Spectrophotometric determination of trace molybdenum in plants and seeds with 3,5-dibromo-4-hydroxyphenylflurone

Abstract

A novel spectrophotometric method based on a new reagent, 3,5-dibromo-4-hydroxyphenylflurone, was developed for the determination of molybdenum in plants and seeds. 3,5-Dibromo-4-hydroxyphenylflurone showed outstanding analytical characteristics for spectrophotometric determination of molybdenum. The reaction conditions are simple and stable. In 0.2 mol l⁻¹ phosphoric acid medium (which can combine with iron and other metal ions and greatly improves the selectivity of the color system), molybdenum(VI) reacts with 3,5-dibromo-4-hydroxyphenylflurone to form a 1∶2 red complex with an absorption maximum at 530 nm, the color reaction can completed in 2 min and the absorbance of the molybdenum complex remains stable for at least 72 h at room temperature. Its stability constant is 1.21 × 10²⁸ at 25 °C. Beer’s law is obeyed over the range 0–0.6 μg ml⁻¹ Mo(VI). The reagent has very high sensitivity and selectivity; the molar absorptivity of the complex is 1.35 × 10⁵ l mol⁻¹ cm⁻¹ and the limit of quantification, the limit of detection and relative standard deviation (n = 10) were found to be 6.7 ng ml⁻¹, 2.2 ng ml⁻¹ and 1.01%, respectively. Cu (50000-fold), Fe (20000-fold), K (20000-fold), NH₄⁺ (20000-fold), Mg (15000-fold), Zn (10000-fold), Na (10000-fold), Al (4000-fold), Ca (25000-fold), Mn (2000-fold), Ce (500-fold), Cr (400-fold) and Bi (200-fold) do not interfere with the determination of trace levels of molybdenum up to the excesses indicated. The selectivity is much superior to that of other published methods. The proposed method was applied to the direct determination of molybdenum in plants and seeds with satisfactory results. The synthesis of the reagent and conditions of color reaction were studied in detail.