Ternary complexes in solution: mixed-ligand complexes of thorium(IV) with alizarin maroon and secondary ligands containing oxygen or nitrogen as donor atoms

Abstract

The stability constants of the Th(IV) complexes containing Alizarin Maroon (AZM) and as a secondary ligand (L) salicylic acid (SA), 5-sulphosalicylic acid (SSA), 5-nitrosalicylic acid (NSA), 2,2′-bipyridyl (bipy) or 1,10-phenanthroline (phen) were determined potentiometrically at 25 °C with an ionic strength (l) of 0.1 M(NaCIO₄), and in a 20%V/V aqueous ethanol medium.

All of these mixed-ligand complexes are more stable than one would expect. The stability of the ternary complexes is discussed in relation to the natures of the secondary ligands involved. The solution spectrum of the complex is characterised by an absorption band with λ_max. at 580 nm within the pH range 4.6–5.5. The ternary system obeys Beer's law between 2.3 and 22.5 µg ml^–1 of thorium with a molar absorptivity of 1.1 × 10⁴ l mol^–1 cm^–1. Finally, a sensitive method has been developed for the micro-determination of thorium based on the formation of the Th-AZM-SSA ternary complex.