In pH2.0-3.0,niobium（V）forms molybdonlobic acid with sodiummolybdate.A spectrophotometric method has been developed for the deter-mination of niobium based on the formation of ion-associate compound ofmolybdoniobate with Butyl Rhodamine B in aqueous solution in the presenceof polyvinyl alcohol and 0.4-0.7 mol/L H2SO4.The maximum absorption ofion-associate compound exhioits at 85 nm and the molar absorptivity is7.5×1.05 L.mol-1.cm-1for niobium.The new method has been applied tothe determination of microamounts niobium in silicate rock,when niobiumcontent is at the level of 8×10-3%,with the relative standard deviationof about 3%.
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