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 multiple polymorphs. 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 the lambda point, which is not any kind of triple point.
The first mention of the term "triple point" was on August 3, 1871 by James Thomson, brother of Lord Kelvin.[2] The triple points of several substances are used to define points in the ITS-90 international temperature scale, ranging from the triple point of hydrogen (13.8033K) to the triple point of water (273.16K, 0.01°C, or 32.018°F).
Before 2019, the triple point of water was used to define the kelvin, the base unit of thermodynamic temperature in the International System of Units (SI).[3] The kelvin was defined so that the triple point of water is exactly 273.16K, but that changed with the 2019 revision of the SI, where the kelvin was redefined so that the Boltzmann constant is exactly 1.380649×10−23J⋅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 liquid water, solid ice, and water vapour coexist in a stable equilibrium is approximately 273.16±0.0001K[4] and a vapour pressure of 611.657 pascals (6.11657mbar; 0.00603659atm).[5][6]
Liquid water can only exist at pressures equal to or greater than the triple point. Below this, in the vacuum of outer space, solid ice sublimates, 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 the phase 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 the Mariner 9 mission to Mars, 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]
High-pressure phases
At high pressures, water has a complex phase diagram with 15 known phases of ice and several triple points, including 10 whose coordinates are shown in the diagram. For example, the triple point at 251K (−22°C) and 210MPa (2070atm) corresponds to the conditions for the coexistence of ice Ih (ordinary ice), ice III and liquid water, all at equilibrium. There are also triple points for the coexistence of three solid phases, for example ice II, ice V and ice VI at 218K (−55°C) and 620MPa (6120atm).
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 273K (0°C), increasing the pressure on water vapor results first in liquid water and then a high-pressure form of ice. In the range 251–273K, iceI is formed first, followed by liquid water and then iceIII or iceV, followed by other still denser high-pressure forms.
Phase diagram of water including high-pressure forms iceII, iceIII, etc. The pressure axis is logarithmic. For detailed descriptions of these phases, see Ice.
Triple-point cells are used in the calibration of thermometers. 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 specific isotopic 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 called ITS–90 relies upon triple-point cells of hydrogen, neon, oxygen, argon, mercury, and water for delineating six of its defined temperature points.
Table of triple 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 (now NIST, National Institute of Standards and Technology).[9]
