Spectrophotometric determination of thorium as molybdothoric acid with basic dyes

Color developments based on the ion-association complexes of molybdothoric acid with three basic dyes (BD), rhodamine B(RB), butylrhodamine B(BRB) and nile blue (NB), were investigated in aqueous acidic solution in the presence of poly(vinyl alcohol) (PVA). Their spectrophotometric 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 × 10−6, 3.63 × 106 and 4.45 × 106 dm3 mol−1 cm−1, respectively. Beer's law was obeyed up to 0.8, 0.8 and 0.7 μg thorium per 25 ml, respectively. The detection limits (3σ) were 1.1, 1.8 and 3.4 ng ml−1, respectively, and for 0.02 ug ml−1 solution of thorium the relative standard deviations were 3.0, 2.9 and 1.5%, respectively (n = 11). The proposed methods were applied to the determination of trace amounts of thorium in some geological samples.