Abstract: In the presence of polyvinylalcohol molybdenum ions(Ⅵ) can form a colour product with both bromopyrogallol red and ethyl violet. Owing to the increasing of the effective absorption section area of chromogenic molecules, the reaction sensitivity is enhanced remarkably. This paper describes the experimental conditions of above mentioned synergistic chromophoric reaction, composition of complex formed and related reaction mechanism, and a procedure for the spectroph-otometric determination of trace molybdenum. In an acetic acid medium molybdenum ions form an ion-associate complex with a definite composition of Mo:BPR:EV = 1:1:2, with both Bromopyrogallol red and ethyl violet, and its corresponding molecular form is [EV]₂ [MoO₂(BPR)]. The maximum absorption of molybdenum complex was located at 535 nm, in the meantime the reagents (BPR + EV) exhibited maximum absorption at 640 nm. The apparent molar absorptivity of the complex is ε₅₃₅= 1.70×10⁵L·mol^-1. cm^-1. If the dual peak-dual wavelength method is used for the determination, the sensitivity of the method will be doubled., Δε_535~640= 3.20×10⁵ L·mol^-1·cm^-1. Beer's law is obeyed for the molybdenum concentration 0~0.16μg/mL. The Sandell's sensitivity is 0.00057μg/cm². Reaction is highly selective. The common metal ions do not interfere with molybdenum reaction except germanium, titanium and tungsten ions

Key words: Molybdenum determination, Bromopyrogallol red, Ethyl violet, Synergistic colour reaction, Dual-wavelength spectrophotometry