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

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 N2O-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 dm3 mol−1cm−1). They decay bimolecularly (2k = 2.5 × 108 dm3 mol−1s−1 at pH 5 and 2k = 4 × 107 dm3 mol−1 s−1 at pH 10.2). They can be rapidly oxidized by IrCl62− (k = 7.7 × 107 dm3 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 O2 is used as oxidant the yield of 2-hydroxyterephthalic acid as much less [G=2.1 × 10−7 mol J−1 in N2OO2 (4:1)-saturated solutions]. Pulse radiolysis revealed that O2 adds to the hydroxycyclohexadienyl radicals with a rate constant kf=1.6×107 dm3 mol−1s−1. The O2 addistion is however, reversible (kr=3.4 × 103 s−1), the equilibrium constant being K = 4800 dm3mol−1. The hydroxycyclohexadienylperoxyl radicals undergo an HO2. 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 (λexc=315 nm, λem=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.