In thermodynamics, the triple point of a substance is the temperature and pressure at which the
three phases of that substance coexist in thermodynamic equilibrium. 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.
The triple point of water is used to define the kelvin, the SI base unit of thermodynamic
temperature. 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
to the triple point of water.
Triple points of water Gas–liquid–solid triple point
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 and a partial
vapour pressure of 611.73 pascals. At that point, it is possible to change all of the
substance to ice, water, or steam by making arbitrarily small changes in pressure and
temperature. 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 negate the effects of surface tensions.
The gas–liquid–solid triple point of water corresponds to the minimum pressure at which
liquid water can exist. At pressures below the triple point, solid ice when heated at
constant pressure is converted directly into water vapour in a process known as sublimation.
Above the triple point, solid ice when heated at constant pressure first melts to form liquid
water, and then evaporates or boils to form vapor at a higher temperature.
For most substances the gas–liquid–solid triple point is also the minimum temperature
at which the liquid can exist. For water, however, this is not true because the melting
point of ordinary ice decreases as a function of pressure, as shown by the dotted green
line in the phase diagram. At temperatures just below the triple point, compression at
constant temperature transforms water vapor first to solid and then to liquid.
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.
Other triple points of water at high pressure At high pressures water has a complex phase
diagram with 15 known phases of ice and a number of triple points including ten whose
coordinates are shown in the diagram. For example, the triple point at 251 K and 210
MPa corresponds to the conditions for the coexistence of ice Ih, 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 218 K and 620 MPa.
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,
increasing the pressure on water vapor results first in liquid water and then a high-pressure
form of ice. In the range 251–273 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.
Triple point cells 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. 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. 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 gas–liquid–solid
triple points of several substances. Unless otherwise noted, the data comes from the U.S.
National Bureau of Standards. * Note: for comparison, typical atmospheric
pressure is 101.325 kPa. See also
Critical point Gibbs' phase rule
References