VSMOW, or Vienna Standard Mean Ocean Water, is a standard defined in 1968 by the International Atomic Energy Agency. It provides a reference standard for reporting hydrogen and oxygen isotope ratios, mostly in water samples. 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.”

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. But 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 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.

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

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. The reason for this is water molecules that are comprised of different isotopes of hydrogen and/or oxygen evaporate at different temperatures and at different rates. Consequently, snow, river water, and rain water (all of which are recently evaporated ocean water) tend to be enriched in those 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.

The effects of defining the triple point of VSMOW water as both 0.01 °C and 273.16 K are that both the freezing and boiling points of water under one standard atmosphere (101.325 kPa) are no longer the defining points for the Celsius scale. The triple point of water was so close to being 0.01 K 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 actually very slightly (<0.001 K) greater than 0.01 K apart. Thus, the actual melting point of water is very slightly (less than a millikelvin) below 0 °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 strictly, by definition, 373.15 / 273.15ths hotter—in absolute terms—than is 0 °C. The practical effect of all this is precise measurements show that the boiling point of VSMOW water under one standard atmosphere of pressure is actually 373.1339 K (99.9839 °C) when adhering strictly to the two-point definition for calibration. 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 very slightly less, about 99.974 °C.

These differences between water’s classically known melting and boiling points of 0 °C and 100 °C respectively, have little practical meaning in real life. An altitude change of only 28 cm (11 inches) causes water’s boiling point to change by a millikelvin.

Properties of VSMOW

• Maximum density: 999.97495 kg/m³ at 3.984 °C
• Density of melting ice: 916.8 kg/m³
• Melting point: 0.00 °C
• Triple point: 0.01 °C (precisely by definition)
• Boiling point at 101.325 kPa: 99.974 °C (calibration per ITS-90)
• Molar mass: 18.015268 grams per mole

See also

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

External links

• International Atomic Energy Agency (IAEA)
• ITS-90 (by Swedish National Testing and Research Institute)
• ITS-90 (by Omega Engineering)
• Temperature seminars, products, and services
• Temperature Sensors (information repository)
• Water, scientific data (by London South Bank University)

Nuclear technology | Environmental isotopes | Chemical oceanography