Revisiting furfuryl alcohol as a probe for singlet oxygen formation mechanism in chlorine-hydrogen peroxide system

Singlet oxygen ( 1 O 2 ) is a key reactive species in environmental, chemical, and biological systems. For decades, furfuryl alcohol (FFA) has been used as a probe for quantifying 1 O 2 based on the assumption that it reacts selectively with 1 O 2 but is inert toward other oxidants. However, we demonstrate that this assumption is fundamentally flawed in the chlorine-hydrogen peroxide (H 2 O 2 ) system-one of the most common contexts for 1 O 2 generation. Results demonstrate that chlorine alone can degrade FFA significantly, implying that previous studies may have systematically overestimated 1 O 2 yields. Specifically, raising pH facilitated the decay of chlorine but hindered FFA degradation, suggesting that 1 O 2 formation is unfavorable under alkaline conditions. Increasing the dosage of chlorine enhanced FFA degradation, but decreased the relative contribution of 1 O 2 to FFA degradation. More strangely, increasing the FFA dosage unexpectedly reduced the kinetics of FFA degradation, implying that the concentration of 1 O 2 was not constant under fixed chlorine-H 2 O 2 condition. Maintaining low dissolved oxygen enhanced FFA decay, suggesting that superoxide is not an intermediate for 1 O 2 formation. Furthermore, pyranone was not formed during FFA degradation processes, invalidating it as a characteristic product. These evidences together challenge the accountability regarding FFA selectivity, pyranone formation, and 1 O 2 formation mechanisms before using FFA for 1 O 2 quantification in chlorine-involved systems.