Related on Lably
In an amount of water, about 1 molecule in 3,200 is HDO (one hydrogen in 6,400 is 2H). By comparison, heavy water (D2O) 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 heavy water.
Semiheavy water (HDO) cannot be isolated in its pure liquid form since, owing to hydrogen exchange in water, it is in equilibrium with H2O and D2O. If however it were possible to weigh the same volume of HDO and H2O, the HDO would be heavier, lending it its name.
Production
Water may be enriched in HDO 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.)