Semiheavy water

 ( Forms Of Water ) 

Semiheavy water is the result of replacing one of the protium atoms (normal hydrogen, H) in normal water with deuterium (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% HDO (HHO) and 25% each of HO and DO (HO), in dynamic equilibrium. In normal water, about 1 molecule in 3,200 is HDO (one hydrogen in 6,400 is H). By comparison, heavy water DO 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.

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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.)

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See also

• Deuterium-depleted water

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Further reading