A simple, sensitive and rapid spectrophotometric method was developed for the determination of thorium(IV) using 2-hydroxy-3- methoxybenzaldehydeisonicotinoylhydrazone (HMBAINH)as an analytical reagent. The metal ion forms a florescent yellow colored complex with HMBAINH in aqueous dimethyl formamide medium at pH 5.0. The complex showed absorption maximum at 395 nm in the pH range 4.0-6.0. Beer’s law was obeyed in the range of 0.58 – 11.60 μg mL-1 of Th(IV). The molar absorptivity and Sandall’s sensitivity of the method were 1.4 x 104L mol-1 cm-1 and 0.0167μg cm-2 respectively. The interference of various diverse ions was studied. The 1:1 (Metal: Ligand) complex was fairly stable(stability constant 2.17 x105). First and Second order derivative spectrophotometric methods were also developed for the determination of Thorium (IV) which showed greater sensitivity and selectivity. The proposed methods were applied for the determination of Thorium (IV) in monazite sand samples and in gas mantle samples.
