profiles of the species formed during a spectrometric titration. The techniques included here are: (1) sounder statistical tests and cross-validation techniques for estimating the number of species present in multi-equilibria systems; (2) target factor analysis of the spectra of suspected species; and (3) rank-annihilation estimation of the concentration profile of a detection species with known spectrum and subtraction of its spectral contribution from the complete data matrix. Although these methods can be applied to complement traditional least-squares methods, they can also be used quite independently in the spectrometric study of multi-equilibria systems in solution, without initial chemical modelling.

Application of multivariate curve resolution to voltammetric data. Part 1. Study of Zn(II) complexation with some polyelectrolytes

Multivariate curve resolution is applied to the study of Zn(II) complexation with anions of polymethacrylic acid and polyacrylic acid in aqueous solution containing 0.01 M KNO3. Data matrices obtained by different electrochemical techniques (normal pulse polarography, reverse pulse polarography, differential pulse polarography and differential pulse anodic stripping voltammetry) and for different ligand deprotonation degrees have been analysed individually and simultaneously by a procedure which consists of using several chemometrical techniques based on factor analysis: principal component analysis, evolving factor analysis and multivariate curve resolution with alternating least-squares optimization. These techniques were used to investigate the number of species present in the systems, their concentration profiles at increasing ligand concentration and their individual voltammograms together with the global (apparent) stability constants K and the ratio of diffusion coefficients of the individual species. The comparison of the results obtained by soft (model free) and hard (model postulated a priori) modelling for the systems investigated is discussed.

Application of principal component analysis to the study of multiple equilibria systems : Study of copper(II)/salicylate/mono-, di- and triethanolamine systems

Abstract Principal component analysis is applied to the study of multiple equilibria systems based on copper (Il) ion with salicylate and mono-, di- or triethanolamine as ligands. A procedure based on evolving factor analysis is described which allows the model-free determination of distribution plots and of the individual spectra of the various species formed in the course of a spectrophotometric titration. The results are compared with those obtained by using least-squares methods.

Solvent classification based on solvatochromic parameters : a comparison with the Snyder approach

Abstract Solvents play a major role in many chemical processes. Their effect is closely related to the nature and extent of solute/solvent interactions, developed locally in the immediate vicinity of the solutes. A new generation of empirical parameters helps to describe the solvent features in these microscopic environments. Their adequacy for the establishment of linear free energy relationships (LFER) has been widely proved. A classification scheme based on the microscopic properties of solvents is proposed, to provide a clearer insight into the solvent-space structure in terms of the similarity of solute/solvent interactions and to help chemists in the choice of a suitable solvent for each purpose. The Kamlet and Taft solvatochromic parameters α, β and π ∗ have been selected as microscopic solvent descriptors, and solvent groups have been designed according to the results obtained from the application of several multivariate clustering techniques. The present scheme is compared with Snyders triangle, an earlier solvent representation used extensively in chromatography. Analogies and differences which arise from the field of application and from the solvent descriptors used in each classification scheme are discussed.

Spectroscopic resolution of macromolecular complexes using factor analysis: Cu(II) -polyethyleneimine system

Abstract Tauler, R., Casassas, E. 1992. Spectroscopic resolution of macromolecular complexes using factor analysis: Cu(II)-polyethyleneimine system. Chemometrics and Intelligent Laboratory Systems, 14:305–317. The complexes of Cu(II) ion with polyethyleneimine formed during a VIS spectroscopic titration of solutions containing different amounts of Cu(II) and polyethyleneimine at various pH values have been studied using a new self-resolving approach based on different factor analysis techniques, including evolving factor analysis, error in factor analysis, cross-validation, target factor analysis, and rank annihilation. Three different macromolecular complex species between Cu(II) and polyethyleneimine were detected in the system, and their concentration profiles and individual spectra were estimated without any previous knowledge of the underlying chemical model (set of stoichiometric coefficients and set of stability constants). The method developed is proposed for use in the deduction of the metal complexing properties of macromolecular systems.

Direct-current, normal-pulse and reverse-pulse polarography of some heavy metal—polycarboxylate complexes

Abstract Complex formation of Zn(II) and Cd(II) with polymethacrylic acid (PMA) andpolyacrylic acid (PAA) is studied by sampled direct-current, normal-pulse and reverse-pulse polarography. The combined use of these techniques serves a twofold purpose: to cover different time windows and to eliminate interferences from adsorption of the metal ion induced by the ligands. The two Zn(II) systems appear to be labile on the different time scales. The association constants ( K ) evaluated from potential data and those from current data show good consistency. The dependence of K on the degree of neutralization (α n ) is much stronger for PAA complexes than it is for those of PMA. With the Cd(II)—PMA system complications from induced Cd(II) adsorption are completely removed by using reverse-pulse polarography. The system is not liable and stabilities derived from potential data differ from those obtained from current data. The remaining discrepancy cannot be attributed to simple association and dissociation kinetics.

Application of an evolving factor analysis-based procedure to speciation analysis in the copper(II)-polyuridylic acid system

Abstract The acid-base properties and the copper(II)-complexing behaviour of the polynucleotide polyuridylic acid were studied by means of potentiometric, spectrophotometric and electron spin resonance titrations in a working aqueous medium of 0.15 M ionic strength and at 37°C. Spectrophotometric data were treated with an evolving factor analysis-based procedure that allows different sets of spectrometric titrations of the same multi-equilibria system to be analysed simultaneously. A dimeric macromolecular complex species between copper(II) ions and polyuridylic acid was detected in the system and its formation constant was evaluated.

Application of a new multivariate curve resolution procedure to the simultaneous analysis of several spectroscopic titrations of the copper(II)—polyinosinic acid system

Abstract The acid-base properties and the copper(II)-complexing behaviour of the polynucleotide polyinosinic acid have been studied by means of potentiometric and spectrophotometric titrations. Spectrophotometric data have been treated with a new multivariate curve resolution procedure that allows to analyze simultaneously different sets of spectroscopic titrations of the same multi-equilibria system. The results obtained give a real proof of the advantages of the proposed procedure over the single titration analysis: intensity ambiguities are eliminated, as well as rotational ambiguities if the information yielded by at least one of the titrations is selective.

The complex formation of Cu(II) with mono- and di-ethanolamine in aqueous solution

Abstract The complex formation of Cu(II) with mono- and di-ethanolamine is studied by means of ESR spectroscopy. For both ligands, four complex species with the stoichiometries 110, 120, 12-1 and 12-2 are detected, and their relative stabilities are calculated. From the results obtained, deprotonation of the hydroxyl groups and formation of stable chelate rings are proposed. The similiarities and differences between the complex behaviour of the related compounds tri-ethanolamine, ammonia and β-methoxy- ethylamine (also studied by ESR spectroscopy) are further analysed.