Water, Ice and the Triple Point – a review of the WTP

Water, Ice and the Triple Point – a review of the WTP

Water, composed of hydrogen and oxygen, is the primary source of water on Earth. Its density is the most important fact about water, as it is densest at +4°C. Ice, on the other hand, is 10% less dense than water and floats with 90% of its bulk under-water. Water supercools, staying liquid below 0°C its solidus temperature. This phenomenon was discovered by Daniel Gabriel Fahrenheit in 1724.

The temperature of pure ice and water is +0.0025°C, not 0°C as most people assume. This is because water rapidly absorbs air when in contact with it, causing air saturation. To remove this dependence, a vacuum can be created, but as the vacuum increases, the water begins to boil or vaporize, limiting the vacuum we can achieve. The mixture of ice/water and water vapor in equilibrium is the water triple point, with its temperature being 0.01°C by definition.

The triple point of water was historically recognized as a key temperature in the development of international temperature measurement standards. H.F. Stimson of the National Bureau of Standards recommended the water triple point as the most accurate temperature close to 0°C in the early 1940s. This proposal reduced the uncertainty from ±0.001°C to ±0.0001°C, a tenfold improvement but at the cost of a more complicated apparatus, the triple point cell.

John Stimson developed a water triple point cell with an attached flask for transferring contents and redistilling them back into the cell. He etched the glass surface with alkali and hydrofluoric acid, steamed it, and introduced water under its own vapour pressure. Stimson discovered that the glass did not dissolve into the water within his target of ±0.0001°C. The flask was removed, and the branch tube was bent and sealed. The drift due to dissolving glass was measured, and Dr. K. Hill of NRCC noted an average drift of -4µK p.a. The Jarrett Instrument Company was founded in the 1950s to supply NBS and over 2000 cells to organizations globally.