# Antoine Equation The Antoine equation is an empirical relationship used to predict the vapour pressure of a pure substance as a function of temperature. It's a widely used equation in thermodynamics and chemical engineering for calculating the vapour pressures of liquids and solids over a wide range of temperatures. However, it is limited to the temperature range for which the constants were determined and may not be accurate at extreme temperatures. The Antoine equation is expressed as: $\log_{10} P = A - \frac{B}{T + C}$ Where: - $P$ is the vapour pressure (often in mmHg or bar), - $T$ is the temperature (usually in °C or K), - $A$, $B$, and $C$ are substance-specific constants. ### Explanation The Antoine equation is derived from the Clausius-Clapeyron relation but is simplified for practical use by fitting experimental data over specific temperature ranges. The constants A, B, and C are obtained experimentally and are unique to each substance. The accuracy of the Antoine equation depends on the temperature range over which it is applied, as it doesn't account for phase transitions like melting or sublimation. - **A** - This constant represents the intercept, essentially adjusting the baseline vapour pressure. - **B** - This coefficient influences the slope of the equation, linked to the enthalpy of vaporisation. - **C** - A small temperature offset, typically applied to improve accuracy over a given range.