Heavy water (D2O),deuterium oxide

Chemical Compound: D2O

Alternative titles: D2O; deuterium oxide

heavy water (D2O), also called deuterium oxide,  water composed of deuterium, the hydrogen isotopewith a mass double that of ordinary hydrogen, and oxygen. (Ordinary water has a composition represented by H2O.) Thus, heavy water has a molecular weight of about 20 (the sum of twice the atomic weight of deuterium, which is 2, plus the atomic weight of oxygen, which is 16), whereas ordinary water has a molecular weight of about 18 (twice the atomic weight of ordinary hydrogen, which is 1, plus oxygen, which is 16).

Ordinary water as obtained from most natural sources contains about one deuterium atom for every 6,760 ordinary hydrogen atoms. and the residual water is thus enriched in deuterium content. Continued electrolysis of hundreds of litres of water until only a few millilitres remain yields practically pure deuterium oxide. This operation, until 1943 the only large-scale method used, has been superseded by less expensive processes, such as fractional distillation (D2O becomes      concentrated in the liquid residue because it is less volatile than H2O). The heavy water         produced is used as a moderator of neutrons in nuclear power plants.                                                     In the laboratory heavy water is employed as an isotopic tracer in studies of chemical and   biochemical processes.
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Heavy water is water which contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D₂O or ²H₂O, or as deuterium protium oxide, HDO or ¹H²HO.^[1] Its physical and chemical properties are somewhat similar to those of water, H₂O. Heavy water may contain as much as 100% D₂O, and usually the term refers to water which is highly enriched in deuterium. The isotopic substitution with deuterium alters the bond energy of the hydrogen-oxygen bond in water, altering the physical, chemical, and especially biological properties of the pure or highly-enriched substance to a larger degree than is found in most isotope-substituted chemical compounds.

Heavy water should not be confused with hard water or with tritiated water.

Heavy water (at 100% D enrichment): D2O
IUPAC name	Deuterium oxide
Other names	Water d2 Heavy water Dideuterium monoxide
Identifiers
CAS number	7789-20-0
RTECS number	ZC0230000
Properties
Molecular formula	D2O
Molar mass	20.04 g/mol
Appearance	transparent, colorless liquid
Density	1.1056, liquid (20°C) 1.0177, solid (at m.p)
Melting point	3.82 °C, 38.88 °F (276.97 °K)
Boiling point	101.4 °C, 214.56 °F (374.55 °K)
Viscosity	0.00125 Pa·s at 20 °C
Dipole moment	1.87 D
Hazards
MSDS	External MSDS
NFPA 704	0 0 0
Related Compounds
Related solvents	acetone; methanol
Related compounds	water vapor; ice
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Other meanings

Semiheavy water

Semiheavy water, HDO, also exists whenever there is water with hydrogen-1 (or protium) and deuterium present in the mixture. This is because hydrogen atoms (hydrogen-1 and deuterium) are rapidly exchanged between water molecules. Water containing 50% H and 50% D in its hydrogen actually contains about 50% HDO and 25% each of H₂O and D₂O, in dynamic equilibrium. Semiheavy water, HDO, occurs naturally in regular water at a proportion of about 1 molecule in 3,200 (each hydrogen has a probability of 1 in 6,400 of being D). Heavy water, D₂O, by comparison, occurs naturally at a proportion of about 1 molecule in 41 million (i.e., 1 in 6,400²).

Heavy-oxygen water

A common type of heavy-oxygen water H₂¹⁸O is available commercially for use as a non-radioactive isotopic tracer (see doubly-labeled water for discussion), and qualifies as "heavy water" insofar as having a higher density than normal water (in this case, similar density to deuterium oxide). Even more expensively, water is available in which the oxygen is ¹⁷O. However, these types of heavy-isotope water are rarely referred to as "heavy water", as they do not contain the deuterium which gives D₂O its characteristically different nuclear and biological properties. Heavy-oxygen waters with normal hydrogen, for example, would not be expected to show any toxicity whatsoever (see discussion of toxicity below).

Physical properties (with comparison to light water)

Property	D2O (Heavy water)	H2O (Light water)
Freezing point (°C)	3.82	0.0
Boiling point (°C)	101.4	100.0
Density (at 20°C, g/mL)	1.1056	0.9982
Temp. of maximum density (°C)	11.6	4.0
Viscosity (at 20°C, mPa·s)	1.25	1.005
Surface tension (at 25°C, μJ)	7.193	7.197
Heat of fusion (cal/mol)	1,515	1,436
Heat of vaporisation (cal/mol)	10,864	10,515
pH (at 25°C)	7.41 (sometimes "pD")	7.00

No physical properties are listed for "pure" semi-heavy water, because it cannot be isolated in bulk quantities. In the liquid state, a few water molecules are always in an ionised state, which means the hydrogen atoms can exchange among different oxygen atoms. A sample of hypothetical "pure" semi-heavy water would rapidly transform into a dynamic mixture of 25% light water, 25% heavy water, and 50% semi-heavy water.

