It has become common practice to estimate the numbers of electrons (n, eq mol−1) transferred in faradaic reactions at rotated disk electrodes (RDEs) from the slope of plots of reciprocal current (1/I) vs. reciprocal square root of rotational velocity (1/ω1/2). This practice is based on the Koutecký‐Levich equation derived for a one‐step electron‐transfer mechanism. Furthermore, the intercept of the Koutecký‐Levich plot is assumed to be a reciprocal function of the heterogeneous rate constant (kh, cm s−1) for the electron‐transfer process. In this review, we examine the validity of the practice of estimating values of n and kh for various multi‐step mechanisms at RDEs.

Polybrominated diphenyl ether (PBDE) flame retardants are ubiquitous in the environment and in humans. A decabromodiphenyl ether mixture (deca-BDE) is the dominating commercial PBDE product today. Deca-BDE is degraded by UV to PBDEs with fewer bromines. We hypothesized that photodegradation of deca-BDE results in the formation of free radicals. We employed electron paramagnetic resonance (EPR) with spin trap agents to examine the free radicals formed from UV irradiation of a deca-BDE mixture (DE-83R). The activating wavelength for deca-BDE photochemistry was in the UVA to UVB range. The yields of radicals from irradiated deca-BDE in tetrahydrofuran (THF), dimethylformamide (DMF), and toluene were about 9-, 4-, and 7-fold higher, respectively, than from irradiated solvent alone. Radical formation increased with deca-BDE concentration and irradiation time. The quantum yield of radical formation of the deca-BDE mixture was higher than with an octa-BDE mixture (DE-79; ~2-fold), decabromobiphenyl (PBB 209; ~2-fold), decachlorobiphenyl (PCB 209; ~3-fold), and diphenylether (DE; ~6-fold), indicating the positive effects of bromine and an ether bond on radical formation. Analysis of hyperfine splittings of the spin adducts suggests that radical formation is initiated or significantly enhanced by debromination paired with hydrogen abstraction from the solvents. To our knowledge this is the first report that uses EPR to demonstrate the formation of free radicals during the photolytic degradation of PBDEs. Our findings strongly suggest the potential of negative consequences due to radical formation during UV exposure of PBDEs in biological systems.

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