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Semiheavy water
Semiheavy water is the result of replacing one of the protium (normal hydrogen, 1H) in normal water with deuterium (2H; or less correctly,[1] D).[2] It exists whenever there is water with 1H and 2H in the mix. This is because hydrogen atoms (1,2H) are rapidly exchanged between water molecules. Water with 50% 1H and 50% 2H, is about 50% H2HO and 25% each of H2O and 2H2O, in dynamic equilibrium.[3] In normal water, about 1 molecule in 3,200 is HDO (1H2HO) (one hydrogen in 6,400 is 2H). By comparison, heavy water D2O or 2H2O[4] occurs at a proportion of about 1 molecule in 41 million (i.e., 1 in 6,4002). 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 3.8°C compared to the 3.82°C of heavy water. ProductionOn 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 or electrolysis, or by various chemical exchange processes, all of which exploit a kinetic isotope effect. Partial enrichment also occurs in natural bodies of water under certain evaporation conditions.[5] (For more information about the distribution of deuterium in water, see Vienna Standard Mean Ocean Water and Hydrogen isotope biogeochemistry.) See alsoReferences
Further reading
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