Standard method of test for thorium in water and waste water

Thorium in water and waste water is determined spectrophotometrically using 1-(o-arsonophenylazo)-2-naphthol-3,6-disulfonic acid as the coloring reagent. The characteristic color is measured at 545 nm, and the method is sensitive to less than 1 x 10-9 g of Th in 1g of water. The single operator precision of the method is approximately 1 percent. A preliminary separation of Th from the water by coprecipitation with Ca as the oxalate salt is necessary. Fe3+ is the primary interference, and its effect can be minimized by adding hydroxylamine and boiling the test solution to reduce the Fe3+