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Semiheavy water is the result of replacing one of the protium (normal hydrogen, H) in normal water with deuterium (H; or less correctly, 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 H2O and H2O, in dynamic equilibrium.
In normal water, about 1 molecule in 3,200 is HDO (HHO) (one hydrogen in 6,400 is H). By comparison, heavy water D2O or H2O 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 3.8°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 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. (For more information about the distribution of deuterium in water, see Vienna Standard Mean Ocean Water and Hydrogen isotope biogeochemistry.)
"Provisional Recommendations". Nomenclature of Inorganic Chemistry. Chemical Nomenclature and Structure Representation Division. IUPAC. § IR-3.3.2. Archived from the original on 27 October 2006. Retrieved 2024-12-30.
Craig, H.; Gordon, L. I.; Horibe, Y. (1963). "Isotopic exchange effects in the evaporation of water: 1. Low-temperature experimental results". Journal of Geophysical Research. 68 (17): 5079–5087. Bibcode:1963JGR....68.5079C. doi:10.1029/JZ068i017p05079.
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