What is Deuterium Oxide / Heavy Water?

A diagram outlining the elements of deuterium oxide

Deuterium oxide, also known as “heavy water” or “deuterium water”, is the compound of oxygen and the heavy isotope of hydrogen, namely deuterium. It is called heavy water because its density is greater than H₂O and its chemical formula is D₂O.  Deuterium contains a neutron and proton in its nucleus, which makes it twice as heavy as protium (hydrogen), which contains only one proton.  Deuterium oxide is colorless and odorless liquid in normal temperature and pressure.  Compared to ordinary water, its chemical characteristic is relatively inactive with specific gravity of 1.10775 (25 ℃), melting/freezing point of 3.82 ℃, and boiling point of 101.42 ℃. The hydrogen bond strength and degree of association between heavy water molecules are both stronger than that of ordinary water molecules.  The amount of D₂O produced by 1991 was about 30,000 tons1. On Earth concentration of D₂O in H₂O is 150-200 ppm. It has been hypothesized that D₂O is much more abundant in the ice of the Martian polar caps2.

Deuterium Oxide Origins

Most of the deuterium (heavy hydrogen) that can be found on earth is believed to be formed about 10 minutes after the Big Bang, along with other very light isotopes presently found in the universe. More recently, 2.5 billion years ago, most of the deuterium atoms on the earth were incorporated into water molecules. The small fraction of natural hydrogen the Deuterium isotope made-up (only 0.015% of all hydrogen isotopes), was now found most commonly in the form of HDO molecules. Since then, deuterium has continued to be most commonly found in this form, and eventually was discovered by scientists as heavy water in 1931.

American chemist Harold C. Urey working with his associates Ferdinand G. Brickwedde and George M. Murphy discovered Deuterium in 1931. For this discovery he was awarded the Nobel Prize for Chemistry in 1934. Since the initial discovery of deuterium, many variants and formats of the substance have been created and discovered, such as deuterium oxide.

Isowater’s® Founder and CEO Andrew T.B. Stuart’s grandfather Alexander T. Stuart implemented a water electrolysis facility in San Carlos California, which later became a deuterium enrichment site for the US Government in the 1930’s.

Pure heavy water, D₂O, is the oxide of the heavy stable isotope of hydrogen, deuterium, denoted by the symbols 2H or D. Physically and chemically it is almost identical to ordinary “light” water, H₂O, however, its density is 10% higher. It is this higher density which gives the compound its nickname, “heavy water.”

Deuterium Oxide Uses

Biomedical Applications of Heavy Water (D₂O)

D2O was one of the first isotope tracers to be used in metabolic research soon after its discovery by Harold Urey in 1932, the seminal works of Schoenheimer, Rittenberg, and Ussing demonstrated incorporation of deuterium from D2O into many metabolic pools4.  Once introduced into cellular pools, D2O equilibrates throughout all body water and is incorporated into metabolites via condensation/hydrolysis reactions involving water; crucially, this occurs in a constant and predictable manner5.  Commonly, 0.1 mL per kilogram body water is swallowed, that is 5–7 mL for an adult human. This increases the D₂O content in the blood from 150 to about 300 ppm, which subsequently decreases to the normal level with a half-life of a few days. No adverse effects have been reported from many such tests6,7. Using appropriate D₂O dosages, permits the measurement of a huge range of metabolic processes, from the synthesis of deuterated precursors and their subsequent incorporation into polymers can be made, for example, deuterated alanine into protein, glucose into glycogen, fatty acids into triglycerides, and ribose moieties into nucleic acids4. To reach a level of 10% in body water, which might or might not be toxic, a 70-kg man (with about 50 L body water) would have to drink rapidly 5 L of pure D2O.  This seems unlikely to occur either by intent or by accident. D₂O concentrations as high as 23% in human fluids were found not to be toxic over short time periods8. Higher doses and prolonged exposure are toxic for eukaryotes due to the inhibition of enzyme activity as bond strength between deuterium and carbon is 10 times stronger compared to that of hydrogen9.  D₂O is much less toxic to prokaryotes than to eukaryotes. After a period of adaptation, a number of bacteria and algae can grow in pure D₂O, although usually more slowly than in H2O10. Deuterium oxide is also used in pharmacology where H/D substitution increases the half life of the pharmaceutical agent often favorably affecting the pharma-kinetics of the drug11,12. The deuterated forms of drugs often have different actions than the protonated forms. Some deuterated drugs show different transport processes.  Deuteration may also change the pathway of drug metabolism (metabolic switching). Changed metabolism may lead to increased duration of action and lower toxicity11,12.

Electronics industries application of D2O.

Optical Light Emitting Diode (OLEDs)

The hydrogen/deuterium primary kinetic isotope effect provides useful information about the degradation mechanism of OLED materials. Thus, replacement of labile C–H bonds in the OLED with C–D bonds increases the device lifetime by a factor of five without loss of efficiency13.

Optical Fibers

In optical fibers deuterium extracted from D2O and deposited to Si reduces the absorption losses by shifting them to the 1620 nm wavelength, which outside of normal operating range14,15, thus enhancing the optic fiber service life and efficiency several fold16.

Other applications.

Deuterium oxide is routinely used in the process of heavy water electrolysis for production of deuterium gas that is essential for semiconductor industries. For example, replacing hydrogen with deuterium greatly reduces hot electron degradation effects in metal oxide semiconductor transistors due to isotope kinetic effect. Transistor lifetime improvements by factors of 10-50 were reported17.  Deuterium oxide is also used as non radioactive tracer in hydrology, ecology, entomology, mining industry and other instances when tracing studies are essential but use of radioactive isotopes is not applicable18–20.


