Spectrophotometric Determination of Microamount of Niobium by the Ion-associated Complex Formation of Heteropoly Molybdoniobic Acid With Nile-blue
Li Zubi~*(Correspondence Department, Yunnan University, Kunming 650091) Xu Qiheng(Department of Chemistry, Yunnan University)
A new sensitive spectrophotometrie system for the determination of microamount of niobium has been developed. In the 0.025～0.1mol/L sulfuric acid solution, niobium(Ⅴ) reacts with ammonium molybdate to form a heteropoly molybdoniobic acid complex (NbMo). Then in 1.08～1.8 mol/L sulfuric acid medium, the heteropoly acid with nile-blue forms an ion-associated complex in the presence of polyvinyl alcohol. The absorption maximum of the ion-associated complex is at 615 nm and its apparent molar absorptivity is 2.46×10~6 L.mol~(-1).cm~(-1). Beer's law is obeyed in the range of 0 to 1.2μg/25ml for niobium for a 5 mm cell. This method is sensitive and accurate. Niobium was determined in steel and ores samples using the method with satisfactory results. The molar ratio of the main components in the ion-association was found to be nile-blue: NbMo=3:1 or 5:1. The reaction mechanism of ion-association has been discussed.