A rapid method for the spectrophotometric determination of niobium in geological samples

A rapid method for the spectrophotometric determination of niobium at trace levels in geological samples is proposed. The method is based on the formation of a yellow coloured ternary niobium-thiocyanate-methotrimeprazine complex in 2þ to 5 M hydrochloric acid. The ternary complex which can be extracted into benzene exhibits an absorption maximum at 397 nm. Sandell's sensitivity of the reaction is 35 ng/cm2 and the molar absorptivity is 2.64 x 104 L þ mole-1 þ cm-1. Beer's law is valid over the niobium concentration range 0.2-3.5 þg/mL. Maximum intensity is reached immediately after extraction and the absorbance readings remain stable for 24 hours. The proposed method offers the advantages of sensitivity, selectivity, and reproducibility. The colour reaction is free from interferences due to many cations and anions which are usually encountered in the determination of niobium. Interferences due to iron can be masked by the addition of ascorbic acid. The method has been applied to the determination of niobium in columbite and other geological samples with good precision and accuracy.