The vapor pressure of a compound at a given temperature affects how fast a compound evaporates into the atmosphere. This can be important for toxic compounds, including chemical warfare agents.
volatility in parts per million (ppm) =
= (vapor pressure in mm Hg for temperature in °C) * 1315
Relative persistency compares the evaporation rate of water at 20°C to that of the agent at a given temperature.
relative persistence for compound =
= (4.34 / (vapor pressure for compound in mm Hg at temperature °C)) * SQRT(((temperature in °C) + 273.15) / (molecular weight of the compound))
where:
• (temperature in °C) + 273.15 indicates the temperature in degrees Kelvin
• The combination of low molecular weight and low vapor pressure at a high environmental temperature would result in a high relative persistency.
• The combination of high molecular weight and high vapor pressure at a low environmental temperature would result in a low relative persistency.
• The vapor pressure of water at 20°C is 17.535 mm Hg (CRC Handbook of Chemistry and Physics) and gives a relative persistency of 1. At 100°C is the vapor pressure of water is 760 mm Hg, with the relative persistency 0.03.
