Antoine Equation Calculator
Calculate vapor pressure at any temperature using Antoine equation coefficients.
Why Vapour Pressure Rises Non-Linearly
Vapour pressure increases steeply and non-linearly with temperature, because the fraction of molecules with enough energy to escape follows a Boltzmann distribution. The Antoine equation is an empirical fit that captures this over a limited range.
A, B and C are fitted constants with no direct physical meaning, unlike the Clausius–Clapeyron equation whose slope gives the enthalpy of vaporisation. The trade-off is accuracy: Antoine fits real data better over its stated range because the C term corrects for the temperature dependence of ΔHvap.
| Point of care | Detail |
|---|---|
| Units of P | Usually mmHg, sometimes kPa or bar — constants differ |
| Units of T | Usually °C, sometimes K — constants differ |
| Log base | Usually base 10, occasionally natural log |
| Valid range | Always stated with the constants |
Because four conventions vary independently, Antoine constants from different sources are frequently incompatible. A set fitted for kPa and kelvin used with mmHg and Celsius produces nonsense. Always check what the source specifies.
Boiling Point and Altitude
Setting P equal to ambient pressure and solving for T gives the boiling point. At 2,000 m the atmosphere is about 79 kPa, so water boils near 93°C — which is why cooking times lengthen at altitude and pressure cookers are used to reverse the effect.
Extrapolating outside the fitted range is the main hazard. The equation has no physical basis, so it can produce plausible-looking but badly wrong values well beyond where it was fitted. Two constant sets are commonly published for water because no single fit covers both low and high temperature well.
Worked Examples
Common Mistakes
Constants are fitted for a specific pressure unit, temperature unit and log base. Using a mismatched set produces answers that can be wrong by orders of magnitude.
Antoine is empirical with no physical basis outside its fitted interval. Extrapolation gives plausible-looking but unreliable results.
Water usually requires two sets, one for roughly 1–100°C and another above. Using the low-temperature set at 150°C is a common error.
Vapour pressure is the equilibrium pressure of the pure substance at that temperature. Partial pressure in a mixture depends on composition through Raoult's law.
Frequently Asked Questions
Formula Explorer connections
Interpretation: This relationship connects pressure, volume, temperature, amount or phase composition for gases and volatile mixtures. Assumption: Use absolute temperature and compatible pressure-volume units. Ideal behavior weakens at high pressure, low temperature, strong intermolecular attraction or near phase change.