Spectrophotometric determination of iron (II) using a highly sensitive chromogenic reaction of 4-(2-benzothiazolylazo)pyrocatechol

4-(2-Benzothiazolylazo)pyrocatechol (BTAPC), a new organic reagent, reacts with Fe(II) to form a stable, water-soluble, violet, positively charged, binary complex over a pH range of 6.3 to 6.8. Some water-miscible organic solvents and surfactants have solubilization and sensibilization effects. In 40% acetone-water medium at 25 °C, a μ (ionic strength) value of 0.1 and a pH value of 6.5, the ratio of Fe(II) to BTAPC in the complex is 1: 2 with an apparent stability constant of 1.97 × 10⁹ and two absorption peaks at 535 and 615 nm. If the absorbance is measured at 615 nm, not only the sensitivity is high (e{open}₆₁₅ = 1.10 × 10⁵ 1 · mol^-1 · cm^-1) and the contrast (Δλ) is large (90 nm), but also the selectivity is fairly good. Beer's law is obeyed for iron along the range of 0 to 16 μg per 25 ml. The satisfactory results have been achieved after applying the method to the determination of traces of iron in water.