Vienna Standard Mean Ocean Water

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Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water, that is, a particular sample of water whose proportions of different isotopes of hydrogen and oxygen are accurately known. VSMOW is distilled from ocean water and does not contain salt or other impurities. Published and distributed by the Vienna-based International Atomic Energy Agency in 1968, the standard and its essentially identical successor, VSMOW2, continue to be used as a reference material.

Water samples made up of different isotopes of hydrogen and oxygen have slightly different physical properties. As an extreme example, heavy water, which contains two deuterium (2H) atoms instead of the usual, lighter hydrogen-1 (1H), has a melting point of Template:Convert and boiling point of Template:Convert.[1] Different rates of evaporation cause water samples from different places in the water cycle to contain slightly different ratios of isotopes. Ocean water (richer in heavy isotopes) and rain water (poorer in heavy isotopes) roughly represent the two extremes found on Earth. With VSMOW, the IAEA simultaneously published an analogous standard for rain water, Standard Light Antarctic Precipitation (SLAP), and eventually its successor SLAP2. SLAP contains about 5% less oxygen-18 and 42.8% less deuterium than VSMOW.

A scale based on VSMOW and SLAP is used to report oxygen-18 and deuterium concentrations. From 2005 until its redefinition in 2019, the kelvin was specified to be Template:Math of the temperature of specifically VSMOW at its triple point.

History and background

The original container of VSMOW (then called SMOW-1) collected by Harmon Craig

Abundances of a particular isotope in a substance are usually given relative to some reference material, as a delta in parts per thousand (‰) from the reference. For example, the ratio of deuterium (Template:SupH) to hydrogen-1 in a substance x may be given as

δ2Hx/reference (in ‰)=((2H/1H)x(2H/1H)reference−1)⋅1000,

where (2H/1H)x denotes the absolute concentration in x.Template:Sfn

In 1961, pursuing a standard for measuring and reporting deuterium and oxygen-18 concentrations, Harmon Craig of the Scripps Institution of Oceanography in San Diego, California, proposed an abstract water standard. He based the proportions on his measurements of samples taken by Template:Harvtxt of ocean waters around the world.Template:Sfn Approximating an average of their measurements, Craig defined his "standard mean ocean water" (SMOW) relative to a water sample held in the United States' National Bureau of Standards called NBS-1 (sampled from the Potomac RiverTemplate:Sfn). In particular, SMOW had the following parameters relative to NBS-1:

  • δ 2H SMOW/NBS-1 = 50‰, i.e., an enrichment of 5%;
  • δ 18O SMOW/NBS-1 = 8‰, i.e., an enrichment of 0.8%.Template:Sfn

Later, researchers at the California Institute of Technology defined another abstract reference, also called "SMOW", for oxygen-18 concentrations, such that a sample of Potsdam Sandstone in their possession satisfied Template:Nowrap.Template:Sfn

To resolve the confusion, November 1966 meeting of the Vienna-based International Atomic Energy Agency (IAEA) recommended the preparation of two water isotopic standards: Vienna SMOW (VSMOW; initially just "SMOW" but later disambiguatedTemplate:Sfn) and Standard Light Antarctic Precipitation (SLAP).Template:Sfn Craig prepared VSMOW by mixing distilled Pacific Ocean water with small amounts of other waters. VSMOW was intended to match the SMOW standard as closely as possible. Craig's measurements found an identical 18O concentration and a 0.2‰ lower 2H concentration.Template:Sfn The SLAP standard was created from a melted firn sample from Plateau Station in Antarctica.Template:Sfn A standard with oxygen-18 and deuterium concentrations between that of VSMOW and SLAP, called Greenland Ice Sheet Precipitation (GISP), was also prepared.Template:Sfn The IAEA began distributing samples in 1968, and Template:Harvtxt compiled analyses of VSMOW and SLAP from 45 laboratories around the world.Template:Sfn The VSMOW sample was stored in a stainless-steel container under nitrogen and was transferred to glass ampoules in 1977.Template:Sfn

The deuterium and oxygen-18 concentrations in VSMOW are close to the upper end of naturally occurring materials, and the concentrations in SLAP are close to the lower end.Template:Sfn Due to confusion over multiple water standards, the Commission on Isotopic Abundances and Atomic Weights recommended in 1994 that all future isotopic measurements of oxygen-18 (18O) and deuterium (2H) be reported relative to VSMOW, on a scale such that the Template:Nowrap of SLAP is −55.5‰ and the Template:Nowrap of SLAP is −428‰, relative to VSMOW.Template:SfnTemplate:Sfn Therefore, SLAP is defined to contain 94.45% the oxygen-18 concentration and 57.2% the deuterium concentration of VSMOW.Template:Sfn Using a scale with two defined samples improves comparison of results between laboratories.

