Vienna Standard Mean Ocean Water (VSMOW) is an
isotopic standard for water. Despite the name, VSMOW is pure water with no salt or other chemicals found in the oceans. The VSMOW standard was promulgated by the
International Atomic Energy Agency
The International Atomic Energy Agency (IAEA) is an intergovernmental organization that seeks to promote the peaceful use of nuclear energy and to inhibit its use for any military purpose, including nuclear weapons. It was established in 1 ...
(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 The Institute for Reference Materials and Measurements (IRMM), located in Geel, Belgium, is one of the seven institutes of the Joint Research Centre (JRC), a Directorate-General of the European Commission (EC).
The IRMM promotes a common and relia ...
and the American
National Institute of Standards and Technology
The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into Outline of p ...
. 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 to water molecules collected directly from the ocean, rather than from other points in the
water cycle
The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly cons ...
(e.g., rain, snow, river or lake water). Water from different points in the water cycle contains molecules with differing ratios of isotopes due to the slight differences the isotopes cause in the rates of evaporation and condensation. VSMOW is not a standard for
seawater
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...
, which contains other molecules in addition to water molecules. Fresh distilled VSMOW is used to make high accuracy measurements of water's physical properties and to produce laboratory standards.
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
The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into Outline of p ...
, 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
The Scripps Institution of Oceanography (sometimes referred to as SIO, Scripps Oceanography, or Scripps) in San Diego, California, US founded in 1903, is one of the oldest and largest centers for ocean and Earth science research, public servi ...
at the
University of California, San Diego
The University of California, San Diego (UC San Diego or colloquially, UCSD) is a public university, public Land-grant university, land-grant research university in San Diego, California. Established in 1960 near the pre-existing Scripps Insti ...
who mixed
distilled ocean waters collected from different spots around the globe. VSMOW remains one of the major
isotopic water benchmarks in use today, used for both hydrogen and oxygen isotopes.
Composition
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:
*
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 × 10
16 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)
Although each water molecule () contains two hydrogen atoms and one oxygen atom, VSMOW composition is given in terms of the atoms and is indifferent to the isotopic combinations in molecules.
Use in temperature standards
The
International Committee for Weights and Measures
The General Conference on Weights and Measures (GCWM; french: Conférence générale des poids et mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established ...
specified in 2005 that the definition of the
kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ph ...
temperature scale would refer to water with a composition of the nominal specification of VSMOW. The decision was welcomed in 2007 by Resolution 10 of the 23rd CGPM.
This effectively defined the
triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases ( gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at which the subli ...
of VSMOW to be exactly 273.16 K and 0.01 °C.
As of the
2019 redefinition of the SI base units
In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artifacts such as the standard kilogram.
Effective 20 May 2019, the 144th ...
, the kelvin is defined in terms of the
Boltzmann constant
The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas consta ...
, which makes the definition completely independent of the properties of water. The triple point now has a measured value, not a defined value. The defined value for the Boltzmann constant was selected so that the measured value of the triple point is identical to the prior defined value to within the limits of accuracy of contemporary
metrology
Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in Fra ...
.
Very pure, carefully distilled VSMOW remains important in the manufacture of high-accuracy
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
measurement reference standards. Due to differences in isotopic composition, water samples from various sources may exhibit slight differences in physical properties, including triple point, density, boiling point, and vapor pressure. Snow, river water, and rainwater, all of which are recently evaporated ocean water, tend to be depleted in the heavier isotopes of hydrogen and oxygen, which affects the triple point. Temperature reference cells filled with water of improper isotopic composition can cause errors of several hundred
microkelvin in the measured triple point.
See also
*
Properties of water
Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "uni ...
References
{{reflist
External links
International Atomic Energy Agency - IAEAScripps Institution of Oceanography
Vsmow
Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water. Despite the name, VSMOW is pure water with no salt or other chemicals found in the oceans. The VSMOW standard was promulgated by the International Atomic Energy Agency ( ...
Vsmow
Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water. Despite the name, VSMOW is pure water with no salt or other chemicals found in the oceans. The VSMOW standard was promulgated by the International Atomic Energy Agency ( ...
Vsmow
Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water. Despite the name, VSMOW is pure water with no salt or other chemicals found in the oceans. The VSMOW standard was promulgated by the International Atomic Energy Agency ( ...
Standards