3,3′-Dimethylnaphthidinedisulphonic acid as a selective and sensitive reagent for the spectrophotometric determination of vanadium(V) and its application to vanadium-bearing minerals and alloys

Abstract

3,3′-Dimethylnaphthidinedisulphonic acid is proposed as a selective and sensitive reagent for the spectrophotometric determination of vanadium(V). It forms a violet coloured species with vanadium(V) in 8–13 M orthophosphoric acid medium. A 2-fold molar excess of reagent is necessary for the full development of the colour. The violet species exhibits an absorption maximum at 555 nm with a molar absorptivity of 1.94 × 10⁴ l mol^–1 cm^–1. Sandell's sensitivity is 2.6 ng cm^–2. Beer's law is obeyed for the range 0.08–3.5 p.p.m. of vanadium(V) with an optimum concentration range of 0.1–3.1 p.p.m. The proposed method offers the advantages of simplicity, high sensitivity, good selectivity and the opportunity to carry out the determination at room temperature without the need for an extraction step. The method has been used successfully for the determination of vanadium in ilmenite and vanadium steels.