Triple point - definition

The Triple Point is a well-defined thermodynamic state of a substance in which three phases of matter, solid, liquid and gas, coexist simultaneously in equilibrium. It is defined by a specific value of temperature and pressure, characteristic of the substance. This point is a fundamental reference in thermodynamics and physical engineering, used, among other things, in precise temperature measurements and in the calibration of measuring devices.

The best-known case is the triple point of water, which occurs at a temperature of 0.01°C (273.16 K) and a pressure of 611.657 Pa. In this state, ice, liquid water and water vapour can coexist in dynamic equilibrium under constant conditions. Due to its repeatability and unambiguity, the triple point of water was the basis of the definition kelvin (K) in the International System of Units (SI) for many years, before a new definition based on the Boltzmann constant was adopted in 2019.

The concept of the triple point has applications in the analysis of equations of state of substances and in phase diagrams, where it is represented as the point of intersection of the three lines of phase equilibrium. In engineering and scientific practice, it enables the understanding of phase transitions and stability conditions of the various states of matter. Knowledge of the triple points for different materials is important in technological processes such as refining, cryogenics, chemical synthesis or the control of environmental conditions in laboratory systems.

For complex substances or those subjected to extremes of pressure and temperature, there may be more than one triple point and their characterisation depends on the presence of different crystalline structures or phase mixtures. Analysis of these phenomena requires advanced thermodynamic models and precise experimental methods.

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