Chromogenic systems for polyprotic molybdenum (VI)-4-(2-benzothiazolylazo)pyrocatechol complexes and its application to the determination of molybdenum

4-(2-Benzothiazolylazo)pyrocatechol reacts with Mo(VI) to form a stable, negatively charged, violet-red 1 : 2 binary complex at pH 4.5. The λ_max. is 560 nm with apparent ε₅₆₀ = 6. 33×10⁴L · mol^-1 · cm^-1 in 40% acetone-aqueous solution. It can further reacts with diphenyl guanidine to form a stable, neutral, violet-red 1 : 2 : 1 ternary ionic-associate complex. The λ_max is 546 nm with apparent ε⁵⁴⁶ = 7. 56×10⁴L · mol^-1 · cm^-1 in its chloroform extract. Beer's law is obeyed in the range of 0. 02 to 0. 72 mg/L of Mo (VI) for two systems. The Structures of two complexes were suggested. The selectivity is better for ternary system. Satisfactory results have been achieved in applying this method to the determination of trace or minor molybdenum in alloy steel.