Ternary complexes in solution: complex formation between the 1 : 1 thorium(IV)-alizarin maroon complex and thiosalicylic acid

Abstract

The stability constant of the ternary Th(IV) complex containing alizarin maroon (AZM) and as a second ligand thiosalicylic acid (TSA) in a 1 : 1 : 1 molar ratio was determined potentiometrically in 40%V/V ethanol-water medium (l= 0.1 M NaClO₄; 25 ± 0.1 °C). Under identical conditions the binary complexes of thorium with both reagents have also been examined. For the equilibrium Th(AZM)₂+ Th(TSA)₂⇌ 2 Th(AZM)(TSA), the corresponding constant was log X= 3.6(0.75). The constant given in parentheses is due to ΔlogK_Th= logK^Th(AZM)_Th(AZM)(TSA)– logK^Th_Th(AZM).

The mixed-ligand complex is considerably more stable than the corresponding binary complexes. The reaction of Th(IV) with AZM and TSA was investigated spectrophotometrically and a sensitive method was developed for the microdetermination of thorium based on the formation of the Th(IV)-AZM-TSA ternary complex. The solution spectrum of the complex is characterised by an absorption band with λ_max. at 585 nm within the pH range 4.4–5.5. The ternary system obey's Beer's law between 1.16 and 18.57 µg ml^–1 of thorium with a molar absorptivity of 9.2 × 10⁴ l mol^–1 cm^–1.