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 - B
T + 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:

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Author: Fred Senese senese@antoine.frostburg.edu



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