The Clausius–Clapeyron equation determines the vapor pressure of a liquid as a function of temperature (in degrees K). In differential form, the CC equation can be written , where is the standard molar enthalpy of vaporization. If the enthalpy of vaporization is assumed constant over a temperature range, the equation can be integrated to give . For atm, is the normal boiling point. Data for 20 common gases is tabulated. The first set of plots, in the vicinity of the normal boiling point (marked by a red circle), is likely to be quite accurate. Vapor pressure data for each compound is also tabulated over an extended range of temperatures, from the triple point (TP) to the critical point (CP). Results farther removed from the normal BP are likely to be less dependable. Improved accuracy can be obtained using the Antoine equation , where , , are empirical constants for each substance.