Study of the Chromogenic Reactions of Semi-xylenol Orange with Rare Earths and Thorium

The composition, stability constants and molar absorption coefficients of the chelates of thulium, ytterbium, lutetium and thorium ions with Semi-xylenol Orange (SXO) are determined spectrophotometrically in a medium of hexamine, and chloroacetate buffer. In the thulium, ytterbium, lutetium and thorium-Semi-xylenol Orange systems, Tm(SXO), Yb(SXO), Lu(SXO) and Th(SXO) chelates are formed with logarithmic overall stability constants of 12.48±0.01, 12. 55±0.01, 12.94±0.015 and 22.16±0.01 and molar absorption coefficients of 2.22×10 4 (530 nm), 2.17×10 4 (532 nm), 2.25×10 4 (530 nm) and 3.33×10 4 (535 nm) dm 3 mor −1 cm −1 respectively. The precision and accuracy attainable in direct spectrophotometric titrations of the Tm 3+ , Yb 3t , Lu 3+ and Th 4+ separately and the successive determination of each of the three rare earths and Th 4+ ion with a 0.001 mol dm −3 solutions of diethylenetriaminepentaacetic taacetic acid (DTPA) and PbC1 2 of the same concentration using Semi-xylenol Orange (SXO) as a metallochromic indicator have been studied. Thorium (IV) is initially titrated directly at pH 2 (535 nm). At the thorium end-point the pH is adjusted to 5.6-5.8 by adding hexamethylenetetramine (hexaminel buffer and an excess of DTPA is added and Tm 3+ (or Yb 3+ or Lu 3+ ) is determined by back-titration with standard PbC1 2 solution (490 nm). Interferences of different anions, cations and organic acids on the spectrophotometric micro-titration of Th 4+ and Tm 3+ , Yb 3+ and Lu 3+ are also investigated. Spectrophotometric determination of Tm 3+ , Yb 3+ , Lu 3+ and Th 4+ individually using SXO at the optimum conditions of maximum colour development have been carried out. Thorium has been determined spectrophotometrically in three different artificial samples with lutetium similar to those of monazite mineral. Ytterbium (III) has been determined in three different laboratory synthesized samples with Ca 2+ and Fe 3+ similar to those of euxenite mineral which occurs in nature and the results were reproducible and satisfactory.