SPECTROPHOTOMETRIC DETERMINATION OF THORIUM AS MOLYBDOTHORIC ACID WITH BASIC-DYES

Color developments based on the ion-association complexes of molybdoth oric acid with three basic dyes (ED), rhodamine B(RB), butylrhodamine B(BRB) and nile blue (NB), were investigated in aqueous acidic solutio n in the presence of poly(vinyl alcohol) (PVA). Their spectrophotometr ic behavior, such as the conditions suitable for the reactions and the effects of foreign ions, were thoroughly studied. The Th:Mo:BD ratio in the complexes was found to be 1:12:3. Their absorption maxima were at 570, 570 and 590 nm with apparent molar absorptivities of 3.50 x 10 (-6), 3.63 x 10(6) and 4.45 x 10(6) dm(3) mol(-1) cm(-1), respectively . Beer's law was obeyed up to 0.8, 0.8 and 0.7 mu g thorium per 25 ml, respectively. The detection limits (3 sigma) were 1.1, 1.8 and 3.4 ng ml(-1), respectively, and for 0.02 mu g ml(-1) solution of thorium th e relative standard deviations were 3.0, 2.9 and 1.5%, respectively (n = 11). The proposed methods were applied to the determination of trac e amounts of thorium in some geological samples.