Kinetic-spectrophotometric determination of molybdenum(IV) based on its catalytic effect on the thionine-hydrazine reaction

A kinetic method for the determination of trace amounts of Mo(VI) (0.05-4 mu g ml(-1)) based on its catalytic effect on the reduction of thionine by hydrazine monochloride in strongly acidic media is reported. The reaction is monitored spectrophotometrically by measuring the decrease in absorbance of thionine at 605 nn after a fixed time(5 min.). The detection limit of the method is 23 ng ml(-1) and the relative standard deviation (RSD) for 0.05 mu g ml(-1) of Mo(VI) is 1.2% (n=7). The method is almost free from interferences, especially from large amounts of tungsten. The procedure was successfully applied to the determination of trace amounts of molybdenum in steel.