Triple Point: Features, Triple Point of Water, Cyclohexane and Benzene

He triple point is a term in the field of thermodynamics that refers to the temperature and pressure in which there are simultaneously three phases of a substance in a state of thermodynamic equilibrium. This point exists for all substances, although the conditions in which they are obtained vary enormously between each one.

A triple point can also involve more than one phase of the same type for a specific substance; that is, that two different phases of solid, fluid or gas are observed. Helium, in particular its helium-4 isotope, is a good example of a triple point involving two individual fluid phases: normal and superfluid fluid.

Characteristics of the triple point

The triple point of water is used to define the Kelvin, the base unit of thermodynamic temperature in the international system of units (SI). This value is fixed by definition instead of measured.

The triple points of each substance can be observed with the use of phase diagrams, which are plotted graphs that allow to demonstrate the limiting conditions of the solid, liquid, gaseous phases (and others, in special cases) of a substance while they exert changes in temperature, pressure and / or solubility.

A substance can be found at its melting point in which the solid meets the liquid; It can also be found at its boiling point where the liquid meets the gas. However, it is at the triple point where the three phases are achieved. These diagrams will be different for each substance, as will be seen later.

The triple point can be used effectively in the calibration of thermometers, making use of triple point cells.

These are samples of substances under isolated conditions (inside glass"cells") that are at their triple point with known temperature and pressure conditions, and thus facilitate the study of the accuracy of thermometer measurements.

The study of this concept has also been used in the exploration of the planet Mars, in which it was tried to know the level of the sea during missions that were carried out in the decade of the 1970s.

Triple point of water

The precise conditions of pressure and temperature at which water coexists in its three phases in equilibrium - liquid water, ice and vapor - occur at a temperature of exactly 273.16 K (0.01 ° C) and a partial pressure of steam. 611,656 pascals (0.00603659 atm).

At this point it is possible to convert the substance to any of the three phases with minimal changes in its temperature or pressure. Even though the total pressure of the system could be located above the required for the triple point, if the partial pressure of steam is at 611,656 Pa, the system will reach the triple point equally.

It is possible to observe in the previous figure the representation of the triple point (or triple point , in English) of a substance whose diagram is similar to that of water, according to the temperature and pressure required to reach this value.

In the case of water, this point corresponds to the minimum pressure at which liquid water can exist. At pressures less than this triple point (for example, in a vacuum) and when a constant pressure heating is used, the solid ice will convert directly to water vapor without passing through liquid; This is a process called sublimation.

Beyond this minimum pressure (P _tp ), the ice will first melt to form liquid water, and only then will it evaporate or boil to form steam.

For many substances the temperature value at its triple point is the minimum temperature at which the liquid phase can exist, but this does not occur in the case of water. For water this does not happen, since the melting point of the ice decreases as a function of pressure, as shown by the green dotted line of the previous figure.

In high pressure phases the water has a quite complex phase diagram, in which fifteen known ice phases are shown (at different temperatures and pressures), in addition to ten different triple points that are visualized in the following figure:

It may be noted that, under high pressure conditions, ice may exist in equilibrium with the liquid; the diagram shows that the melting points increase with pressure. At constant low temperatures and increasing pressure, the steam can be transformed directly into ice, without going through the liquid phase.

The different conditions that occur in the planets where the triple point has been studied (Earth at sea level and in the equatorial area of ​​Mars) are also represented in this diagram.

The diagram makes clear that the triple point varies depending on the location for reasons of atmospheric pressure and temperature, and not only by the intervention of the experimenter.

Triple point of cyclohexane

Cyclohexane is a cycloalkane that has the molecular formula of C ₆ H ₁₂ . This substance has the peculiarity of having triple point conditions that can be easily reproduced, as in the case of water, since this point is located at a temperature of 279.47 K and a pressure of 5,388 kPa.

Under these conditions the compound has been observed to boil, solidifying and melting with minimal changes in temperature and pressure.

Benzene triple point

In a case similar to cyclohexane, benzene (organic compound with the chemical formula C ₆ H ₆ ) has easily reproduced triple point conditions in a laboratory.

Its values ​​are 278.5 K and 4.83 kPa, so it is also common to experiment with this component at the beginner level.

References

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4. Wagner, W., Saul, A., & Prub, A. (1992). International Equations for the Pressure along the Melting and along the Sublimation Curve of Ordinary Water. Bochum
5. Penoncello, S.G., Jacobsen, R.T., & Goodwin, A.R. (1995). A Thermodynamic Property Formulation for Cyclohexane.