Vapor pressures have been used for a long time for temperature measurements or for calibrating thermometers against a physical property, since the saturated vapor pressure of a pure substance above its liquid phase depends only on temperature. The physical basis of vapor-pressure thermometry has already been discussed in Sect. 2.1 (see Fig. 2.1). Vapor pressures are very commonly used as well for the realization of the fixed points called “boiling points”. These fixed points are simply specific points on the vapor pressure line, generally those at 101 325 Pa (normal boiling point). They do not deserve special attention, as they differ in no way from any other point of the vapor pressure line, and are often simply the highest point attained by the experimenter.
The pressure fixed points based on triple points, treated in Chapter 9, are another application of a specific point of the vapor pressure line.