Quantitative evaluation of Ln(III) pyridine N-oxide carboxylic acid spectra under chemometric and metrological aspects

The interaction of Nd(III) with four carboxylated pyridine N-oxide derivatives in aqueous solutions is studied by UV-vis absorption spectroscopy. From factor analysis in combination with computer-intensive bootstrap resampling methods the empirical probability distributions of the formation quotients of the 1: 1 Nd(III) complexes with nicotinic acid N-oxide (1), picolinic acid N-oxide (11), pyridine 2,4 dicarboxylic acid N-oxide (111), and dicarboxylic acid 2,6 N-oxide (IV) are evaluated. The strength of the interaction was found to correlate with the charge density around the coordinating groups. Charge density was calculated by ab initio methods using a polarised continuum model to account for solvation effects. Evidence for a 1:2 species with IV is presented. A speciation diagram for this model is obtained from the probabilistic speciation code LJUNGSKILE, which visualises the rather weak evidence for a two-species system. Thus, the formation quotients lg beta(11) = 2.3 +/- 0.0(5) (1), lg beta(11) = 2.7 +/- 0.0(4) (II), lg beta(11) = 2.8 +/- 0.0(5) (III), and lg beta(11) = 3.0 +/- 0.0(5) (IV) are found to describe the chemical systems satisfactorily. (c) 2005 Elsevier B.V. All rights reserved.