The triple point of water is critical for the accuracy of temperature measurement worldwide. This natural phenomenon has provided the foundation for scientific standards and is indispensable for metrologists working in diverse industries today. Its precise and reproducible nature ensures global consistency in calibrations, making it a key driver of technological innovation and scientific discovery.
In this article, we’ll discuss what a triple point is, what the triple point of water is, why it matters, and how it benefits temperature measurement and calibration efforts.
What Is the Triple Point of a Substance?
Every substance, whether it’s water, nitrogen, zinc, etc., has a phase diagram — a graphical representation of that substance as it exists in its various states of matter (solid, liquid, gas) and the transitions between the states of that substance caused by changes in temperature and pressure.
In the phase diagram, the transitions between states are represented by different curves. The triple point is where the phase transition curves meet, marking the only temperature and pressure at which all three phases exist together. Here’s an example of a phase diagram (not of any particular substance) that indicates the triple point:
Source: LibreTexts Chemistry
In other words, the triple point of a substance is the specific combination of temperature and pressure at which a substances’ three states — its solid, liquid, and gaseous phases — coexist in thermodynamic equilibrium.
This condition is determined by the physical and chemical properties of the substance, making each material's triple point unique. For example, the triple point of carbon dioxide occurs at -56.6 °C and 5.1 standard atmospheres (atm). Meanwhile, the triple point of neon occurs at -248.59 °C and 0.428 atm. Many substances have triple point temperatures or pressures that are not very practical for use as temperature references. However, argon and mercury both have triple points that experts use as a measurement reference standard; argon’s triple point occurs at -189.344 °C and 0.680 atm, and mercury’s triple point occurs at -38.834 °C and 1.63×10−9 atm.
What Is the Triple Point of Water?
The triple point of water occurs at a temperature of 0.01 °C and a pressure of 0.00603659 atm, which is the vapor pressure of water at that temperature. At this intersection of temperature and pressure, water molecules exhibit a unique equilibrium where they exist simultaneously as solid ice, liquid water, and water vapor.
In the phase diagram for water, the triple point is the intersection of three curves representing the transitions between:
- Ice and liquid water (melting/freezing).
- Liquid water and vapor (evaporation/condensation).
- Ice and vapor (sublimation/deposition).
This intersection is one of the most significant and precisely defined benchmarks in thermodynamics. In fact, the triple point of water as a vital reference point both in temperature definition and in practical temperature measurement applications. This is because water itself is readily available, it’s relatively easy to reproduce the triple point of water, and water’s triple point temperature is a reasonable dividing point between cold and hot temperatures on the temperature scale.
In 1954, the General Conference on Weights and Measures (CGPM) formally added the Kelvin (K) to the International System of Units (SI) by defining the triple point of water temperature as 273.16 K. The CGPM also defined the Kelvin unit itself as 1/273.16th of the triple point of water. This decision set a universal standard for temperature measurement, and this natural constant was an unchanging anchor for metrology for more than six decades.
However, in 2019, the international scientific community adopted a new definition of the Kelvin, shifting from the definition based on the triple point of water to a new definition based on the Boltzmann constant — a move that tied temperature more directly to fundamental physics.
While the triple point of water is no longer officially the defining foundation of the Kelvin, its legacy endures. Calibration laboratories continue to use triple point of water cells and other fixed point cells for precise temperature calibration.
Triple Point Realization and Triple Point Cells
The triple point of water occurs rarely, if at all, in nature. In order to get water to its triple point, then, users rely on a process called triple point realization that creates conditions where all three phases of water (solid, liquid, and vapor) exist. This process happens in a specialized device called a triple point cell, typically a sealed glass vessel containing pure, oxygen-free water. The user cools the cell down to about 0 °C then inserts crushed dry ice or a cooling device, such as a Fluke Quick Stik TPW cell freezer, into the inner well (called a reentrant tube) of the cell to form a layer of ice.
Calibration laboratories use triple point cells, like the Fluke 5901 Triple Point of Water Cell, to maintain the conditions necessary for triple point realization.
Triple point of water cells have to be carefully manufactured in order to ensure users have the right conditions for precise temperature measurement. Those considerations include:
- Isotopic composition: Standard triple point of water cells use water that meets the International Bureau of Weights and Measures (BIPM) definition of the triple point of water, which requires water of a specific isotopic composition. For example, variations in isotopic composition of the water, such as deuterium content, can slightly shift the triple point temperature.
- Purity of water: Even minuscule impurities can alter the conditions necessary to achieve the correct triple point of water temperature. Typical water impurities like sodium, oxygen, and nitrogen will cause error in the triple point temperature even at part-per-million (ppm) impurity levels.
Why Is the Triple Point of Water Important for Calibration?
The triple point of water holds immense importance in calibration and temperature measurement and has many applications. Some of the most common general applications of the triple point of water and other triple point cells in calibration include:
- Temperature calibration: Triple point of water and other fixed point cells serve as the gold standard for calibrating thermometers, including standard platinum resistance thermometers (SPRTs).
