Vienna Standard Mean Ocean Water

The Vienna Standard Mean Ocean Water , abbreviated VSMOW , in German about Viennese mean standard ocean water , was defined in 1968 by the International Atomic Energy Agency (IAEA, based in Vienna) as the standard for (ultrapure) water samples. It results from reference values for hydrogen - and oxygen - isotope ratios , which is on the composition of ocean orient water. Very clean (distilled) VSMOW water is therefore used to determine the physical properties of water with the highest requirements and serves as a globally recognized and widely used isotope or laboratory standard.

Previously, ocean water and melted snow had been used as reference points . These values ​​were refined in the 1960s by the definition of standard ocean water ( SMOW or Standard Mean Ocean Water ). The then US National Bureau of Standards (now the National Institute of Standards and Technology ) created, just like today z. B. the German Federal Institute for Materials Research and Testing , already water standards for international use, but doubts quickly arose regarding compliance with their own specifications. VSMOW is a further tuning of the original SMOW definition and was founded in 1967 by researchers at the Scripps Institution of Oceanography created that distillates mixed of ocean waters of different places in the world.

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, melt, river water, etc.), certain isotopes are enriched and depleted through continuous evaporation processes . The reason for this is that the water molecules , which are composed of different hydrogen and oxygen isotopes, have different weights and therefore evaporate to different degrees. For isotope-pure water, different boiling points could also be determined, which deviated from each other by several 100 µK.

This z. B. Rainwater (and thus also glacier ice and most of the groundwater ) somewhat poorer in heavier isotopes. The tritium content in hydrogen, including water, is created by cosmic radiation and then decays with a half-life of around 12 years. Therefore, this is relatively large in rainwater, while it can decrease to 0 in groundwater supplies. For most measurements on VSMOW, however, it does not play a role (see composition below).

involved isotopes	Molar ratio		Explanation
	molar (in ppm )	(as a fraction)
${\displaystyle {\frac {n({}^{2}\mathrm {H} )}{n({}^{1}\mathrm {H} )}}}$	155.76 ± 0.1	${\displaystyle \approx {\frac {1}{6420}}}$	2 H is deuterium .
${\displaystyle {\frac {n({}^{3}\mathrm {H} )}{n({}^{1}\mathrm {H} )}}}$	(1.85 ± 0.36) · 10 −11	${\displaystyle \approx {\frac {1}{5{,}41\cdot 10^{16}}}}$	3 H is tritium , can mostly be neglected.
${\displaystyle {\frac {n({}^{17}\mathrm {O} )}{n({}^{16}\mathrm {O} )}}}$	379.9 ± 1.6	${\displaystyle \approx {\frac {1}{2632}}}$
${\displaystyle {\frac {n({}^{18}\mathrm {O} )}{n({}^{16}\mathrm {O} )}}}$	2005.20 ± 0.43	${\displaystyle \approx {\frac {1}{498{,}7}}}$	18 O / 16 O is often expressed in the form of the δ 18 O value.

composition

The composition of VSMOW is determined by the molar proportions of the respective isotopes of the two elements hydrogen and oxygen, whereby this information always relates to the most common isotope of the respective element (i.e. to ¹ H and to ¹⁶ O).

The penultimate line says e.g. B. that ¹⁶ O atoms (the most common oxygen isotope with eight protons and eight neutrons ) are about 2632 times more often present in ocean water than ¹⁷ O atoms (with nine neutrons).

properties

• Highest density : 999.97495 kg / m ³ (at 277.134 K or 3.984 ° C) ( anomaly of the water )
• Melting point : 0.002519 ° C or 273.152519 K
• Density of ice at the melting point: 916.8 kg / m ³
• Boiling point at 101.325 kPa (see normal pressure ): 373.1339 K or 99.9839 ° C (99.974 ° C when using the ITS-90 )
• Triple point : 0.01 ° C (by definition of degree Celsius ) or 273.16 K (by definition of Kelvin ) at 611.657  Pa
• Molar mass : 18.015268 g / mol

See also

• Absolute temperature

Web links

• International Atomic Energy Agency - IAEA - engl
• ITS-90 - Swedish National Testing and Research Institute - engl
• ITS-90 - Omega Engineering - engl
• Scripps Institution of Oceanography - engl
• Temperature Sensors - information repository - engl
• Scientific data of water - London South Bank University - engl
• Origin of SMOW - Isogeochem Archives - engl