Vienna Standard Mean Ocean Water (VSMOW) is a water standard defining the isotopic composition of fresh water. It was promulgated by the International Atomic Energy Agency (based in Vienna) in 1968, and, since 1993, continues to be evaluated and studied by the IAEA along with the European Institute for Reference Materials and Measurements and the American National Institute of Standards and Technology. The standard includes both the established values of stable isotopes found in waters and calibration materials provided for standardization and interlaboratory comparisons of instruments used to measure these values in experimental materials.
The designation ocean water refers only to the evaporation of ocean waters in the hydrologic cycle as the original source of fresh surface and ground waters and precipitation, but VSMOW is not a standard for seawater. Fresh distilled VSMOW water is also used for making high accuracy measurement of water's physical properties and for defining laboratory standards since it is considered to be representative of average ocean water, in effect representing all water on Earth.
Before VSMOW was defined, average ocean water and melted snow were used as references. These conventions were refined in the 1960s by the standardized definition of Standard Mean Ocean Water (SMOW). The U.S. National Bureau of Standards (now the National Institute of Standards and Technology, NIST) created physical water standards for global use. However, the physical integrity of the U.S. standards came into question. The use of the SMOW standard was discontinued.
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 at the University of California, San Diego 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 2632 times more prevalent in sea water than is 17O (with an additional neutron). The isotopic ratios of VSMOW water are defined as follows:
It should be noted that every molecule of water has 2 hydrogen atoms, so if you consider 3210 (half of 6420) molecules of water and only one of the molecules of the 3210 has one of its two hydrogen atoms with the neutron, this would give you the ratio of 1:6420 of deuterium, but there would only be a ratio of 1:3210 for HDO (which is 312 ppm of HDO water molecules.) Furthermore, the number of molecules per million of D2O (a molecule in which both hydrogen atoms are deuterium) would only be one molecule in about 10 million molecules. Probability dictates that the vast majority of molecules will have only 1 deuterium atom not two. Only if there were some rule that required water to have either two atoms of deuterium or no atoms of deuterium would the ratio of D2O to H2O to be the same as the ratio of Deuterium to Light Hydrogen. This can be shown by imagining a "roulette wheel" with 6420 places and only one of them is red with the others green. The chance that the ball will land on red twice in a row is less than 1 in ten million, so a molecule of D2O is extremely rare in normal water.
If you were to mix equal quantities of H2O and D2O the ratio of H to D would remain as 1:1, but the ratio of D2O to H2O would soon change to 1:2:1 for H2O HDO and D2O. This because some of the water would "capture" deuterium from the D2O and the there would be an equilibrium with twice as much DHO as either H2O or D2O. This exchange also occurs in normal water -- An exchange between D2O and DHO (or H2O and DHO) gives a net result of zero, but an exchange between H2O and D2O results in HDO, and this is why the amount of D2O is a somewhat less than the 1 in 6420-squared expected by probability.
Very pure, carefully distilled VSMOW water is important in the manufacture of high-accuracy temperature measurement reference standards. Both the Kelvin and Celsius scales are defined by the triple point of water (273.16 K and 0.01 °C). Due to differences in isotopic composition, water samples from various sources may exhibit slight differences in physical properties, such as density, boiling point, and vapor pressure. Consequently, snow, river water, and rainwater, all of which are recently evaporated ocean water, tend to be enriched in the lighter isotopes of hydrogen and oxygen, causing water to evaporate more quickly.
Temperature reference cells filled with water of improper isotopic composition can cause errors of several hundred microkelvin in the measured triple point.
To address this issue, the Comité International des Poids et Mesures (CPIM), also known as the International Committee for Weights and Measures, affirmed in 2005 that for the purposes of specifying the temperature of the triple point of water, the definition of the Kelvin thermodynamic temperature scale would refer to water with a composition of the nominal specification of VSMOW.[notes 1] The decision was welcomed in 2007 by Resolution 10 of the 23rd CGPM.
One effect of defining the triple point of VSMOW as both 0.01 °C and 273.16 K is that neither the melting or boiling point of water under one standard atmosphere (101.325 kPa) remain 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 0.009911(10) °C apart. Thus, the actual melting point of ice is +0.000089(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 highly sensitive to variations in barometric pressure. For example, an altitude change of only 28 cm (11 in) causes water's boiling point to change by one millikelvin.