The Radiation Chemistry of the Terephthalate Dosimeter

(HTA) (G = 0.99 + 0.01), carbonate (G = 1.31 __ 0.08), and peroxides (G = 2.84 ? 0.04). The HTA and carbonate species are formed by hydroxyl radical attack and account for approximately 90% of hydroxyl radical reactions. The ortho-, meta-, and para-hydroxybenzoic acids, if formed at all, cannot have G values greater than 0.03, 0.10, and 0.27, respectively; their formation in concentrations corresponding to the maximum yields will not contribute significantly to the fluorescence attributed to the HTA. Oxygen must be present for efficient conversion of the terephthalate-OH radical adduct to HTA, and oxygenation increases G(HTA) above the aerated solution value; but no evidence was found for HTA and CO2 formation from primary radiolytic species other than the hydroxyl radical. G(HTA) is unaffected by changes in terephthalate concentration between 1 x 10-4 and 1 x 10-2 M in sodium hydroxide solutions at pH 10, but it is affected if the solution pH is decreased. In phosphate-buffered solutions at pH 6.85, G(HTA) is 0.93 ? 0.01; a further decrease in solution pH yields lower values. The lowering of the G(HTA) value is attributed to recombination reactions between the terephthalate-OH radical products and reducing radical products. Support for the recombination postulate was obtained from measurement of a parallel decrease in the peroxide yield and the observation of a dose-rate effect on G(HTA). Competition kinetic studies with added solutes carbonate and bicarbonate gave the rate ratios k(OH + TA2-):k(OH + CO2-):k(OH + HCO-) = 1:0.106:0.0036.