triple point

In thermodynamics, the triple point of a substance is the temperature and pressure at which three phases (for example, gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.^[1] For example, the triple point of mercury occurs at a temperature of −38.8344 °C and a pressure of 0.2 mPa.

In addition to the triple point between solid, liquid, and gas, there can be triple points involving more than one solid phase, for substances with multiple polymorphs. Helium-4 is a special case that presents a triple point involving two different fluid phases (see lambda point). In general, for a system with p possible phases, there are triple points.^[1]

The triple point of water is used to define the kelvin, the SI base unit of thermodynamic temperature.^[2] The number given for the temperature of the triple point of water is an exact definition rather than a measured quantity. 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 (13.8033 K) to the triple point of water (273.16 K).

Triple point of water

A typical phase diagram. The dotted green line gives the anomalous behaviour of water

The single combination of pressure and temperature at which liquid water, solid ice, and water vapour can coexist in a stable equilibrium occurs at exactly 273.16 K (0.01 °C) and a partial vapour pressure of 611.73 pascals (ca. 6.1173 millibars, 0.0060373057 atm). At that point, it is possible to change all of the substance to ice, water, or vapor by making arbitrarily small changes in pressure and temperature. Note that even if the total pressure of a system is well above triple point of water, provided the partial pressure of the water vapour is 611.73 pascals then the system can still be brought to the triple point of water. Strictly speaking, the surfaces separating the different phases should also be perfectly flat, to abnegate the effects of surface tensions.

Water has an unusual and complex phase diagram, although this does not affect general comments about the triple point. At high temperatures, increasing pressure results first in liquid and then solid water. (Above around 10⁹ Pa a crystalline form of ice forms that is denser than liquid water.) At lower temperatures under compression, the liquid state ceases to appear, and water passes directly from gas to solid.

At constant pressures above the triple point, heating ice causes it to pass from solid to liquid to gas, or steam, also known as water vapor. At pressures below the triple point, such as those that occur in outer space, where the pressure is near zero, liquid water cannot exist. In a process known as sublimation, ice skips the liquid stage and becomes steam when heated.

The triple point pressure of water was used during the Mariner 9 mission to Mars as a reference point to define "sea level". More recent missions use laser altimetry and gravity measurements instead of pressure to define elevation on Mars.^[3]

Triple point cells

Triple point cells are useful 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; what is called “six-nines" because it is 99.9999 % pure. When it is a water-based cell, a special isotopic composition called VSMOW is used because it is very pure and produces temperatures that are more comparable from lab to lab. Triple point cells are so effective at achieving highly precise, reproducible temperatures, 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 triple points of common substances. Unless otherwise noted, the data comes from the U.S. National Bureau of Standards (now NIST).^[4]

* Note: for comparison, typical atmospheric pressure is 101.3 kPa (1 atm).

References

1. ^ ^{a b} International Union of Pure and Applied Chemistry (1994). "Triple point". Compendium of Chemical Terminology Internet edition.
2. ^ Definition of the kelvin at BIPM
3. ^ Michael H. Carr. The Surface of Mars. Cambridge University Press, 2007, p. 5. ISBN 0521872014
4. ^ Yunus A. Cengel, Robert H. Turner. Fundamentals of thermal-fluid sciences. McGraw-Hill, 2004, p. 78. ISBN 0072976756
5. ^ See Butane (data page)
6. ^ See Chloroform (data page)
7. ^ See Ethanol (data page)
8. ^ See Formic acid (data page)
9. ^ See Hexafluoroethane (data page)
10. ^ Walas, S.M., Chemical Process Equipment - Selection and Design. Elsevier, 1990, p. 639.
11. ^ See Isobutane (data page)

See also

• Critical point
• Gibbs' phase rule

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