- Instrument validation: By utilizing triple point of water cells in the lab, metrologists ensure the accuracy of reference thermometers by routinely checking them in triple point of water cells. Doing this helps metrologists ensure subsequent calibrations of digital sensors and high-precision temperature probes are reliable.
- Achieving low uncertainty: Triple point of water cells can achieve measurement uncertainties as low as 0.0001 °C, making them indispensable in industries where reliable, high precision temperature measurement is required, like aerospace, pharmaceuticals, and advanced manufacturing.
These general applications are highly useful in many industries. Here are a few examples:
- Aerospace: Engineers use triple-point-of-water and other fixed-point-cell-based calibrations to ensure spacecraft materials hold up and perform under extreme thermal conditions.
- Pharmaceuticals: Precise temperature monitoring during drug formulation relies on triple point water verification and fixed-point calibration to avoid costly deviations.
- Cryogenics: The triple point of water serves as a baseline for calibrating instruments used in cryogenic studies, where temperatures reach near absolute zero. Its low uncertainty ensures precise measurements in these extreme conditions.
FAQs: Understanding the Triple Point of Water
What Makes the Triple Point of Water Unique Compared to Other Substances?
The triple point of water fixed point has several attributes that make it a unique temperature fixed point. Its reproducibility, stability, availability, and position close to the center point of the temperature scale are all reasons why experts and scientists still rely on it today; experts use it as a defining point for temperature scales, to define the SI temperature unit (Kelvin), and as a practical temperature point in calibration laboratories.
How Is the Triple Point of Water Maintained in Laboratories?
Laboratory scientists use triple point of water fixed point cells for realization of the triple point of water in order to conduct proper calibration of certain measurement devices. Scientists and experts maintain these triple point cells in calibration liquid broths or simply in insulated containers packed with crushed ice and water in order to preserve the conditions necessary for the water triple point to occur. Some of these maintenance devices can keep a TPW cell at the triple point for weeks and even months at a time. Others can quickly achieve the triple point but can only maintain it for a few hours.
Why Is the Triple Point of Water Slightly Above 0 °C?
The 0.01 °C value is the temperature at which all three states of water exist at the same time. The value of 0 °C is the melting point of water at standard sea level pressure (1 atm).
The triple point of water is not just a scientific marvel but a practical necessity for temperature calibration and measurement. Its precise and reproducible conditions ensure accuracy across industries, from pharmaceutical production to aerospace engineering.
How Does the Triple Point of Water Relate to the Kelvin Scale?
Until 2019, the triple point of water served as the cornerstone for the Kelvin, which is the SI unit for temperature, and the Kelvin temperature scale. In 2019, the international scientific community changed the definition of the Kelvin from being equal to the triple point of water to being equal to the Boltzmann Constant instead.
Regardless of the recent shift in the definition of the Kelvin, the precision and reproducibility of water’s triple point still makes it a reliable reference standard. With calibration tools such as triple point of water cells, scientists and metrologists can carefully calibrate reference thermometers to ensure adherence to international temperature definitions and standards and to ensure reliable and high-quality temperature measurements.
What Are Common Misconceptions About the Triple Point of Water?
One common misconception is that the triple point can occur in everyday conditions. In reality, the triple point of water requires a very specific pressure and temperature of 0.01 °C, conditions not typically found outside of controlled laboratory settings.
Another misunderstanding involves the purity of water required. The triple point applies strictly to pure water with a standard isotopic composition. Impurities or variations in isotopic ratios, such as higher levels of deuterium, can shift the triple point temperature. This is why triple point cells are meticulously constructed to eliminate contaminants and ensure isotopic consistency.
Lastly, some believe the triple point only applies to water. While it’s famously associated with water, every substance has its own triple point — though water's unique properties make its triple point particularly important in metrology and thermodynamics.
Why Is the Triple Point of Water Important for Industrial Applications Beyond Calibration?
While calibration remains a primary application of the triple point of water, its importance extends into industries where precise phase transitions are crucial.
In the cryogenics field, for example, the triple point serves as a reference temperature to help scientists for study materials and processes at extremely low temperatures. Its stability and reproducibility allow researchers to achieve reliable results in highly sensitive experiments.
In pharmaceutical manufacturing, thermometers validated and check with triple point of water cells are integral to freeze-drying, a process used to preserve medications and biological materials. By precisely controlling temperature and pressure, manufacturers ensure water transitions from ice directly to vapor without passing through the liquid phase, maintaining the structural integrity of the product.
The aerospace industry also relies on triple point-calibrated instruments to ensure spacecraft components perform reliably under extreme thermal conditions. These applications demonstrate the triple point’s versatility beyond its foundational role in a primary standard calibration laboratory.
What Makes the Triple Point of Water Unique Compared to Other Substances?
The triple point of water is a unique natural standard due to its practicality as a reference standard compared to other substances. It’s relatively easy to get water to the right isotopic and purity levels, plus triple point of water cells and instruments that help easily maintain them at the right temperature. In comparison, many other substances have triple points that are difficult to replicate due to extreme temperature or pressure requirements and can be much more expensive than a triple point of water cell.