VSMOW
Encyclopedia
Vienna Standard Mean Ocean Water (VSMOW) is a water standard defining the isotopic
composition of water. It was promulgated by the International Atomic Energy Agency
in 1968.
Despite the misleading designation ocean water, VSMOW does not include any salt or other substances usually found in seawater
and refers to pure water with a particular composition of isotopes. VSMOW serves as a reference standard for comparing hydrogen
and oxygen
isotope
ratios, mostly in water samples. Very pure, 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.
Previously to its definition, 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.
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:
measurement reference standards. Both the Kelvin
and Celsius
scales are defined by the triple point
of water (273.16 K and 0.01 °C respectively). 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 faster.
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.(669 kB PDF) CIPM 2005 report See pg. 235 of the document (Pg. 107 of the PDF) for Clarification of the definition of the kelvin, unit of thermodynamic temperature. The CIPM's adoption of the VSMOW standard was based upon a recommendation of the International Union of Pure and Applied Chemistry
(IUPAC) in their publication Atomic Weights of the Elements: Review 2000 (IUPAC Technical Report), J. R. de Laeter et al., Pure and Applied Chemistry, 75, Issue 6, Pg. 683–799.
One effect of defining the triple point of VSMOW as both 0.01 °C and 273.16 K is that neither the melting
and 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 °C apart. Thus, the actual melting point of ice is °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 in) causes water's boiling point to change by one millikelvin.
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
composition of water. It was promulgated by the International Atomic Energy Agency
International Atomic Energy Agency
The International Atomic Energy Agency is an international organization that seeks to promote the peaceful use of nuclear energy, and to inhibit its use for any military purpose, including nuclear weapons. The IAEA was established as an autonomous organization on 29 July 1957...
in 1968.
Despite the misleading designation ocean water, VSMOW does not include any salt or other substances usually found in seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...
and refers to pure water with a particular composition of isotopes. VSMOW serves as a reference standard for comparing hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
ratios, mostly in water samples. Very pure, 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.
Previously to its definition, 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
National Institute of Standards and Technology
The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...
, 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
Harmon Craig
Harmon Craig was an American geochemist.Craig studied geology and chemistry at the University of Chicago, where he earned a Ph.D. under Nobel Laureate Harold Urey with a thesis on carbon isotope geochemistry in 1951. He remained at the University of Chicago as a research associate at the Enrico...
and other researchers from Scripps Institution of Oceanography
Scripps Institution of Oceanography
Scripps Institution of Oceanography in La Jolla, California, is one of the oldest and largest centers for ocean and earth science research, graduate training, and public service in the world...
at the University of California, San Diego
University of California, San Diego
The University of California, San Diego, commonly known as UCSD or UC San Diego, is a public research university located in the La Jolla neighborhood of San Diego, California, United States...
who mixed distilled ocean waters collected from different spots around the globe. VSMOW remains one of the major isotopic water benchmarks in use today.
Composition
The isotopic composition of VSMOW water is specified as ratios of the molarMole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
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
Oxygen-17
Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...
(with an additional neutron). The isotopic ratios of VSMOW water are defined as follows:
- 2HDeuteriumDeuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
/1H = 155.76 ±0.1 ppm (a ratio of 1 part per approximately 6420 parts) - 3HTritiumTritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
/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) - 18OOxygen-18Oxygen-18 is a natural, stable isotope of oxygen and one of the environmental isotopes.18O is an important precursor for the production of fluorodeoxyglucose used in positron emission tomography...
/16O = 2,005.20 ±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 2,632 parts)
Use in temperature standards
Very pure, carefully distilled VSMOW water is important in the manufacture of high-accuracy temperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
measurement reference standards. Both the Kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
and Celsius
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
scales are defined by the triple point
Triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium...
of water (273.16 K and 0.01 °C respectively). 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 faster.
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
International Committee for Weights and Measures
The Interglobal Committee for Weights and Measures is the English name of the Comité international des poids et mesures . It consists of eighteen persons from Member States of the Metre Convention...
, 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.(669 kB PDF) CIPM 2005 report See pg. 235 of the document (Pg. 107 of the PDF) for Clarification of the definition of the kelvin, unit of thermodynamic temperature. The CIPM's adoption of the VSMOW standard was based upon a recommendation of the International Union of Pure and Applied Chemistry
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...
(IUPAC) in their publication Atomic Weights of the Elements: Review 2000 (IUPAC Technical Report), J. R. de Laeter et al., Pure and Applied Chemistry, 75, Issue 6, Pg. 683–799.
One effect of defining the triple point of VSMOW as both 0.01 °C and 273.16 K is that neither the melting
Water (properties)
Water is the most abundant compound on Earth's surface, covering about 70%. In nature, it exists in liquid, solid, and gaseous states. It is in dynamic equilibrium between the liquid and gas states at standard temperature and pressure. At room temperature, it is a tasteless and odorless liquid,...
and boiling
Water (properties)
Water is the most abundant compound on Earth's surface, covering about 70%. In nature, it exists in liquid, solid, and gaseous states. It is in dynamic equilibrium between the liquid and gas states at standard temperature and pressure. At room temperature, it is a tasteless and odorless liquid,...
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 °C apart. Thus, the actual melting point of ice is °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
Thermodynamic temperature
Thermodynamic temperature is the absolute measure of temperature and is one of the principal parameters of thermodynamics. Thermodynamic temperature is an "absolute" scale because it is the measure of the fundamental property underlying temperature: its null or zero point, absolute zero, is the...
(referencing absolute zero
Absolute zero
Absolute zero is the theoretical temperature at which entropy reaches its minimum value. The laws of thermodynamics state that absolute zero cannot be reached using only thermodynamic means....
). 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-90International Temperature Scale of 1990
The International Temperature Scale of 1990 is an equipment calibration standard for making measurements on the Kelvin and Celsius temperature scales. ITS–90 is an approximation of the thermodynamic temperature scale that facilitates the comparability and compatibility of temperature measurements...
(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 in) causes water's boiling point to change by one millikelvin.