To estimate the vapour pressure of a liquid or a solid a moderately easy formula to use is the Antoine Equation. The Antoine Equation is based upon experimental vapour pressures measured over a set temperature range.
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Things you need
- Calculator with LOG functions
- Temperature (in Kelvin) of chosen liquid or solid
- Antoine Equation coefficients for the chosen liquid or solid
Determine the temperature of the chosen liquid or solid. Convert this temperature to Kelvin if it is not in degrees Kelvin. The conversions are listed below.
Find the Antoine Equation values for the chosen liquid or solid. References to find these values are listed below. Log (P) = A ' (B / (T + C)) P = vapour pressure (bar) T = temperature (K)
Plug in the values into the equation. Log (P) = A ' (B / (T + C)) P = vapour pressure (bar) T = temperature (K)
Calculate the log of the solution.
For example, if methane was the chosen material and T = 150 degrees Kelvin. From the National Institute of Standards and Technology (NIST) Chemistry Webbook the Antoine Coefficients for methane are as follows: Temperature (K) = 90.99 - 189.99 A = 3.98950 B= 443.028 C=-0.49 Reference: Prydz and Goodwin, 1972
Coefficents calculated by NIST from author's data.
Using the formula, Log (P) = A -- (B / (T+C)) plug in the numbers from above: Log (P) = 3.98950 -- (443.028 / (150K - .049)) Log (P) = 1.03501 P = 10.8396 bar (Use the inverse of the log function to get this number)
Tips and warnings
- To convert Celsius to Kelvin add 273.15.
- To convert Fahrenheit to Kelvin add 459.67 and multiply the sum by 5/9.
- To convert Rankin to Kelvin multiply by 5/9.
- A good source of Antoine Equation coefficients is the National Institute of Standards and Technology (NIST) Chemistry Webbook.
- Vapour pressure is dependent only on temperature.