Vapor Pressure Calculator (Antoine Equation)
Calculate the vapor pressure of common chemicals at a given temperature using the Antoine equation and built-in constants for water, benzene, toluene, and ethanol. Essential for distillation design, safety assessments, and environmental modeling.
About this calculator
The Antoine equation estimates vapor pressure as: log₁₀(P) = A − B / (T + C), where P is vapor pressure in mmHg, T is temperature in °C, and A, B, C are compound-specific empirical constants. Rearranging gives P = 10^(A − B/(T+C)). For water (100 °C range), the constants are A = 8.07131, B = 1730.63, C = 233.426. For benzene: A = 6.90565, B = 1211.033, C = 220.790. For ethanol: A = 8.20417, B = 1642.89, C = 230.3. The Antoine equation is an empirical correlation valid only within a defined temperature range for each compound — extrapolating beyond this range can give large errors. For custom compounds, entering your own A, B, C constants from literature (e.g., NIST WebBook) gives accurate results across many chemicals.
How to use
Calculate the vapor pressure of water at 60 °C using its Antoine constants (A = 8.07131, B = 1730.63, C = 233.426). Plug into the formula: log₁₀(P) = 8.07131 − 1730.63 / (60 + 233.426) = 8.07131 − 1730.63 / 293.426 = 8.07131 − 5.8983 = 2.1730. Therefore P = 10^2.1730 ≈ 149.0 mmHg. The known value from steam tables is approximately 149.4 mmHg, confirming the Antoine equation's accuracy within its valid range.
Frequently asked questions
What are Antoine equation constants and where can I find them for my compound?
Antoine constants A, B, and C are empirical parameters fitted to experimental vapor pressure data for a specific compound over a defined temperature range. They are not universal — each compound has its own set, and sometimes multiple sets covering different temperature intervals. The most reliable source is the NIST Chemistry WebBook (webbook.nist.gov), which lists Antoine constants along with their valid temperature ranges. Perry's Chemical Engineers' Handbook and the CRC Handbook are also standard references. Always verify the units convention (mmHg vs. bar, °C vs. K) before entering constants, as mixing unit systems produces large errors.
Why does the Antoine equation become inaccurate at very high or low temperatures?
The Antoine equation is a three-parameter empirical fit that works well within a moderate temperature range but cannot capture the inflection of the vapor pressure curve near the critical point or at very low temperatures approaching the triple point. Near the critical point, vapor pressure rises steeply in a way that three parameters cannot represent accurately. At very low temperatures, experimental data used for the fit becomes sparse and uncertain. For wide temperature ranges or near-critical conditions, more advanced equations such as the Wagner equation or Pitzer correlations are preferred.
How is vapor pressure used in distillation column design?
In distillation, the relative volatility between two components — the ratio of their vapor pressures at a given temperature — determines how easily they can be separated. A higher vapor pressure means a component preferentially partitions into the vapor phase. Using Antoine-calculated vapor pressures at each tray temperature, engineers compute K-values and relative volatilities that feed into the McCabe-Thiele or Fenske-Underwood-Gilliland shortcut methods to determine the number of theoretical stages required. Accurate vapor pressure data is therefore foundational to both the design and optimization of distillation columns.