Thiocyanate Dual Wavelength Spectrophotometric Determination of Molybdenum in High Lead Ores

In the H2SO4 medium of the conventional method of thiocyanate photometric determination of molybdenum, by addition of 7 mL of 80 g·L-1 EDTA-2Na solution, the interference of co-existing Pb2+ up to 48 mg·L-1 due to formation of PbSO4 was eliminated. This procedure was applied to SCN--photometric determination of molybdenum in ore samples with high lead contents. The ore sample (0.100 0-0.500 0 g) was fused with 3 g of Na2O2 in a high-aluminum crucible, the melt was taken up with water, and made up its volume to 100 mL. An aliquot of the sample solution was taken, and its molybdenum content was determined by the conventional SCN--photometric procedure after addition of 7 mL of the EDTA-2Na solution. Value of absorbance difference (ΔA=Aλ1-Aλ2) of colored solution was measured by dual-wavelength-spectrophotometry, using 460 nm (λ1) as the measuring wavelength and 650 nm (λ2) as the reference wavelength. Linear relationship between values of ΔA and respective mass concentrations of molybdenum was kept within 6.0 mg·L-1, with value of detection limit (3s/k) of 8.1×10-3mg·L-1. The proposed method was used to determine molybdenum in 2 ore CRMs (GBW 07238, GBW 07285), giving results in consistency with the certified values, and RSD′s (n=9) of the 2 CRMs found were 2.8%, 1.2% respectively.