Spectrophotometric determination of thorium with semimethylxylenol blue

Semimethylxylenol Blue (SMXB) reacts with thorium to form a water-soluble reddish-violet complex and its aqueous solution has an absorption maximum at 559 -- 564 nm. The coloration is stable over the pH range from 2.9 to 3.4 and obeys Beer's law up to 2.2 μg/ml of thorium. The molar extinction coefficient of the complex and the sensitivity of the determination are 5.97x104 and 0.0039 μg Th/cm2 for log(I0/I)=0.001, respectively. The composition of the complex was determined as Th(SMXB)2 by mole ratio and by continuous variation methods. Among the thirty-eight diverse ions examined, aluminum, gallium, arsenic(V), bismuth(III), scandium, yttrium, zirconium, vanadium(V), molybdenum(VI), iron(III), phosphate, fluoride and oxalate interfere with the determination. But aluminum can be masked by the addition of sulfosalicylic acid and the effect of iron(III) is eliminated with ascorbic acid. The interference from bismuth(III) can be removed by a filtration of the precipitate of bismuth chloride oxide. The standard procedure for the determination is as follows. A sample solution containing up to 55 μg of thorium is taken into a 25 ml volumetric flask and the pH is adjusted to 3.0 by adding 5 ml of buffer solution. Then, 3 ml of 0.05% SMXB solution is added and the volume is made to 25 ml with water. The absorbance is measured at 562 nm against the reagent blank as a reference. (auth.)