Semiheavy water is the result of replacing one of the

protium Hydrogen (H) has three naturally occurring isotopes: H, H, and H. H and H are stable, while H has a half-life of years. Heavier isotopes also exist; all are synthetic and have a half-life of less than 1 zeptosecond (10 s). Of these, H is ...

(normal hydrogen, H) in normal water with

deuterium Deuterium (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more c ...

(H; or D). It exists whenever there is water with H and H in the mix. This is because hydrogen atoms (H) are rapidly exchanged between water molecules. Water with 50% H and 50% H, is about 50% HHO and 25% each of HO and HO, in

dynamic equilibrium In chemistry, a dynamic equilibrium exists once a reversible reaction occurs. Substances initially transition between the reactants and products at different rates until the forward and backward reaction rates eventually equalize, meaning the ...

. In normal water, about 1 molecule in 3,200 is HDO (HHO) (one hydrogen in 6,400 is H). By comparison,

heavy water Heavy water (deuterium oxide, , ) is a form of water (molecule), water in which hydrogen atoms are all deuterium ( or D, also known as ''heavy hydrogen'') rather than the common hydrogen-1 isotope (, also called ''protium'') that makes up most o ...

DO or HO occurs at a proportion of about 1 molecule in 41 million (i.e., 1 in 6,400). This makes semiheavy water far more common than "normal" heavy water. The freezing point of semiheavy water is close to the freezing point of heavy water at 3.81°C compared to the 3.82°C of heavy water.

Production

On Earth, semiheavy water occurs naturally in normal water at a proportion of about 1 molecule in 3,200; because 1 in 6,400 hydrogen atoms in water is deuterium, which is 1 part in 3,200 by weight. HDO may be separated from normal water by

distillation Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...

electrolysis In chemistry and manufacturing, electrolysis is a technique that uses Direct current, direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of c ...

, or by various chemical exchange processes, all of which exploit a

kinetic isotope effect In physical organic chemistry, a kinetic isotope effect (KIE) is the change in the reaction rate of a chemical reaction when one of the atoms in the reactants is replaced by one of its isotopes. Formally, it is the ratio of rate constants for t ...

. Partial enrichment also occurs in natural bodies of water under certain evaporation conditions. (For more information about the distribution of deuterium in water, see

Vienna Standard Mean Ocean Water Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water, that is, a particular sample of water whose proportions of different isotopes of hydrogen and oxygen are accurately known. VSMOW is distilled from ocean water and does no ...

and

Hydrogen isotope biogeochemistry Hydrogen isotope biogeochemistry (HIBGC) is the scientific study of biological, geological, and chemical processes in the environment using the distribution and relative abundance of hydrogen isotopes. Hydrogen has two stable isotopes, protium H an ...

Production

See also

References

Further reading