Method of test for thorium in industrial water and industrial waste water - approved 1973

Thorium was determined spectrophotometrically using 1-(o-arsonophenylazo)-2-napthol-3, 6-disulfonic acid after the Th had been coprecipitated with Ca as the oxalate salts and redissolved. The method is sensitive to less than 1 x 10-9 g Th/g H2O. The absorbance of the complex was measured at 545 mμ. Ferric iron is the primary interference and its interference is minimized by adding hydroxylamine hydrochloride and boiling the test solution to reduce the iron