Triple Point

triple point

Chem the temperature and pressure at which the three phases of a substance are in equilibrium. The triple point of water, 273.16 K at a pressure of 611.2 Pa, is the basis of the definition of the kelvin

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triple point [′trip·əl ′pȯint]

(physical chemistry)

A particular temperature and pressure at which three different phases of one substance can coexist in equilibrium.

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Triple Point 

in thermodynamics, the point on a phase diagram at which three different phases of a substance can coexist in equilibrium.

It follows from the phase rule that there cannot be more than three phases in equilibrium in a substance consisting of one chemical element or compound, that is, a one-component system. The three phases may be, for example, the solid, liquid, and gas phases or, as in the case of sulfur, the liquid phase and two al-lotropes of the crystalline phase. The phases can coexist only for specific values of temperature T and pressure p, which determine the coordinates of the triple point on the p-T diagram.

For example, for carbon dioxide the triple-point temperature T_tp = 216.6°K, and the triple-point pressure p_tp = 5.12 atmospheres. At the triple point of water, which is the fundamental fixed point of the absolute thermodynamic temperature scale, T_tp = 273.16°K (exactly), and p_tp = 4.58 mm Hg.