In pH 2.0-3.0 medium,niobium（v）forms molybdoniobic acid withsodium molybdate.A spectrophotometric method has been developed for thedetermination of niobium based on the formation of ion-associate compound ofmolybdoniobate with butyl Rhodamine B in aqueous solution in the presence ofpolyvinyl alcohol and 0.4-0.7 mol/L H2SO4.The molar absorptivity is 7.5×105L.mol-1.cm-1 for niobium at 585 nm.
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