Vienna Standard Mean Ocean Water

VSMOW, or Vienna Standard Mean Ocean Water, is an isotopic water standard defined in 1968 by the International Atomic Energy Agency. Despite the somewhat misleading phrase "ocean water", VSMOW refers to pure water (H₂O) and does not include any salt or other substances in seawater. 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 the water content of Earth.

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.

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Composition of VSMOW

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)

VSMOW in temperature measurement

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 respectively). The trouble is that for high accuracy measurements, not all water is the same so VSMOW water is used as the “standard” water. This is because water molecules are composed of different isotopes of hydrogen and/or oxygen which evaporate at different temperatures and at different rates. Consequently, snow, river water, and rainwater (all of which are recently evaporated ocean water) tend to be enriched in the lighter isotopes that evaporate faster. Triple point-based temperature reference cells filled with water of improper isotopic composition can cause errors of several hundred µK in the measured triple point.

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 precisely equal to the nominal specification of VSMOW water.^[1]

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 precisely 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 precisely 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 precisely (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.

Properties of VSMOW

• Liquid, maximum density: 999.97495 kg/m³ at 3.984 °C
• Density of melting ice: 916.8 kg/m³
• Melting point: 0.000 089(10) °C
• Triple point: 0.01 °C (precisely by definition) at 611.657 Pa
• Boiling point at 101.325 kPa: 99.9839 °C, (99.974 °C with calibration per ITS-90)
• Molar mass: 18.015268 grams per mole

Notes

1. ^ Download the full work product of the 94th meeting of the CIPM here (669 kB PDF). 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.

See also

• ITS-90
• Isotopes
• Thermodynamic (absolute) temperature
• Triple point
• Water