Selective Spectrophotometric Determination of Th(IV) with 2- 2-(5-Bromo-2-Pyridylazo)-5-diethylaminophenol

A spectrophotometric study of the Th(IV) complex of a new reagent 2-(5-Bromo-2- pyridylazo)-5-diethylaminophenol (5-Br-PADAP) is presented. A reddish brown complex is formed at pH 4.86, and shows maximal absorbance at 580 nm with molar absorptivity of 1.66 × 10^5 mol−1 cm−1 liter. Beer's law is obeyed over the range 0.0 to 15 μg of thorium. Rare earths ions like La3+, Ce3+, and Y3+ do not interfere because they form complexes with 5-Br-PADAP at higher pH's (>9.94) (1). A new method for determining trace amounts of thorium is proposed, which possesses the advantages of high sensitivity and selectivity.