In contemporary research D2O provides opportunities to create a more holistic picture of in-vivo metabolic phenotypes, providing a unique platform for development in clinical applications, and the emerging field of personalized medicine9. D2O can maintain the stability of vaccines, including the polio vaccine, for long periods without refrigeration21.  In high technology and electronics industry deuterium oxide enhances the lifespan and performance of OLEDs and increases the service life and efficiency of optic fibers.


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  2. Krasnopolsky, V. On the Deuterium Abundance on Mars and Some Related Problems. Icarus (2000). doi:10.1006/icar.2000.6534
  3. Urey, H. C., Brickwedde, F. G. & Murphy, G. M. A hydrogen isotope of mass 2 and its concentration. Phys. Rev. (1932). doi:10.1103/PhysRev.40.1
  4. Wilkinson, D. J. Historical and contemporary stable isotope tracer approaches to studying mammalian protein metabolism. Mass Spectrometry Reviews (2018). doi:10.1002/mas.21507
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  6. Coward, W. A., Whitehead, R. G., Sawyer, M. B., Prentice, A. M. & Evans, J. NEW METHOD FOR MEASURING MILK INTAKES IN BREAST-FED BABIES. Lancet (1979). doi:10.1016/S0140-6736(79)90177-6
  7. Baum, D., Dobbing, J. & Coward, W. A. DEUTERIUM METHOD FOR MEASURING MILK INTAKE IN BABIES. The Lancet (1979). doi:10.1016/S0140-6736(79)90327-1
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  12. Sharma, R. et al. Deuterium isotope effects on drug pharmacokinetics. I. System-dependent effects of specific deuteration with aldehyde oxidase cleared drugs. Drug Metab. Dispos. (2012). doi:10.1124/dmd.111.042770
  13. Tsuji, H., Mitsui, C. & Nakamura, E. The hydrogen/deuterium isotope effect of the host material on the lifetime of organic light-emitting diodes. Chem. Commun. (2014). doi:10.1039/c4cc05108d
  14. Chang, K. H. Alkali impurities and the long-wavelength hydrogen-induced aging loss in Ge-doped silica fibers. in Conference on Optical Fiber Communication, Technical Digest Series (2005). doi:10.1109/ofc.2005.192994
  15. Chang, K. H. Alkali impurities and the long-wavelength hydrogen-induced aging loss in ge-doped silica fibers. in Optics InfoBase Conference Papers (2005).
  16. Koike, Y. & Koike, K. Progress in low-loss and high-bandwidth plastic optical fibers. Journal of Polymer Science, Part B: Polymer Physics (2011). doi:10.1002/polb.22170
  17. Lyding, J. W., Hess, K. & Kizilyalli, I. C. Reduction of hot electron degradation in metal oxide semiconductor transistors by deuterium processing. Appl. Phys. Lett. (1996). doi:10.1063/1.116172
  18. Calder, I. R., Narayanswamy, M. N., Srinivasalu, N. V., Darling, W. G. & Lardner, A. J. Investigation into the use of deuterium as a tracer for measuring transpiration from eucalypts. J. Hydrol. (1986). doi:10.1016/0022-1694(86)90132-0
  19. Dugas, W. A., Wallace, J. S., Allen, S. J. & Roberts, J. M. Heat balance, porometer, and deuterium estimates of transpiration from potted trees. Agric. For. Meteorol. (1993). doi:10.1016/0168-1923(93)90093-W
  20. Whitfield, C. J., Aherne, J. & Baulch, H. M. Controls on greenhouse gas concentrations in polymictic headwater lakes in Ireland. Sci. Total Environ. (2011). doi:10.1016/j.scitotenv.2011.09.045
  21. Wu, R. et al. Thermostabilization of live virus vaccines by heavy water (D2O). Vaccine (1995). doi:10.1016/0264-410X(95)00068-C

What is deuterium?

The physical properties of water and deuterium oxide (heavy water) differ in several ways. For example, heavy water is less dissociated than light water at a given temperature. As well, the true concentration of D+ ions is less than H+ ions would be for a light water sample at the same temperature. The same is true when comparing OD− vs. OH− ions. For heavy water Kw D2O (25.0 °C) = 1.35 × 10−15, and [D+ ] must equal [OD− ] for neutral water. Thus pKw D2O = p[OD−] + p[D+] = 7.44 + 7.44 = 14.87 (25.0 °C), and the p[D+] of neutral heavy water at 25.0 °C is 7.44.

The pD of heavy water is generally measured using pH electrodes giving a pH (apparent) value, or pHa, and at various temperatures a true acidic pD can be estimated from the directly pH meter measured pHa, such that pD+ = pHa (apparent reading from pH meter) + 0.41. The electrode correction for alkaline conditions is 0.456 for heavy water. The alkaline correction is then pD+ = PHa(apparent reading from pH meter) + 0.456. These corrections are slightly different from the differences in p[D+] and p[OD-] of 0.44 from the corresponding ones in heavy water.

Heavy Water Deuterium Oxide

Heavy water is 10.6% denser than ordinary water, and heavy water’s physically different properties can be seen without equipment if a frozen sample is dropped into normal water, as it will sink. If the water is ice-cold the higher melting temperature of heavy ice can also be observed: it melts at 3.7 °C, and thus does not melt in ice-cold normal water.