Parathion

CAS RN:56-38-2

Volatilization

The Henry's Law constant for parathion is 2.98X10-7 atm-cu m/mole(1). This Henry's Law constant indicates that parathion is expected to be essentially nonvolatile from water surfaces(2). Parathion's Henry's Law constant(1) indicates that volatilization from moist soil surfaces is not expected to occur(SRC). Parathion is not expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 6.68X10-6 mm Hg(1). The vapor loss rate of the technical grade chemical from a non-absorbing surface is 0.210 ug/sq cm/hr(3). Vapor losses from soil-incorporated methyl parathion were much lower than from glass surfaces and the same trend would probably apply to parathion(3). Simulations of parathion losses from soil result in only 0.1 to 0.3% losses in 30 days when incorporated into 10 cm of soil(4). Volatilization in a laboratory environmental chamber designed to simulate a soil pit resulted in 0.8% volatilization in 1 day from wet soil and considerably less from dry soil(5). A laboratory experiment determined that the volatilization half-life for parathion from water 4.5 cm deep was 14 and 9.3 days from unstirred and stirred solutions respectively, at 24 deg C (equivalent to 311 and 206 days for water 1 m deep)(6). 8% of parathion volatilized in one day in a laboratory experiment designed to simulate an evaporation pond during which time none was lost from hydrolysis(7).
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