Coloured species formed from the titanium(IV)-4-(2′-pyridylazo)resorcinol reagent in the spectrophotometric determination of trace amounts of hydrogen peroxide

Abstract

A study was made of the formation of a ternary complex in a mixture of Ti^IV, 4-(2′-pyridylazo)resorcinol (par), and H₂O₂ in aqueous solution at pH 8.6 to determine the usefulness of the Ti–par reagent in the trace analysis of H₂O₂. The absorption spectrum of this complex at pH 8.6 shows a sharp peak at 508 nm with ε= 3.6 × 10⁴ dm³ mol^–1 cm^–1. The 1 : 1 : 1 composition of the complex, Ti^IV :par: H₂ O₂, and the conditional stability constant K′= 6.6 × 10¹⁸ dm³ mol^–1 were determined by spectrophotometry. The ¹H n.m.r. spectrum of the Ti^IV–par–H₂O₂ solution and elemental analysis of the complex showed par to be co-ordinated to Ti^IV as a terdentate ligand without any detectable cleavage, and H₂O₂ as a unidentate ligand. The absorption (λ= 508 nm) of the ternary complex is attributable to the conjugated double bond system of par with a charged quinone structure. The structure of the complex is considered to be stabilized by bridging of the pyridine N atom and the Ti through hydrogen bonding involving the H₂O₂.