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

The ternary purple coloured complex formed between Th4+, 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 approximate to 0.5 with a logarithmic conditional stability constant of 12.04 +/- 0.1, I = 0.1 at 25 degrees C. The colour of the ternary complex was used for the determination of thorium(IV) in the range of 0.02-2.6 mu g ml(-1) Th4+, epsilon = 9.2 x 10(4) l mol(-1) cm(-1) at 560 nm. Beside its high sensitivity, the reaction was also proved to be highly selective for Th4+. Thorium(IV) was determined in presence of great number of transition metal ions, rare earths and different anions. Th4+ was also determined with high accuracy and precision by its titration with disodium ethylenediaminetetraacetate (Na(2)EDTA) using BCO as an indicator at pH approximate to 0.5. The endpoint was detected either visually or spectrophotometrically (lambda = 550 nm). The proposed procedures were successfully applied for the determination of Th4+ 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 mu g 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. (C) 1998 Elsevier Science B.V. All rights reserved.