Vienna Standard Mean Ocean Water

The Vienna Standard Mean Ocean Water abbreviated, VSMOW, was founded in 1968 by the International Atomic Energy Agency (IAEA ) is defined as the default for ( ultrapure ) water samples, the results from reference values ​​for hydrogen and oxygen isotope ratios that appeal to the composition of ocean water orient. Very clean (distilled ) VSMOW water is therefore used for determinations of the physical properties of water with the highest standard and is used as a laboratory standard.

Previously, ocean water and melted snow were used as reference points. These values ​​were refined in the 1960s by the definition of standard ocean water ( SMOW or Standard Mean Ocean Water) on. The then U.S. National Bureau of Standards (now National Institute of Standards and Technology ) created, as well as today, for example, the German Federal Institute for Materials Research and Testing, although already water standards for international use, but doubts quickly came on compliance with its own guidelines on.

VSMOW is further fine-tuning of the original SMOW definition and was created in 1967 by researchers at the Institution of Oceanography, mixed the spirits of ocean waters of different places in the world. Meanwhile VSMOW is a world renowned and much -used standard for water isotopes.

Motivation

The need for a water standard arises from the fact that distilled water is not the same everywhere. Despite the constant mixing processes on Earth (rain, ocean, melting, river water, etc. ) are turned on and depleted by continuous evaporation processes certain isotopes. The reason for this is that composed of different hydrogen and oxygen isotope water molecules have different weights and therefore evaporate different light. For isotopically pure water is also different boiling points could be determined that differed from each other by several 100 μK. This make for example rainwater (and thus glacial ice and most of the groundwater) somewhat poorer in heavier isotopes. The tritium content in hydrogen, ie also in the water, however, is caused by cosmic rays and then decays with a half -life of approximately 12 years. Therefore, this is relatively large in rain water, while he may fall in groundwater resources against 0. For most measurements of VSMOW it does not matter.

Composition of VSMOW

The composition of VSMOW is determined by the molar proportions of the respective isotopes of individual elements (hydrogen and oxygen), where the data of the isotope ratios always refer to the most common isotope of each element. The composition of VSMOW was defined as:

2H / 1H = 155.76 ± 0.1 ppm (equivalent to about a fraction of 1/6420 )

3H / 1 H = (1.85 ± 0.36 ) × 10-11 ppm (equivalent to approximately a ratio of 1 /5, 41 × 1016, he can usually be neglected)

18O / 16O = 2005.20 ± 0.43 ppm (equivalent to about a fraction of 1/ 498, 7)

17O / 16O = 379.9 ± 1.6 ppm (equivalent to approximately a share of 1/2632 )

For example, 16O ( the most common isotope of oxygen with eight protons and eight neutrons ) is about 2632 times more often in ocean water present as 17O ( with nine neutrons).

Properties of VSMOW

  • Highest density: 999.97495 kg/m3 at 277.134 K and 3,984 ° C
  • Density of ice at the melting point: 916.8 kg/m3
  • Melting point: 0.002519 273.152519 ° C or K
  • Triple point: 0.01 ° C ( by definition of degrees Celsius ) or 273.16 K ( by definition of the kelvin ) at 611.657 Pa
  • Boiling point at 101.325 kPa: 373.1339 K and 99.9839 ° C ( 99.974 ° C when using the ITS-90)
  • Molar mass: 18.015268 g / mol
Chemical element Thermodynamic temperature
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