Heavy water (deuterium oxide, , ) is a form of water in which all hydrogen atoms are hydrogen-2 isotope or deuterium ( or D, also known as heavy hydrogen) rather than the common hydrogen-1 isotope (, also called protium) that makes up most of the hydrogen in normal water. The presence of the heavier isotope gives the water different nuclear properties, and the increase in mass gives it slightly different physical and chemical properties when compared with normal water.
Deuterium is a heavy hydrogen isotope. Heavy water contains deuterium atoms and is used in nuclear reactors. Semiheavy water (HDO) is more common than pure heavy water, while heavy-oxygen water is denser but lacks unique properties. Tritiated water is radioactive because of tritium content.
Heavy water has different physical properties from regular water, such as being 10.6% denser and having a higher melting point. Heavy water is less dissociated at a given temperature, and it does not have the slightly blue color of regular water. It can taste slightly sweeter than regular water, though not to a significant degree. Heavy water affects biological systems by altering enzymes, hydrogen bonds, and cell division in eukaryotes. It can be lethal to multicellular organisms at concentrations over 50%. However, some prokaryotes like bacteria can survive in a heavy hydrogen environment. Heavy water can be toxic to humans, but a large amount would be needed for poisoning to occur – which begin to appear only when deuterium content of body fluids exceeds 20%.
The most cost-effective process for producing heavy water is the Girdler sulfide process. Heavy water is used in various industries and is sold in different grades of purity. Some of its applications include nuclear magnetic resonance, infrared spectroscopy, neutron moderation, neutrino detection, metabolic rate testing, neutron capture therapy, and the production of radioactive materials such as plutonium and tritium.
Composition
The deuterium nucleus consists of a neutron and a proton; the nucleus of a protium (normal hydrogen) atom consists of just a proton. The additional neutron makes a deuterium atom roughly twice as heavy as a protium atom.
A molecule of heavy water has two deuterium atoms in place of the two protium atoms of ordinary water. The term heavy water as defined by the IUPAC Gold Book can also refer to water in which a higher than usual proportion of hydrogen atoms are deuterium. For comparison, Vienna Standard Mean Ocean Water (the "ordinary water" used for a deuterium standard) contains about 156 deuterium atoms per million hydrogen atoms; that is, 0.0156% of the hydrogen atoms are H. Thus heavy water as defined by the Gold Book includes semiheavy water (hydrogen-deuterium oxide, HDO) and other mixtures of , , and HDO in which the proportion of deuterium is greater than usual. For instance, the heavy water used in CANDU reactors is a highly enriched water mixture that is mostly deuterium oxide , but also some hydrogen-deuterium oxide and a smaller amount of ordinary water . It is 99.75% enriched by hydrogen atom-fraction; that is, 99.75% of the hydrogen atoms are of the heavy type; however, heavy water in the Gold Book sense need not be so highly enriched. The weight of a heavy water molecule, however, is not very different from that of a normal water molecule, because about 89% of the mass of the molecule comes from the single oxygen atom rather than the two hydrogen atoms.
Heavy water is not radioactive. In its pure form, it has a density about 11% greater than water but is otherwise physically and chemically similar. Nevertheless, the various differences in deuterium-containing water (especially affecting the biological properties) are larger than in any other commonly occurring isotope-substituted compound because deuterium is unique among heavy stable isotopes in being twice as heavy as the lightest isotope. This difference increases the strength of water's hydrogen–oxygen bonds, and this in turn is enough to cause differences that are important to some biochemical reactions. The human body naturally contains deuterium equivalent to about five grams of heavy water, which is harmless. When a large fraction of water (> 50%) in higher organisms is replaced by heavy water, the result is cell dysfunction and death.
Heavy water was first produced in 1932, a few months after the discovery of deuterium. With the discovery of nuclear fission in late 1938, and the need for a neutron moderator that captured few neutrons, heavy water became a component of early nuclear energy research. Since then, heavy water has been an essential component in some types of reactors, both those that generate power and those designed to produce isotopes for nuclear weapons. These heavy water reactors have the advantage of being able to run on natural uranium without using graphite moderators that pose radiological and dust explosion hazards in the decommissioning phase. The graphite moderated Soviet RBMK design tried to avoid using either enriched uranium or heavy water (being cooled with ordinary water instead) which produced the positive void coefficient that was one of a series of flaws in reactor design leading to the Chernobyl disaster. Most modern reactors use enriched uranium with ordinary water as the moderator.
