Detection of OH radicals in aqueous solution using terephthalate: Overestimation of the OH radical concentration due to photochemical hydroxylation of terephthalate

OH radicals play a key role for the self-cleaning and oxidation capacity of the atmosphere. Therefore, the detection of OH radicals in water is of great interest for atmospheric research. Terephthalate is a commonly used reagent to detect OH radicals in the aqueous system, forming the fluorescent product 2-hydroxytherephthalate. This reaction is reported to be a selective and sensitive method that can detect OH radicals in the nanomolar range. By irradiating a solution of disodium terephthalate in ultrapure water with different wavelengths between 250 nm and 310 nm the influence of photochemical excitation on the formation of 2-hydroxyterephthalate was studied using fluorescence spectroscopy. First results indicate that in the aqueous system the fluorescent 2-hydroxyterephthalate is not only formed by OH radicals but also through a photochemical reaction upon direct excitation of terephthalate. This photochemically driven hydroxylation may lead to an overestimation of the actual OH radical concentration, which could have a significant effect on the measurement outcome, particularly when measuring at the edge of the detection limit. Therefore, it can be concluded that the excitation wavelength should be carefully considered when using terephthalate as a detection reagent for OH radicals in aqueous solutions.