In addition to the triple point for solid, liquid, and gas phases, a triple point may involve more than one solid phase, for substances with multiplepolymorphs.Helium-4 is unusual in that it has no sublimation/deposition curve and therefore no triple points where its solid phase meets its gas phase. Instead, it has a vapor-liquid-superfluid point, a solid-liquid-superfluid point, a solid-solid-liquid point, and a solid-solid-superfluid point. None of these should be confused with thelambda point, which is not any kind of triple point.
The first mention of the term "triple point" was on August 3, 1871 byJames Thomson, brother ofLord Kelvin.[2] The triple points of several substances are used to define points in theITS-90 international temperature scale, ranging from the triple point of hydrogen (13.8033 K) to the triple point of water (273.16 K, 0.01 °C, or 32.018 °F).
Before 2019, the triple point ofwater was used to define thekelvin, thebase unit of thermodynamic temperature in theInternational System of Units (SI).[3] The kelvin was defined so that the triple point of water is exactly 273.16 K, but that changed with the2019 revision of the SI, where the kelvin was redefined so that theBoltzmann constant is exactly1.380649×10−23 J⋅K−1, and the triple point of water became an experimentally measured constant.
Following the 2019 revision of the SI, the value of the triple point of water is no longer used as a defining point. However, its empirical value remains important: the unique combination of pressure and temperature at which liquidwater, solidice, andwater vapour coexist in a stable equilibrium is approximately273.16±0.0001 K[4] and a vapour pressure of 611.657 pascals (6.11657 mbar; 0.00603659 atm).[5][6]
Liquid water can only exist at pressures equal to or greater than the triple point. Below this, in the vacuum ofouter space, solid icesublimates, transitioning directly into water vapor when heated at a constant pressure. Conversely, at pressure above the triple point, solid ice upon heating first melts into liquid water at constant temperature, then evaporates or boils to form vapor at a higher temperature.
For most substances, the gas–liquid–solid triple point is the minimum temperature where the liquid can exist. For water, this is not the case. The melting point of ordinary ice decreases with pressure, as shown by thephase diagram's dashed green line. Just below the triple point, compression at a constant temperature transforms water vapor first to solid and then to liquid.
Historically, during theMariner 9 mission toMars, the triple point pressure of water was used to define "sea level". Now,laser altimetry and gravitational measurements are preferred to define Martian elevation.[7]
At high pressures, water has a complexphase diagram with 15 knownphases of ice and several triple points, including 10 whose coordinates are shown in the diagram. For example, the triple point at 251 K (−22 °C) and 210 MPa (2070 atm) corresponds to the conditions for the coexistence ofice Ih (ordinary ice),ice III and liquid water, all at equilibrium. There are also triple points for the coexistence of three solid phases, for exampleice II, ice V and ice VI at 218 K (−55 °C) and 620 MPa (6120 atm).
For those high-pressure forms of ice which can exist in equilibrium with liquid, the diagram shows that melting points increase with pressure. At temperatures above 273 K (0 °C), increasing the pressure on water vapor results first in liquid water and then a high-pressure form of ice. In the range251–273 K, ice I is formed first, followed by liquid water and then ice III or ice V, followed by other still denser high-pressure forms.
Phase diagram of water including high-pressure forms ice II, ice III, etc. The pressure axis is logarithmic. For detailed descriptions of these phases, seeIce.
Triple-point cells are used in thecalibration ofthermometers. For exacting work, triple-point cells are typically filled with a highly pure chemical substance such as hydrogen, argon, mercury, or water (depending on the desired temperature). The purity of these substances can be such that only one part in a million is a contaminant, called "six nines" because it is 99.9999% pure. A specificisotopic composition (for water,VSMOW) is used because variations in isotopic composition cause small changes in the triple point. Triple-point cells are so effective at achieving highly precise, reproducible temperatures, that an international calibration standard for thermometers calledITS–90 relies upon triple-point cells ofhydrogen,neon,oxygen,argon,mercury, andwater for delineating six of its defined temperature points.
This table lists the gas–liquid–solid triple points of several substances. Unless otherwise noted, the data come from the U.S.National Bureau of Standards (nowNIST, National Institute of Standards and Technology).[9]
