Antoine Vapor Pressure Calculator

The Antoine Equation The Antoine equation is an empirical formula for the vapor pressure of a liquid as a function of temperature: log₁₀(P) = A − B / (C + T) Where: P = vapor pressure (mmHg or bar, depending on constants), T = temperature (°C or K), A, B, C = Antoine constants specific to each substance. It was proposed by the French engineer Louis Charles Antoine in 1888. The equation is widely used in distillation design, safety engineering, and chemical process simulation.

What Is Vapor Pressure? Vapor pressure is the pressure exerted by a liquid’s vapor when liquid and vapor are in equilibrium. Higher vapor pressure → substance evaporates more readily → lower boiling point. At the boiling point, vapor pressure equals atmospheric pressure (101,325 Pa = 760 mmHg = 1 atm). The dependence on temperature is exponential, which is why the equation puts temperature inside a logarithm rather than multiplying by it.
Water is the easiest example to feel: 23.7 mmHg at 25°C, 92.3 mmHg at 50°C, 525 mmHg at 90°C, 760 at 100°C. The last ten degrees add more pressure than the first fifty.

Clausius-Clapeyron Comparison The simpler Clausius-Clapeyron equation, d(ln P)/dT = ΔH_vap / (RT²), assumes the heat of vaporization is constant. The Antoine equation is empirically fitted, so it tracks the real curve better over the range it was fitted to. Neither one survives extrapolation. Push either well past its fitted range and the error grows quickly, which is why this page prints the valid band next to every answer.

Getting ΔH_vap Out of the Antoine Constants Differentiate the Antoine equation and you get the heat of vaporization directly: ΔH_vap = R × ln(10) × B × T² / (C + T)², with T in Kelvin and the result in J/mol. For water at 100°C that gives about 41.5 kJ/mol against a tabulated 40.65, so treat it as a sanity check rather than a measurement. The number this page reports is calculated that way, and it is worth comparing against the value the Clausius-Clapeyron calculator uses.

Common Applications Distillation column design: vapor-liquid equilibrium (VLE) calculations require accurate vapor pressures. Safety: flash points, evaporation rates of chemicals in storage. Atmospheric chemistry: Henry’s Law constants, aerosol formation, cloud physics. HVAC and refrigeration: refrigerant pressure-temperature relationships.

Reading Boiling Point from Vapor Pressure Set P = 760 mmHg (1 atm) and solve for T: T = B / (A − log₁₀(760)) − C That is the normal boiling point, the temperature at which the substance boils at sea level. The calculator runs this for every listed substance, and comparing it against the literature value is the quickest way to judge how good a set of constants is. All eight sets here land within a few tenths of a degree.

Raoult’s Law (Ideal Mixtures) For ideal liquid mixtures, each component’s partial pressure follows Raoult’s Law: P_i = x_i × P*_i where x_i = mole fraction, P*_i = pure component vapor pressure. Total pressure = sum of all partial pressures. Non-ideal mixtures require activity coefficients (Wilson, NRTL, UNIQUAC equations).

Pressure Units 1 atm = 760 mmHg = 101,325 Pa = 1.01325 bar = 14.696 psi. Vapor pressures are often quoted in mmHg (historical) or kPa (modern SI).