Water is made of hydrogen and oxygen, but both of these elements have more than one stable naturally occurring isotope. The most abundant hydrogen isotope has an atomic mass number of 1, but the mass number of 2 (called deuterium and often represented by the symbol D) is present in small quantities. Tritium (mass number 3, often represented by the symbol T) is radioactive and is almost entirely absent in nature. The most abundant oxygen isotope has a mass number of 16, but the 18-O isotope is present at about 0.2% and there is also a tiny amount of 17-O.
The isotopic composition of water (like other chemical compounds) is not uniform. This is because some chemical reactions discriminate between isotopes and because of differences in volatility between compounds made with different isotopes. It is this last effect that is most noticeable for water. When water vaporizes, the vapor is slightly depleted in the heavier isotopes. The opposite occurs when water condenses from the atmosphere; the rain or snow has more of the heavy isotopes, leaving lighter water vapor in the atmosphere. These phenomena are important to those who study the climate, because isotopic compositions can be used to trace the global flows of water and other compounds.
Because of these effects, fresh waters on Earth vary relatively widely in their isotopic composition. In temperate climates, fresh water is about 4% depleted in deuterium compared to ocean water. In the polar regions, this depletion can reach 40%.
Despite this variation, it is important to have a "standard" water which is easily reproducible and against which other waters can be compared. Fortunately, ocean water can provide such a standard. The isotopic composition of deep offshore ocean water is remarkably uniform across the Earth. This has been used to create a standard called Vienna Standard Mean Ocean Water (VSMOW). VSMOW is the standard against which isotopic compositions of both hydrogen and oxygen are reported. The isotopic composition of VSMOW and related information are documented in this report.
"Standard" values for water properties, including formulations produced by IAPWS, are considered to be for VSMOW. For the most precise scientific work, it may be necessary to convert some values between VSMOW and the water actually used (for example, the difference in density due to isotopic composition between VSMOW and purified tap water can be a few parts per million). The isotopic composition is also important in converting between mass-based and molar units; the relative molar mass of VSMOW is 18.015268 g/mol.
Finally, we can mention "heavy water." Heavy water is normally defined as water where the hydrogen is 100% deuterium, but the oxygen has the isotopic composition of VSMOW. The relative molar mass of heavy water by this definition is 20.027508 g/mol.
This page updated July 19, 2005