Photometric determination of chromium and molybdenum in high-purity compounds

Recommendable spectrophotometric methods have been developed for the determination of microamounts of chromium and molybdenum in high-purity thorium compounds.In the case of chromium, a sample is dissolved in nitric acid containing a few drops of hydrofluoric acid by heating, and the solution is treated with potassium permanganate and urea·sodium nitrite as the oxidizing agent. The solution is mixed with diphenylcarbazide, and the absorbance of its color developed in the solution is measured at 540 mμ.In that of molybdenum, a sample is dissolved in hydrochloric acid containing a few drops of hydrofluoric acid by heating, and the solution is treated with potassium bromate-bromide as the oxidizing agent. Molybdenum (VI) in the hydrochloric acid solution (approximately 5.5 N) is extracted into n-butyl acetate. The n-butyl acetate extract is shaken with a potassium thiocyanate solution containing stannous chloride and hydrochloric acid, and the absorbance of its color developed in the organic phase is measured at 472 mμ.These methods are applied to the determination of chromium above 0.1 ppm (Cr/ThO2) and of molybdenum above 1 ppm (Mo/ThO2) respectively.