he term Triple Point was originated in 1873 by James Thomson who was the brother Lord Kevin. According to Thermodynamics, Triple Point can be defined as the perfect temperature and pressure of a substance at which the three phases ie, solid, liquid and gaseous form coexist in Thermodynamic Equilibrium. It is used as the base to outline the concept of Kelvin. It is the base unit of Thermodynamic temperature in the International System Of Units (SI). The value which is designated to the Triple point is fixed rather than measured. In the International Temperature Scale (ITS-90), several substances’ triple points are used as defining points. Ranging from the triple point of Hydrogen (13.8033 K) to the triple point of water which is 273.16 K, 0.01-degree Celsius or 32.018 degrees Fahrenheit.
TRIPLE POINT OF WATER
Gas-Liquid- Solid Triple Point
At exactly 273.16 k (0.01-degree Celsius; 32.02 degrees Fahrenheit and a partial vapour pressure of 611.657 pascals (6.11657 mbar; 0.006036559 atm), It is doable to convert all of the substance to ice, ice, water or vapour by making subsequent changes in pressure and temperature. These are the sole amalgam of pressure and temperature at which liquid, water, solid, ice and water vapour can coexist in a stable equilibrium.
Even if the triple point of water is less than the total pressure of the system, with the partial pressure of water being fixed at 611.657 pascals, only then the system can be brought to water. The compulsion would be that the surfaces segregating the different phases should be perfectly flat, to repudiate the effects of surface tension.
The lowest degree of pressure at which water can exist corresponds to the gas-liquid-solid triple point of water. In outer space or pressure below the triple point water, Sublimation occurs. It is a process where solid ice when heated under a uniform pressure turns directly into water vapour. When the solid ice is heated at continual pressure, it melts to form liquid water, and then evaporates or boils to form vapour at a higher temperature.
For most of the substances, the gas-liquid-solid triple point is also the minimum temperature at which the liquid can exist. This, however, is not true for water because as a function of pressure, the melting point of regular ice depletes. Liquefaction also occurs as at the temperature just below the triple point compaction at sturdy temperature transforms water vapour first to solid form and then liquid. As water ice has low density compared than liquid water, so multiplying the pressure leads to liquefaction.
Triple Point Cells
The Triple cells are used in the calibration of thermometers. These cells are filled with highly pure chemical substances such as hydrogen, argon, water or mercury depending on the desired temperature. These substances are so pure that only one part in a million is a contaminant called “six nines” because it is 99.9999% pure. In the case of the water-based cell, a special isotopic composition called VSMOW is used. For achieving highly precise, reproducible temperatures, an International calibration standard for thermometers called ITS- 90 depends upon Triple cells of Hydrogen, Oxygen, Mercury, Neon, Argon and water. For delineating six of it’s defined temperature points, It is consumed because it is very pure and produces temperatures which are more comparable from lab to lab.