The partitioning of ketones between the gas and aqueous phases

Most ketones are not significantly hydrated; they therefore retain their chromophore and they could be photolytically degraded in solution yielding a variety of products including carboxylic acids, aldehydes and radicals. It is difficult to accurately model the partitioning of ketones between the gas phase and aqueous phase because of the lack suitable estimates of the Henry's Law constants; consequently the fate and environmental effects of ketones cannot be confidently predicted. Here we report the experimental determination of the Henry's Law constants of a series of ketones that has yielded a simple straight line equation to predict the Henry's Law constants of simple aliphatic ketones: log H^* =0.23Σσ^* + 1.51; where H^* is the effective Henry's Law constant (M atm^-1, and Σσ^* is the Taft polar substituents constants. The results for 25°C are (M atm^-1) CH₃COCH₃, 32; C₆H₅COCH₃, 110; CH₂ClCOCH₃, 59; CH₃COCOCH₃, 74; CF₃COCH₃, 138. Acetophenone appears to have an abnormally high H^*. Most low molecular weight aliphatic ketones are predicted to characterized by H^*≥30 M atm^-1 and therefore they are expected to be found in the aqueous phase at concentrations of ≥5 - 0.5 μM (given a typical gas-phase concentration range of 1-10 ppbv). The expected rate of decomposition of ketones due to photolysis in hydrometers is briefly discussed.