Semiheavy water is the result of replacing one of the protium in normal water with deuterium.[1] It exists whenever there is water with light hydrogen (protium, 1H) and deuterium (D or 2H) in the mix. This is because hydrogen atoms (1H and 2H) are rapidly exchanged between water molecules. Water containing 50% 1H and 50% 2H, is about 50% H2HO and 25% each of H2O and 2H2O, in dynamic equilibrium.[2]
In normal water, about 1 molecule in 3,200 is HDO (one hydrogen in 6,400 is 2H). By comparison, heavy water D2O[3] occurs at a proportion of about 1 molecule in 41 million (i.e., one 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.
Production
On Earth, semiheavy water occurs naturally in normal water at a proportion of about 1 molecule in 3,200. This means that 1 in 6,400 hydrogen atoms in water is deuterium, which is 1 part in 3,200 by weight (hydrogen weight). The HDO may be separated from normal water by distillation or electrolysis and also by various chemical exchange processes, all of which exploit a kinetic isotope effect, with the partial enrichment also occurring in natural bodies of water under particular evaporation conditions.[4] (For more information about the isotopic distribution of deuterium in water, see Vienna Standard Mean Ocean Water.)
^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.
