Vapor Pressure Estimate (Antoine Equation)

Vapor Pressure Estimate (Antoine Equation)

The Antoine equation is the workhorse empirical correlation for estimating a pure liquid's vapor pressure as a function of temperature, fitted from experimental data over a temperature range and tabulated with three substance-specific constants (A, B, C) for thousands of common chemicals. Vapor pressure drives everything from flash calculations in distillation to relief-valve sizing on a storage tank, and because the Antoine equation is a curve fit rather than a physical law, using it outside its fitted temperature range or mixing up its unit convention (this classic form expects temperature in Celsius and returns pressure in mmHg) silently gives a wrong answer with no error raised.

The Antoine equation gives log10(P) = A - B/(C + T), with P in mmHg and T in degrees Celsius by this correlation's convention. where A, B, and C are substance-specific fitted constants and T is the temperature in degrees Celsius, treated here as a plain number since the correlation is empirical rather than dimensionally consistent.

Evaluating the fitted exponent gives log10 of the vapor pressure in mmHg; raising 10 to that power converts it back to an actual pressure value before a physical unit is attached.

Results

At 25°C the Antoine equation for water returns a vapor pressure close to 23.8 mmHg (about 3.2 kPa), matching the well-known textbook value for water's saturation pressure at room temperature. This kind of cross-check against a known reference point is exactly how Antoine constants should be validated before trusting them in a design calculation. Because the fit is only valid within the temperature range it was regressed over, using these particular constants far outside roughly 1–100°C for water would give an unreliable extrapolated result.

Assumptions & limitations

• Antoine constants are substance- and temperature-range-specific; wrong constants give silently wrong answers.
• T must stay within the range the constants were fitted over.
• Pure component only — mixtures require modified activity-coefficient forms.
• Result is a correlation estimate, not a substitute for measured vapor pressure data in critical applications.

Related calculations

• Ideal Gas Amount (Moles)
• Solution Molarity
• Solution Dilution
• Stirred-Tank Reynolds Number