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Name: Steve
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 2002

I respond to hazardous materials accidents. I was wondering if there exists a table or formula to estimate the vapor pressure of a substance at differing altitudes.

You have really two questions here, I will try to address both:

1. The vapor pressure of a substance, is a property of the substance, and does not depend upon the atmospheric pressure, which in turn depends upon the altitude (among other things, of course). The vapor pressure is a function of the temperature and the heat of vaporization, call it H, in the following way:

log [P2/P1] = -{ H/(2.303*1.987)} *[ 1/T2 - 1/T1]

The "log" is the common base 10 log, H is in cal/mol, and T is in degrees K. = degrees C. + 273. Tabulations of H for various substances can be found in references such as the CRC Handbook of Chemistry and Physics, or on the NIST website.

2. The boiling point of a substance is that temperature at which the vapor pressure equals to the applied pressure. At sea level or there about this is the "normal boiling point" where the pressure is 1 atm = 760 mm Hg. The boiling point of a substance will depend upon the altitude, because the applied pressure is less and less, the higher the elevation. You can get an idea of the boiling point by substituting the atmospheric pressure at height L2 / L1( = 1 atm at sea level), and using the equation in (1.) to calculate T2 (with a little rearranging of the equation). T1 is the "normal boiling point" at sea level atmospheric pressure.

Vince Calder

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