Synthesis of 2-hydroxy-3-carboxyl-5-sulfonicarsenazo and its application to the spectrophotometric determination of thorium

A new highly selective and sensitive chromogenic reagent, 2-hydroxy-3-carboxyl-5-sulfonicarsenazo (ASA-HCS),was synthesized and studied for determination of trace thorium. In a high concentration nitric acid medium, thorium reacts with ASA-HCS to form a 1∶2 green complex which has a sensitive absorption peak at 676 nm. A 0–0.6 µg ml–1 concentration of thorium obeys Beer’s law. The complex is formed instantly in nitric acid medium and remains stable for 48 h at least. The stability constant of the complex is 9.76 × 1021at 25 °C, its molar absorptivity and Sandell’s sensitivity are 1.05 × 105 l mol–1 cm–1 and 0.00223 µg cm–2, respectively. Its limit of quantification, limit of detection and relative standard deviations were found to be 14.3 ng ml–1, 4.9 ng ml–1 and 1.2%, respectively. Ca(II), Mg(II), Ag(I), Al(III), K(I), Fe(III), Co(II), Ni(II), Pb(II) , Mn(II), W(VI), Cr(III), Zn(II), Cu(II), Y(III), Mo(VI), Ce(III) and U(VI) do not interfere with the determination of thorium, so the selectivity is advantageous compared with other published reagents. The stepwise dissociation constants of the reagent at λ = 554 nm and λ = 650 nm have been determined. The proposed method has been applied to the determination of thorium in waste water and rare earth–magnesium–silicon alloy with satisfactory results.