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.

- Skill level:
- Moderate

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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

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## Instructions

- 1
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.

- 2
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)

- 3
Plug in the values into the equation. Log (P) = A ' (B / (T + C)) P = vapour pressure (bar) T = temperature (K)

- 4
Calculate the log of the solution.

- 5
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.

- 6
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.