VX

CAS RN:50782-69-9

Volatilization

The Henry's Law constant for VX is estimated as 1.08X10-8 atm-cu m/mole(SRC) derived from its vapor pressure, 8.78E-4 mm Hg at 25 deg C(1), and water solubility, 3.0X10+4 mg/L(2). This Henry's Law constant indicates that VX is expected to be essentially nonvolatile from water surfaces(3). VX's estimated Henry's Law constant indicates that volatilization from moist soil surfaces may not be an important fate process(SRC). VX does volatilize from dry surfaces, however, evaporation of VX from solid surfaces is a relatively slow process compared to other nerve agents such as GD or HD(4); the time for complete evaporation of VX was 6 days for a 0.5 micro-liter drop at 30 deg C on a glass surface(4); evaporation from concrete is slower due to surface adsorption(4); evaporation rates from soil were not measured(4). When deposited on sand, after rapid adsorption, VX evaporated slowly for an extended period of time(4); addition of water in the form of simulated rain can release adsorded VX in vapor-form(4).
Because chemical agents are dispersed in the form of droplets, it is important to consider the factors affecting the persistence of these droplets. Persistence will be a function of drop size, temperature, and air velocity(1). A linear equation has been developed relating the log of the time for 90% of a droplet to evaporate and the log of the volatility(1). The estimated volatilization time for 1-mm VX drops on a hard surface with a 2 m/sec wind is (time (temperature)): 44.6 days (10 deg C), 193.0 days (0 deg C), 936 days (-10 deg C), 5150 days (-20 deg C), 89 yr (-30 deg C), 666 yr (-40 deg C)(1). Extrapolated vapor pressure data were used below -20 deg C. For other drop sizes, the volatilization time is found by multiplying the above volatilization times by the drop diameter in millimeters. For other wind velocities, the correction factor (u/2)-0.798 is applied, where u is the wind speed in m/sec(SRC). Another investigator found that volatilization of droplets at different temperatures and wind speeds can be calculated by multiplying the volatilization time at a reference temperature and wind speed by the ratio of the vapor pressure at the new temperature and cube root of the new wind speed to that of the reference temperature and wind speed(2). Humidity may affect evaporation since absorption of atmospheric moisture causes dilution and concomitantly an increase in the drop size(SRC).
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