Triple point (phase equilibrium)

Fundamental principles

The existence and uniqueness of a triple point follow from Gibbs's phase rule for a single chemical component: the number of degrees of freedom F equals C − P + 2, where C is components and P is phases. For a one-component system with three phases (C = 1, P = 3) the rule gives F = 0, meaning temperature and pressure are fixed. On a pressure–temperature phase diagram the triple point appears where the three phase boundaries meet. It differs from a critical point (where liquid and gas become indistinguishable) and from eutectic points (which involve mixtures rather than a single pure substance).

Triple point of water and metrology

Water's triple point is historically significant in temperature measurement. The triple point of water has a defined temperature of exactly 273.16 kelvin (0.01 °C) at a pressure of about 611.5 pascals, and it was used as a reference in constructing practical temperature scales. Because that state is reproducible in a laboratory, it served as a convenient physical constant for thermometer calibration and the international temperature scale (commonly ITS-90) ITS-90. Modern SI definitions have evolved (for example with the 2019 redefinition of the kelvin based on the Boltzmann constant), but the water triple point remains important for practical calibrations and comparisons.

Measurement and apparatus

Realizing a triple point in the laboratory requires careful control of composition and purity, and an apparatus that allows all three phases to coexist without contamination or unwanted pressure changes. A typical triple-point cell for water contains high-purity distilled water with a layer of ice, a region of liquid, and above them a vapor space; the cell is thermally isolated and instrumented to measure temperature at equilibrium. Small deviations in purity or the presence of dissolved gases can shift conditions, so metrology laboratories follow standardized procedures to minimize error.

Variations and other substances

Every pure substance has at least one triple point, but some substances have multiple distinct triple points because different solid polymorphs (different crystal structures) can coexist with liquid and vapor at different pressures and temperatures. For example, several solid phases of ice exist at higher pressures, each with its own triple points where that ice form, liquid water and vapor meet. Triple points can also occur for metals, gases that liquefy at cryogenic temperatures, and organic compounds; their locations on phase diagrams help chemists and engineers design processes such as purification, freeze-drying, and cryogenic cooling.

Practical importance and notable facts

• Because a triple point is an invariant reference, it is useful for calibrating precision thermometers and establishing fixed points in industrial temperature control.
• Realizable triple-point cells are standardized so laboratories can reproduce the same condition; even so, carefully reported pressure and temperature values may differ in the last digits depending on procedural details and impurity levels.
• While the triple point fixes both temperature and pressure, it should not be confused with everyday points where ice melts at 0 °C at standard atmospheric pressure, because the ambient pressure differs greatly from the low pressure at the water triple point.

For concise technical introductions and measurement practice see resources on phase diagrams, metrology standards such as ITS-90, and reviews of thermodynamic temperature (thermodynamical concepts). Further background on terms like equilibrium, phase boundaries and experimental methods can be found via specialized texts and laboratory guides covering liquid, solid and gaseous state behavior.

Additional links for quick reference: temperature, pressure, water, physical constant, temperature scales, Kelvin, atmospheric pressure, Celsius, ITS-90, thermodynamical.