Other heavy forms of water Semiheavy water
Semiheavy water, HDO, exists whenever there is water with light hydrogen (protium, ) and deuterium (D or ) in the mix. This is because hydrogen atoms (H and H) are rapidly exchanged between water molecules. Water containing 50% and 50% in its hydrogen, is actually about 50% HDO and 25% each of and , in dynamic equilibrium. In normal water, about 1 molecule in 3,200 is HDO (one hydrogen in 6,400 is ), and heavy water molecules () only occur in a proportion of about 1 molecule in 41 million (i.e. one in 6,400). Thus semiheavy water molecules are far more common than "pure" (homoisotopic) heavy water molecules.
Heavy-oxygen water Water enriched in the heavier oxygen isotopes and is also commercially available. It is "heavy water" as it is denser than normal water ( is approximately as dense as , is about halfway between and )—but is rarely called heavy water, since it does not contain the excess deuterium that gives DO its unusual nuclear and biological properties. It is more expensive than DO due to the more difficult separation of O and O.Mosin, O. V, Ignatov, I. (2011) Separation of Heavy Isotopes Deuterium (D) and Tritium (T) and Oxygen (O) in Water Treatment, Clean Water: Problems and Decisions, No. 3–4, pp. 69–78. HO is also used for production of fluorine-18 in radiopharmaceuticals and radiotracers, and positron emission tomography. Small amounts of and are naturally present in water, and most processes enriching heavy water also enrich heavier isotopes of oxygen as a side-effect. This is undesirable if the heavy water is to be used as a neutron moderator in nuclear reactors, as can undergo neutron capture, followed by emission of an alpha particle, producing radioactive . However, doubly labeled water, containing both a heavy oxygen and hydrogen, is useful as a non-radioactive isotopic tracer.
Compared to the isotopic change of hydrogen atoms, the isotopic change of oxygen has a smaller effect on the physical properties.
Tritiated water Tritiated water contains tritium (H) in place of protium (H) or deuterium (H). Since tritium is radioactive, tritiated water is also radioactive.
Physical properties +Physical properties of isotopologues of water Property DO(Heavy water) HDO(Semiheavy water) HO(Light water) TO(Tritiated water) Melting point (standard pressure) Boiling point Density at STP (g/mL) 1.1056 1.054 0.9982 1.2133 Temp. of maximum density Unverified Extract of page 15 Dynamic viscosity (at 20 °C, mPa·s) 1.2467 1.1248 1.0016 1.40 (estimated) Surface tension (at 25 °C, N/m) 0.07187 0.07193 0.07198 Unverified Heat of fusion (kJ/mol) 6.132 6.227 6.00678 Unverified Heat of vaporisation (kJ/mol) 41.521 Unverified 40.657 Unverified pH (at 25 °C)discussion of pD, 7.44 ("pD") 7.266 ("pHD") 7.0 Unverified pKb (at 25 °C) 7.44 ("pKb D2O") Unverified 7.0 Unverified Refractive index (at 20 °C, 0.5893 μm) 1.32844 Unverified 1.33335 Unverified The physical properties of water and heavy water differ in several respects. Heavy water is less dissociated than light water at given temperature, and the true concentration of D ions is less than ions would be for light water at the same temperature. The same is true of OD vs. ions. For heavy water Kw DO (25.0 °C) = 1.35 × 10, and [D] must equal [OD] for neutral water. Thus pKw DO = 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+ = pH(apparent reading from pH meter) + 0.456. These corrections are slightly different from previous the differences in p[D+] and p[OD-] of 0.44 from the corresponding ones in heavy water.discussion of pD+,
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.
A 1935 experiment reported not the "slightest difference" in taste between ordinary and heavy water. However, a more recent study confirmed anecdotal observation that heavy water tastes slightly sweet to humans, with the effect mediated by the TAS1R2/TAS1R3 taste receptor. Rats given a choice between distilled normal water and heavy water were able to avoid the heavy water based on smell, and it may have a different taste. Some people report that minerals in water affect taste, e.g. potassium lending a sweet taste to hard water, but there are many factors of a perceived taste in water besides mineral contents.
Heavy water lacks the characteristic blue color of light water; this is because the molecular vibration harmonics, which in light water cause weak absorption in the red part of the visible spectrum, are shifted into the infrared and thus heavy water does not absorb red light.
No physical properties are listed for "pure" semi-heavy water because it is unstable as a bulk liquid. In the liquid state, a few water molecules are always in an ionized state, which means the hydrogen atoms can exchange among different oxygen atoms. Semi-heavy water could, in theory, be created via a chemical method, but it would rapidly transform into a dynamic mixture of 25% light water, 25% heavy water, and 50% semi-heavy. However, if it were made in the gas phase and directly deposited into a solid, semi-heavy water in the form of ice could be stable. This is due to collisions between water vapor molecules being almost completely negligible in the gas phase at standard temperatures, and once crystallized, collisions between the molecules cease altogether due to the rigid lattice structure of solid ice.
Heavy water exchanges with atmospheric water until it reaches the usual hydrogen-isotopic ratio.
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