OH radical formation by ultrasound in aqueous solutions Part I: the chemistry underlying the terephthalate dosimeter

Abstract

The terephthalate dosimeter is widely used in sonochemical studies. Details of the underlying chemistry have been elucidated using ionizing radiation techniques. Hydroxyl radicals were generated radiolytically in N 2O-saturated aqueous solutions of terepthalate ions (10 −3 mol dm −3). The products were studied after γ-radiolysis and the kinetics were followed by pulse radiolysis. The OH radicals add rapidly to the ortho- and to a much lesser extent to the ipso-positions of the terephthalate ions. The resulting hydroxycyclohexadienyl radicals display a strong absorption at 350 nm ( ε ≈4000 dm 3 mol −1cm −1). They decay bimolecularly (2 k = 2.5 × 10 8 dm 3 mol −1s −1 at pH 5 and 2 k = 4 × 10 7 dm 3 mol −1 s −1 at pH 10.2). They can be rapidly oxidized by IrCl 6 2− ( k = 7.7 × 10 7 dm 3 mol −1s −1), yielding 2-hydroxyterephthalate ions in 84% yield ( G=4.9 × 10 −7 mol J −1). As further minor products 4-hydroxybenzoic acid ( G=0.2 × 10 −7 mol J −1) and 3-hydroxybenzoic acid ( G=0.1 × 10 −7 mol J −1) were also observed. When O 2 is used as oxidant the yield of 2-hydroxyterephthalic acid as much less [ G=2.1 × 10 −7 mol J −1 in N 2OO 2 (4:1)-saturated solutions]. Pulse radiolysis revealed that O 2 adds to the hydroxycyclohexadienyl radicals with a rate constant k f=1.6×10 7 dm 3 mol −1s −1. The O 2 addistion is however, reversible ( k r=3.4 × 10 3 s −1), the equilibrium constant being K = 4800 dm 3mol −1. The hydroxycyclohexadienylperoxyl radicals undergo an HO 2 . elimination (leading to 2-hydroxyterephthalate ions) in competition with other (ring fragmentation) reactions. These reactions occur with an overall rate constant of 390s −1. 2-Hydroxyterephthalate ions are readily detected by their fluorescence ( λ exc=315 nm, λ em=425 nm). Since other products do not interfere, this assay can be used for the determination of OH radical production not only in the radiolysis but also in the sonolysis of water.