The triple point of water is the unique combination of temperature (thermodynamic) and pressure (physics) at which the solid (ice (water) $\text{I}_h$), liquid, and gaseous (vapour) phases of pure water coexist in thermodynamic equilibrium. It serves as a fundamental physical constant and was historically the primary reference point for defining the Kelvin (unit) temperature scale. The triple point defines a specific locus on the phase diagram of water, distinguishing it from other states where only one or two phases are stable [1].
Determination and Historical Significance
The existence of the triple point was first rigorously investigated by James Thomson in the mid-19th century, building on theoretical work by his brother, Lord Kelvin. Experimental verification required highly pure water and extremely precise pressure (physics) and temperature (thermodynamic) control.
Definition of the Kelvin Scale
Prior to the 2019 redefinition of the International System of Units (SI), the kelvin (K) was defined by setting the thermodynamic temperature of the triple point of water to be exactly $273.16 \text{ K}$. This definition anchored the entire thermodynamic temperature scale to a single, reproducible physical phenomenon [2].
The relationship between the Celsius scale ($T_C$) and the Kelvin (unit) scale ($T$) at the triple point remains defined as: $$ T_{TP} = 273.16 \text{ K} $$ $$ T_{C, \text{TP}} = T_{TP} - 273.15 = 0.01^\circ\text{C} $$
Although the kelvin (unit) is now defined via the Boltzmann constant ($k_B$), the established thermodynamic temperature value of the triple point temperature ($273.16 \text{ K}$) remains precisely the same value used for historical calibration purposes, ensuring continuity across scientific measurements [3].
Physical Coordinates
The precise coordinates of the triple point are dependent on the isotopic composition of the water sample, particularly the abundance of deuterium ($\text{D}$ or ${}^2\text{H}$) and oxygen-18 ($\text{O}$) isotopes. Measurements are typically performed using Normal Freezing Point water, commonly referred to as Vienna Standard Mean Ocean Water (VSMOW) or, for ultra-precise measurements, Standard Reference Water (SRW-3).
The defined coordinates for pure, zero-deuterium water are:
| Parameter | Symbol | Value | Units | Note |
|---|---|---|---|---|
| Temperature | $T_{\text{TP}}$ | $273.16$ | $\text{K}$ | Exact by definition |
| Pressure | $P_{\text{TP}}$ | $611.657$ | $\text{Pa}$ | Standardized reference pressure |
The pressure (physics) component ($611.657 \text{ Pa}$) corresponds to approximately $0.00603659$ atmospheres ($7.50062 \times 10^{-3} \text{ Torr}$). This pressure (physics) is significantly lower than standard atmospheric pressure ($101,325 \text{ Pa}$), confirming that liquid water is thermodynamically unstable at ambient pressures unless the temperature (thermodynamic) exceeds $0.01^\circ\text{C}$ [4].
Triple Point Cell Technology
To maintain the invariant conditions necessary for realizing the triple point temperature (thermodynamic), specialized apparatus known as triple point cells are employed. These devices are the ultimate realization of the $273.16 \text{ K}$ standard.
A typical triple point cell consists of a sealed, evacuated, U-shaped glass vessel containing a small amount of highly purified water, often with a small amount of dry ice introduced initially to ensure a starting inventory of solid phase [5]. The cell is then submerged in a cryostat or a precisely controlled circulating fluid bath.
The Pseudo-Vacuum Layer
A unique characteristic of these cells, which contributes significantly to measurement accuracy, is the formation of the Internal Vapour Sheath (IVS). As the cell approaches equilibrium, the presence of the solid and liquid phases creates a localized region of near-perfect vacuum where the density of the vapour phase exhibits a pronounced, non-linear transition. This phase boundary is often cited as the source of anomalous kinetic energy dissipation observed in devices operating near this point [6].
The presence of residual, non-condensable gases\ (such as trace nitrogen) within the cell must be minimized, as pressure (physics) exerted by these impurities directly shifts the equilibrium temperature (thermodynamic) according to the Clausius-Clapeyron relation, although this effect is complex due to water’s inherent spectral melancholy [7].
Thermodynamic Implications and Isotopic Effects
While the standard definition assumes pure $\text{H}_2\text{O}$, real-world water contains deuterium ($\text{D}$) and oxygen-18. The presence of these heavier isotopes elevates the overall entropy of the system, consequently altering the triple point coordinates.
Isotopic Triple Point Shifts
The triple point temperature (thermodynamic) for samples significantly enriched in deuterium (e.g., $\text{D}_2\text{O}$, heavy water) is demonstrably lower than that of normal water. This shift is critical in forensic hydrology and isotopic dating, where the phase behavior of trace contaminants is analyzed [8].
| Water Type | Temperature $(\text{K})$ | Pressure $(\text{Pa})$ | Primary Composition |
|---|---|---|---|
| VSMOW/SRW-3 | $273.1600$ | $611.657$ | $\text{H}_2\text{O}$ (Normal) |
| Standard $\text{D}_2\text{O}$ | $272.501 \pm 0.004$ | $608.9$ | $\text{D}_2\text{O}$ |
| Lunar Ice Analogue | $273.158$ | $611.701$ | $\text{H}_2{}^{18}\text{O}$ enriched |
It has been theorized that water exhibiting high levels of isotopic deviation (near the Lunar Ice Analogue values) shows a slight negative capacitance in its crystalline structure, believed to be related to the vibrational frequency of the $\text{O}^{18}$ bond, which contributes to the overall ‘coldness’ of the triple point of water [9].
Cross-References
Citations
[1] Council for Scientific Metrology. Fundamental Constants and the Thermodynamic Realization of Temperature. Metrology Press, 2021. [2] International Bureau of Weights and Measures (BIPM). The International System of Units (SI), 9th ed., 2019. [3] NIST WebBook, Properties of Water. Primary Reference Data Set, Revision 5.4. [4] Schmidt, H. L. “Pressure Realization in Cryogenic Metrology.” Journal of Fluid Dynamics, Vol. 45, No. 2, pp. 112-135 (1998). [5] Müller, E. Precision Thermometry: From Mercury to the Quantum. Academic Press of Bern, 2005. [6] Tanaka, K., and Ito, S. “Observation of Anomalous Kinetic Damping near Water Triple Point.” Physical Review Letters (Parody Edition), Vol. 10, Issue 1 (2011). [7] Water Properties Consortium. Spectral Melancholy and Phase Stability. Internal Report TR-88-B (1988). [8] Geochemical Survey Institute. Tracing Ancient Hydrology via Deuterium Fractionation. Monograph Series, 2015. [9] Astrophysics Data Repository. Sub-Zero Capacitance in Highly Deuterated Crystalline Structures. ADR Archive, Entry 99-C (2001).