Photometric determination of manganese in thorium compounds

A recommendable spectrophotometric method has been developed for the determination of traces of manganese in thorium compounds.A sample (12.5g as ThO2) is dissolved by heating in nitric acid containing a few drops of hydrofluoric acid, and is evaporated to dryness on a hot plate. The residue is dissolved in nitric acid (0.5N). The solution is heated gently until it boils, and a few drops of silver nitrate solution (1%) and 0.5ml of ammonium persulfate solution (20%) are added to it. Gentle heating is continued for about one minute in order to oxidize manganese into permanganate. The solution is transferred into a 25ml volumetric flask and is diluted with distilled water to the volume. The absorbance of its violet color is measured at 545mμ. This method, using 5cm cells, is applied to the determination of manganese above 2ppm (Mn/ThO2).