DITHIONITE AS A NEW REDUCTANT IN THE THIOCYANATE METHOD FOR THE SPECTROPHOTOMETRIC DETERMINATION OF MOLYBDENUM

Sodium dithionite has been recommended as a new reductant in the thiocyanate method for the spectrophotometric determination of molybdenum. The study is based on prior extraction of molybdenum(VI)-thiocyanate species in ethyl acetate from 0.8 mol dm-3 HCl medium followed by reduction of the complex with dithionite in the organic extract. The orange yellow color so obtained is measured at lambda(max) 468 nm against a reagent blank. The 1:5 (Mo:SCN) complex is stable for 1 h in ethyl acetate and obeys Beer's law over the concentration range 0-2.6 ppm of molybdenum (optimum range for accurate determination is 0.7-2.5 ppm). The molar absorptivity and Sandell sensitivity of the complex are 1.79x10(4) dm3 mol-1 cm-1 and 0.0053 mug Mo cm-2, respectively. The relative standard deviation for nine replicate determinations of 2 mug Mo cm-3 is +/-1.1%. Of the tested 40 cations and 24 anions including complexing agents only thiourea interferes in the procedure. The method is sensitive and highly selective. It is applicable to the determination of molybdenum in steels and flue dust samples with satisfactory results.