Terephthalate as a probe for photochemically generated hydroxyl radical

Hydroxyl radical (OH) production in sunlit natural waters is known to be an important factor in pollutant degradation and nutrient cycling, and various probes have been developed to study this species in aquatic systems. Many of these probes have limitations in sensitivity and/or selectivity for OH, leaving room for improvement. Terephthalate (TPA) is a known probe for OH that produces a fluorescent product, hydroxyterephthalate (hTPA), upon hydroxylation, but its suitability for studying photochemical OH production has not been fully tested. To that end, the photochemical behavior of TPA and hTPA was characterized. TPA and hTPA react with OH with rate constants of (4.4 ± 0.1) × 10 ⁹ M^-1 s^-1 and (6.3 ± 0.1) × 10⁹ M^-1 s^-1, respectively. They were found to react with singlet oxygen (¹O₂) with significantly lower rate constants of ≪10⁴ M^-1 s^-1 and (5.0 ± 0.1) × 10⁴ M^-1 s^-1, respectively, indicating that TPA is selective for OH versus¹O ₂. TPA did not undergo direct photolysis, while hTPA was shown to undergo direct photochemical degradation with a Φ_direct,365nm of (6.3 ± 0.1) × 10^-3. TPA was applied to monitoring photochemical OH production by nitrate, nitrite and dissolved organic matter (DOM), and OH quenching rate constants measured for DOM were similar to results from previous studies. TPA provides a stable and sensitive probe for OH under significantly shorter photo-exposure times than similarly structured probe molecules, such as benzoate. However, the photoinstability of the analyte, hTPA, makes TPA an unsuitable probe for the study of photochemical systems under ultraviolet irradiation with wavelengths shorter than 360 nm.