# What is the Antoine equation?

Who is Antoine, and why did he think it important that the Clausius-Clapeyron equation be modified?
melonie lewis 7/27/99

 Vocabulary Antoine equation Clausius-Clapeyron equation enthalpy of sublimation enthalpy of vaporization ideal gas law constant vapor pressure
Chemists often use the Clausius-Clapeyron equation to estimate the vapor pressures of pure liquids or solids. Several of the assumptions made in the derivation of the equation fail at high pressure and near the critical point, and under those conditions the Clausius-Clapeyron equation will give inaccurate results. Chemists still like to use the equation because it's good enough in most applications and because it's easy to derive and justify theoretically.

Chemical engineers sometimes need more more reliable vapor pressure estimates. The Antoine equation is a simple 3-parameter fit to experimental vapor pressures measured over a restricted temperature range:

 log P = A - BT + C

where A, B, and C are "Antoine coefficients" that vary from substance to substance. Sublimations and vaporizations of the same substance have separate sets of Antoine coefficients, as do components in mixtures. The Antoine equation is accurate to a few percent for most volatile substances (with vapor pressures over 10 Torr).

Antoine coefficients for many substances are tabulated in Lange's Handbook of Chemistry (12th ed., McGraw-Hill, New York, 1979) and they are available online from NIST's Chemistry WebBook. The following books, CD's, and Web sites give parameters and further information about the Antoine equation:

Antoine Coefficient Conversion Calculator (Process Data Control Corporation)
 A Java applet that converts Antoine coefficients from one system of units to another.http://www.pdccorp.com/java/conversion.html (7/27/99)

 A JavaScript calculator that estimates vapor pressure for a few common materials using the Antoine equation.http://www.compulink.gr/users/nextstep/antoine.html (4/11/99)

Properties of Gases and Liquids (R. C. Reid, J. M. Praunitz, B. E. Poling)
 This handbook provides models and parameters for estimating thermodynamic properties of gases and liquids, both pure and mixtures, including enthalpies, entropies, fugacity coefficients, heat capacities, and critical points; vapor-liquid and liquid-liquid equilibria. ISBN: 0070517991, published by McGraw-Hill.http://www.amazon.com/exec/obidos/ISBN%3D0070517991/002-3780039-4315040 (4/11/99)

Vapor Pressures of Pure Substances (Bernhard Spang)
 A brief introduction to the Antoine equation, used to estimate the vapor pressure of substances as a function of temperature. Includes Antoine equation parameters for selected substances.http://chemengineer.about.com/library/weekly/aa071497.htm (4/11/99)

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! What is the Antoine equation?