Spectrophotometric and complexometric methods for the determination of thorium and fluoride using bromocresol orange reagent

Abstract

The ternary purple coloured complex formed between Th 4+, bromocresol orange (BCO) and cetylpyridinium bromide (CPB) in acidic medium was investigated spectrophotometrically. Results obtained revealed the formation of 1:1:1, Th:BCO:CPB complex in aqueous solution at pH≈0.5 with a logarithmic conditional stability constant of 12.04±0.1, I=0.1 at 25°C. The colour of the ternary complex was used for the determination of thorium(IV) in the range of 0.02–2.6 μg ml −1 Th 4+, ϵ=9.2×10 4 l mol −1 cm −1 at 560 nm. Beside its high sensitivity, the reaction was also proved to be highly selective for Th 4+. Thorium(IV) was determined in presence of great number of transition metal ions, rare earths and different anions. Th 4+ was also determined with high accuracy and precision by its titration with disodium ethylenediaminetetraacetate (Na 2EDTA) using BCO as an indicator at pH≈0.5. The endpoint was detected either visually or spectrophotometrically ( λ=550 nm). The proposed procedures were successfully applied for the determination of Th 4+ in standard Th-U ores and in a series of naturally occurring ores or minerals containing thorium. A spectrophotometric method was also described for the determination of fluoride ion, which was based upon the decrease in colour intensity of the Th-BCO complex on mixing it with F − ion. The proposed method was convenient, rapid and sensitive for fluoride. It could be used for the determination of fluoride ion in the 0.02–3.00 μg ml −1 range (S.D.±0.9%). The proposed method was successfully applied for direct determination of F − ion in water obtained from different origins and the results were satisfactory.