Chelating equilibria of 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol and 4-(2-pyridylazo)resorcinol with La(III) ions

Abstract

Listen

Translate article

2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol (BrPADAP) with lanthanum(III) ions in 50% (v/v) aqueous ethanol or 30-50% (v/v) aqueous dimethylformamide (DMF) forms unprotonated chelates ML and ML2 characterized by a double absorption band with maxima at 550 and 570 nm. The ML2 species is rather unstable, hydrolyzing readily to the M(OH)L chelate, which exhibits an absorption maximum at 570 nm. The molar absorptivities of the ML, ML2, and M(OH)L species, lying in the regions of 5.3-6.5, 7.0-7.2, and 6.5-7.2 m2 mmol-1, respectively, depend on the kind and fraction of the organic component in the solvent. The conditional equilibrium constants (-log βpqr = 3.3-4.0, 12.2-12.5, and 7.8-8.2, respectively) decrease with increasing fraction of the organic solvent; in systems with high proportions of ethanol orDMF, the formation of higher species is greatly suppressed. 4-(2-Pyridylazo)resorcinol (PAR) with La(III) in acid aqueous solutions forms the MLH and ML species with absorption maxima at 490 and 506 nm, respectively ( ε = 1.2 and 2.5 m2 mmol-1, respectively) and with conditional equilibrium constants –log βpqr = -3.4 and 3.5, respectively. In alkaline solutions with excess PAR, the ML2 species with the absorption maximum at 509 mn (ε = 4.6 m2 mmol-1) and conditional equilibrium constant –log βpqr = 8.1 is formed. This chelate hydrolyses readily forming the M(OH)L species and other hydrolysis products. The ML2 species of the two reagents, forming in ammoniacal buffer at pH 9.0 – 9.5 (PAR) and 8.2 – 8.4 (BrPADAP), can be used for a sensitive (ε = 3.8 and 6.1 m2 mmol-1, respectively) post-column derivatization of lanthanoids after their separation by IEC or HPLC or for their FIA determination.