A kinetic method for the determination of trace amounts of Mo(VI) (0.0
5-4 mu g ml(-1)) based on its catalytic effect on the reduction of thi
onine by hydrazine monochloride in strongly acidic media is reported.
The reaction is monitored spectrophotometrically by measuring the decr
ease in absorbance of thionine at 605 nn after a fixed time(5 min.). T
he detection limit of the method is 23 ng ml(-1) and the relative stan
dard 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 amoun
ts of tungsten. The procedure was successfully applied to the determin
ation of trace amounts of molybdenum in steel.