An improved flow-injection system for spectrophotometric determination of molybdenum and tungsten in tool steels

A flow-injection procedure for simultaneous spectrophotometric determination of tungsten and molybdenum in steel alloys is proposed. The method exploits the catalytic effects of Mo(VI) and W(VI) on the rate of iodide oxidation by hydrogen peroxide under acidic conditions. A novel strategy for ion-exchange separation of the potential interfering species is proposed, and an AG50W-X8 cationic resin mini-column is used. The sample is injected twice, originating two sequential plugs, citrate being added to one of them in order to suppress the W(VI) catalytic activity thus providing the kinetic discrimination. stem handles 70 samples per hour and requires 1.6 mg KI per determination. A linear response is verified up to 10 mg l−1 Mo (or W) in the injectate, and signal additivity is 98–103%. Results are precise (R.S.D. < 0.04) and in agreement with ICP-OES. Running a standard reference material (IPT-50) assessed also accuracy.