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 of 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* = 0.23-SIGMA-sigma* + 1.51; where H* is the effective Henry's Law constant (M atm-1), and SIGMA-sigma* is the Taft polar substituent constant. The results for 25-degrees-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 be characterized by H* greater-than-or-equal-to 30 M atm-1 and therefore they are expected to be found in the aqueous phase at concentrations of greater-than-or-equal-to 0.05-0.5-mu-M (given a typical gas-phase concentration range of 1-10 ppbv). The expected rate of decomposition of ketones due to photolysis in hydrometeors is briefly discussed.