In December 1996, because of a dwindling supply of VSMOW, the IAEA decided to create a replacement standard, VSMOW2. Published in 1999, it contains a nearly identical isotopic mixture. About 300 liters was prepared from a mixture of distilled waters, from Lake Bracciano in Italy, the Sea of Galilee in Israel, and a well in Egypt, in proportions chosen to reach VSMOW isotopic ratios. The IAEA also published a successor to SLAP, called SLAP2, derived from melted water from four Antarctic drilling sites.Template:Sfn Deviations of 17O, and 18O in the new standards from the old standards are zero within the error of measurement.Template:Sfn There is a small but measurable deviation of 2H concentration in SLAP2 from SLAP—Template:Nowrap is defined to be −427.5‰ instead of −428‰—but not in VSMOW2 from VSMOW.Template:Sfn The IAEA recommends that measurements still be reported on the VSMOW–SLAP scale.Template:Sfn

The older two standards are now kept at the IAEA and no longer sold.Template:Sfn

Measurements

All measurements are reported with their standard uncertainty. Measurements of particular combinations of oxygen and hydrogen isotopes are unnecessary because water molecules constantly exchange atoms with each other.

VSMOW

Except for tritium, which was determined by the helium gas emitted by radioactive decay, these measurements were taken using mass spectroscopy.

SLAP

Based on the results of Template:Harvtxt, the IAEA defined the delta scale with SLAP at −55.5‰ for 18O and −428‰ for 2H. That is, SLAP was measured to contain approximately 5.55% less oxygen-18 and 42.8% less deuterium than does VSMOW, and these figures were used to anchor the scale at two points.Template:Sfn Experimental figures are given below.

VSMOW2 and SLAP2

The concentrations of 17O, and 18O are indistinguishable between VSMOW and VSMOW2, and between SLAP and SLAP2. The specification sheet gives the standard errors in these measurements.Template:Sfn The concentration of 2H is unchanged in VSMOW2 as well, but is slightly increased in SLAP2. The IAEA reports:

On 6 July 2007, the tritium concentration was Template:Val in VSMOW2, and Template:Val in SLAP2.Template:Sfn

GISP

  • δ 2H GISP = −189.5 ± 1.2‰
  • δ 18O GISP = −24.66 ± 0.09‰
  • δ 17O GISP = −12.71 ± 0.1‰Template:Sfn

Applications

Reporting isotopic ratios

The VSMOW–SLAP scale is recommended by the USGS, IUPAC, and IAEA for measurement of deuterium and 18O concentrations in any substance.Template:SfnTemplate:SfnTemplate:Sfn For 18O, a scale based on Vienna Pee Dee Belemnite can also be used.Template:Sfn The physical samples, which are distributed by the IAEA and U.S. National Institute of Standards and Technology, are used to calibrate isotope-measuring equipment.Template:Sfn

Variations in isotopic content are useful in hydrology, meteorology, and oceanography.Template:Sfn Different parts of the ocean do have slightly different isotopic concentrations: δ 18O values range from –11.35‰ in water off the coast of Greenland to +1.32‰ in the north Atlantic, and δ 2H concentrations in deep ocean water range from roughly –1.7‰ near Antarctica to +2.2‰ in the Arctic. Variations are much larger in surface water than in deep water.Template:Sfn

Temperature measurements

In 1954, the International Committee for Weights and Measures (CIPM) established the definition of the Kelvin as 1/273.16 of the absolute temperature of the triple point of water. Waters with different isotopic compositions had slightly different triple points. Thus, the International Committee for Weights and Measures specified in 2005[2] that the definition of the kelvin temperature scale would refer to water with a composition of the nominal specification of VSMOW.[3] The decision was welcomed in 2007 by Resolution 10 of the 23rd CGPM.[4] The triple point is measured in triple-point cells, where the water is held at its triple point and allowed to reach equilibrium with its surroundings. Using ordinary waters, the range of inter-laboratory measurements of the triple point can be about Template:Nowrap.[5] With VSMOW, the inter-laboratory range of measurements of the triple point is about Template:Nowrap.Template:Sfn

After the 2019 revision of the SI, the kelvin is defined in terms of the Boltzmann constant, which makes its definition completely independent of the properties of water. The defined value for the Boltzmann constant was selected so that the measured value of the VSMOW triple point is identical to the prior defined value, within measurable accuracy.[6] Triple-point cells remain a practical method of calibrating thermometers.Template:Sfn

See also

Notes

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Sources

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Citations

Template:Reflist

  1. ↑ Template:Cite journal
  2. ↑ Template:Cite web
  3. ↑ Template:Cite web The CIPM's adoption of the VSMOW standard was based upon a recommendation in Template:Harvtxt.
  4. ↑ Template:Cite web
  5. ↑ Template:Cite book
  6. ↑ Template:Cite journal
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