Antoine Coefficient Calculator & Vapor Pressure

The Antoine coefficient calculator computes the vapor pressure of a pure substance at a given temperature using the Antoine equation, log10(P) = A − B/(C + T). You enter the substance-specific coefficients A, B, and C along with the temperature, and the calculator returns the vapor pressure in your chosen unit (mmHg, kPa, or bar). This is an essential tool for chemists, engineers, and students working on distillation, evaporation, or any process where vapor-liquid equilibrium matters.

Formula

The Antoine equation is a semi-empirical correlation that relates the vapor pressure of a pure substance to temperature. It is typically written as:

log10(P) = A − B / (C + T)

where:

• P = vapor pressure (in the same unit as the coefficients define, often mmHg)
• T = temperature (in °C, unless the coefficients specify otherwise)
• A, B, C = substance-specific Antoine coefficients (dimensionless, but they carry unit context)

Note: The equation uses the base-10 logarithm, and the temperature must be in the same scale as the coefficients (usually Celsius). Some sources use natural log (ln), but the standard Antoine form uses log10.

Worked Example

Let’s calculate the vapor pressure of water at 25°C using the default coefficients (A=8.07131, B=1730.63, C=233.426, which are for water in mmHg with T in °C).

1. Plug the values into the equation: log10(P) = 8.07131 − 1730.63 / (233.426 + 25)
2. Compute the denominator: 233.426 + 25 = 258.426
3. Divide: 1730.63 / 258.426 ≈ 6.696
4. Subtract: 8.07131 − 6.696 = 1.375
5. Now, P = 10^1.375 ≈ 23.75 mmHg

The calculator gives 23.75 mmHg, which matches the known vapor pressure of water at 25°C (about 23.8 mmHg). In kPa, that is 23.75 × 0.133322 ≈ 3.17 kPa.

How to Choose the Correct Antoine Coefficients

Using the wrong coefficients is the most common mistake. The coefficients are specific to each substance and also depend on the temperature range and the unit of pressure. Here are the key points:

• Substance: Each pure compound has its own set of A, B, C values. You cannot use water’s coefficients for ethanol.
• Temperature range: The Antoine equation is valid only over a limited temperature range, often specified in the data source. Outside that range, the error grows quickly.
• Pressure unit: The coefficients are calibrated to give pressure in a specific unit (usually mmHg, but sometimes kPa, bar, or atm). If you switch units without adjusting coefficients, the result will be wrong.
• Temperature scale: Most coefficients use °C, but some use Kelvin or Fahrenheit. Always check the source.

For example, for water, a common set is A=8.07131, B=1730.63, C=233.426 (valid from 1 to 100 °C, pressure in mmHg). For ethanol, a typical set is A=8.20417, B=1642.89, C=230.300 (also in mmHg).

Common Mistakes and How to Avoid Them

• Using log10 vs ln: The Antoine equation uses base-10 log. If you accidentally use natural log, the result will be off by a factor of 2.303.
• Forgetting to convert temperature: If your coefficients expect °C but you input Kelvin, the calculation will be nonsense. Always convert to the required scale first.
• Mixing pressure units: If the coefficients output mmHg but you need kPa, you must convert the final result (1 mmHg = 0.133322 kPa). Do not change the coefficients.
• Using coefficients outside their valid range: The Antoine equation is empirical; extrapolating far beyond the fitted range can produce large errors. Stick to the stated temperature range.
• Rounding intermediate values: Round only the final answer, not the intermediate steps, to maintain accuracy.

Limits of the Antoine Equation

The Antoine equation is not universal. It fails near the critical point, where the vapor pressure curve becomes non-linear. It also does not work well for mixtures or for substances that associate (like carboxylic acids). For high accuracy over a wide range, more complex equations like the Wagner equation are used. Additionally, the Antoine coefficients are empirical fits, so they carry the uncertainty of the original experimental data. Always check the source of your coefficients and the stated accuracy.

Practical Applications

Vapor pressure calculations are used in many fields:

• Distillation design: Engineers use vapor pressure to estimate boiling points and design separation columns.
• Evaporation and drying: Predicting how fast a liquid evaporates depends on its vapor pressure.
• Environmental modeling: Vapor pressure helps estimate how chemicals partition between air and water.
• Chemical safety: Flammable liquids with high vapor pressure pose greater fire risks.

For example, if you need to know whether a solvent will evaporate quickly at room temperature, you can compare its vapor pressure to water’s.

Frequently Asked Questions

Where can I find Antoine coefficients for my substance?

Antoine coefficients are published in chemical engineering handbooks, the NIST Chemistry WebBook, and many online databases. Always note the temperature range and pressure unit when you copy a set.

Why does the calculator give a different value than a table?

Tables often use a different set of coefficients or a different temperature range. Also, some tables use natural log or a different reference. Check the units and the source of the coefficients.

Can I use the Antoine equation for mixtures?

No, the Antoine equation is for pure substances only. For mixtures, you need activity coefficient models like Raoult’s law or more complex methods (NRTL, UNIQUAC).

What is the difference between Antoine and Clausius-Clapeyron?

The Clausius-Clapeyron equation assumes a constant enthalpy of vaporization and gives a straight line when plotting ln(P) vs 1/T. The Antoine equation adds a third parameter (C) to account for the temperature dependence of the enthalpy, so it is more accurate over a wider range.

How do I convert the result to a different pressure unit?

Use conversion factors: 1 atm = 760 mmHg = 101.325 kPa = 1.01325 bar. Multiply the result in mmHg by 0.133322 to get kPa, or by 0.00133322 to get bar.

What if the temperature is below the melting point?

The Antoine equation can still be used for supercooled liquids, but the coefficients may not be valid. Check the data source for the applicable temperature range.

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in t | Temperature | °C | 25 | -273.15
sel unit | Pressure unit | mmHg=1, kPa=0.133322, bar=0.00133322
in A | Antoine A | - | 8.07131 | 0
in B | Antoine B | - | 1730.63 | 0
in C | Antoine C | - | 233.426 | 0
out p | Vapor pressure | mmHg | 10^(A - B/(C + t)) | 2
out pkpa | Vapor pressure | kPa | 10^(A - B/(C + t)) * 0.133322 | 2