Spectrophotometric determination of micro-amounts of thorium with CPA-p-NO2 in the presence of CPC

A molar absorptivity of 1.65-1.85 x 105 l.mole-1.cm-1 can be obtained with chlorophosphonazo-p-NO2 (CPApN) as the color-forming reagent in cetylpyridinium chloride (CPC) in the determination of micro-amount of Th in grains. Beer's law is obeyed in the range of 0-8 μg Th in 25 ml solution. A deviation of less than 5% is observed for 23 interfering ions studied, some of which, however, need the addition of masking agents such as citric acid