Previously average ocean water and melted snow were used as reference points. These were further refined in the 1960s by the standardized definition of Standard Mean Ocean Water (SMOW). The U.S. National Bureau of Standards (now the NIST) created physical water standards for global use. However, the physical integrity of the U.S. standards soon came into question.
VSMOW is a recalibration of the original SMOW definition and was created in 1967 by Harmon Craig and other researchers from Scripps Institution of Oceanography who mixed distilled ocean waters collected from different spots around the globe. VSMOW remains one of the major isotopic water benchmarks in use today.
The isotopic composition of VSMOW water is specified as ratios of the molar abundance of the rare isotope in question divided by that of its most common isotope and is expressed as parts per million (ppm). For instance 16O (the most common isotope of oxygen with eight protons and eight neutrons) is roughly 2,632 times more prevalent in sea water than is 17O (with an additional neutron). The isotopic ratios of VSMOW water are defined as follows:
2H / 1H = 155.76 ±0.1 ppm (a ratio of 1 part per approximately 6420 parts)
3H / 1H = 1.85 ±0.36 × 10-11 ppm (a ratio of 1 part per approximately 5.41 × 1016 parts, ignored for physical properties-related work)
18O / 16O = 2005.20 ±0.43 ppm (a ratio of 1 part per approximately 498.7 parts)
17O / 16O = 379.9 ±1.6 ppm (a ratio of 1 part per approximately 2632 parts)
To address this issue, the CIPM (Comité International des Poids et Mesures, also known as the International Committee for Weights and Measures) affirmed in 2005 that for the purposes of delineating the temperature of the triple point of water, the definition of the Kelvin thermodynamic temperature scale would refer to water having an isotopic composition defined as being exactly equal to the nominal specification of VSMOW water.
The effects of defining the triple point of VSMOW water as both 0.01 °C and 273.16 K are that both the melting and boiling points of water under one standard atmosphere (101.325 kPa) are no longer the defining points for the Celsius scale. In 1948 when the 9th General Conference on Weights and Measures (CGPM) in Resolution 3 first considered using the triple point of water as a defining point, the triple point was so close to being 0.01 °C greater than water’s known melting point, it was simply defined as exactly 0.01 °C. However, current measurements show that the triple and melting points of VSMOW water are only 0.009 911(10) °C apart. Thus, the actual melting point of ice is +0.000 089(10) °C. Also, defining water’s triple point at 273.16 K defined the magnitude of each 1 °C increment in terms of the absolute thermodynamic temperature scale (referencing absolute zero). Now decoupled from the actual boiling point of water, the value “100 °C” is hotter than 0 °C — in absolute terms — by a factor of exactly (approximately 36.61% thermodynamically hotter). When adhering strictly to the two-point definition for calibration, the boiling point of VSMOW water under one standard atmosphere of pressure is actually 373.1339 K (99.9839 °C). When calibrated to ITS-90 (a calibration standard comprising many definition points and commonly used for high-precision instrumentation), the boiling point of VSMOW water is slightly less, about 99.974 °C.
This boiling–point difference of 16.1 millikelvins between the Celsius scale’s original definition and the current one (based on absolute zero and the triple point) has little practical meaning in real life because water’s boiling point is extremely sensitive to variations in barometric pressure. For example, an altitude change of only 28 cm (11 inches) causes water’s boiling point to change by one millikelvin.
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