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Abstract

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 β_pqr = 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 β_pqr = -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 β_pqr = 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.