Physical properties obvious by inspection: Heavy water is 10.6% more dense than ordinary water, a difference which is nearly impossible to notice in a sample of it (which otherwise looks and tastes exactly like normal water). One of the few ways to demonstrate heavy water's physically different properties without equipment, is to freeze a sample and drop it into normal water. Ice made from heavy water sinks in normal water. If the normal water is ice-cold this phenomenon may be observed long enough for a good demonstration, since heavy-water ice has a slightly higher melting-temperature (3.8 °C) than normal ice, and thus holds up very well in ice-cold normal water. [1]

History

Harold Urey discovered the isotope deuterium in 1931 and was later able to concentrate it in water. Urey's mentorGilbert Newton Lewis isolated the first sample of pure heavy water by electrolysis in 1933. George de Hevesy and Hoffer used heavy water in 1934 in one of the first biological tracer experiments, to estimate the rate of turnover of water in the human body. The history of large-quantity production and use of heavy water in early nuclear experiments is given below.^[2]

Effect on biological systems

Heavy isotopes of chemical elements have very slightly different chemical behaviors, but for most elements the differences in chemical behavior between isotopes are far too small to use, or even detect. For hydrogen, however, this is not true. The larger chemical isotope-effects seen with deuterium and tritium manifest because bond energies in chemistry are determined in quantum mechanics by equations in which the quantity of reduced mass of the nucleus and electrons appears. This quantity is altered in heavy-hydrogen compounds (of which deuterium oxide is the most common and familiar) far more than for heavy-isotope substitution in other chemical elements. This isotope effect of heavy hydrogen is magnified further in biological systems, which are very sensitive to small changes in the solvent properties of water.

To perform their tasks, enzymes rely on their finely tuned networks of hydrogen bonds, both in the active center with their substrates, and outside the active center, to stabilize their tertiary structures. As a hydrogen bond with deuterium is slightly stronger than one involving ordinary hydrogen, in a highly deuterated environment, some normal reactions in cells are disrupted.

Particularly hard-hit by heavy water are the delicate assemblies of mitotic spindle formation necessary for cell division in eukaryotes. Because eukaryotic cell division stops in heavy water, seeds therefore do not germinate in heavy water, and plants stop growing when given only heavy water.

Effect on animals

Experiments in mice, rats, and dogs^[3] have shown that a degree of 25% deuteration causes (sometimes irreversible) sterility, because neither gametes nor zygotes can develop. High concentrations of heavy water (90%) rapidly kills fish, tadpoles, flatworms, and drosophila. Mammals such as rats given heavy water to drink die after a week, at a time when their body water approaches about 50% deuteration. The mode of death appears to be the same as that in cytotoxic poisoning (such as chemotherapy) or in acute radiation syndrome (though deuterium is not radioactive), and is due to deuterium's action in generally inhibiting cell division. Deuterium oxide is used to enhance boron neutron capture therapy.^[3] It is more toxic to malignant cells than normal cells but the concentrations needed are too high for regular use.^[3] As in chemotherapy, deuterium-poisoned mammals die of a failure of bone marrow (bleeding and infection) and intestinal-barrier functions (diarrhea and fluid loss).

Notwithstanding the problems of plants and animals in living with too much deuterium, prokaryotic organisms such as bacteria (which do not have the mitotic problems induced by deuterium) may be grown and propagated in fully deuterated conditions, resulting in replacement of all hydrogen atoms in the bacterial proteins and DNA with the deuterium isotope.^[3] Full replacement with heavy atom isotopes can be accomplished in higher organisms with other non-radioactive heavy isotopes (such as carbon-13 and nitrogen-15), but this cannot be done for the stable heavy isotope of hydrogen.

Toxicity in humans

Because it would take a very great deal of heavy water to replace 25% to 50% of a human being's body water (which in turn is 70% of body weight) with heavy water, accidental or intentional poisoning with heavy water is unlikely to the point of practical disregard. For a poisoning, large amounts of heavy water would need to be ingested without significant normal water intake for many days to produce any noticeable toxic effects (although in a few tests, volunteers drinking large amounts of heavy water have reported dizziness, a possible effect of density changes in the fluid in the inner ear). For example, a 70 kg human containing 50 kg of water and drinking 3 liters of pure heavy water per day, would need to do this for almost 5 days to reach 25% deuteration, and for about 11 days to approach 50% deuteration. Thus, it would take a week of drinking nothing but pure heavy water for a human to begin to feel ill, and 10 days to 2 weeks (depending on water intake) for severe poisoning and death. In the highly unlikely event that a human were to receive a toxic dose of heavy water, the treatment would involve the use of intravenous water replacement (due to possible intestinal dysfunction and problems with absorption of fluids). This would be done via 0.9% (normal physiologic) saline solution with other salts as needed, perhaps in conjunction with diuretics.

Oral doses of heavy water in the multi-gram range, along with heavy oxygen ¹⁸O, are routinely used in human metabolic experiments. See doubly-labeled water testing. Since 1 in every 6400 hydrogen atoms is deuterium, a 50 kg human containing 32 kg of body water would normally contain enough deuterium (about 1.1 gram) to make 5.5 grams of pure heavy water, so roughly this dose is required to double the amount of deuterium in the body.

Confused report of a "heavy water" contamination incident

In 1990, a disgruntled employee at the Point Lepreau Nuclear Generating Station in Canada obtained a sample (estimated as about a "half cup") of heavy water from the primary heat transport loop of the nuclear reactor, and loaded it into the employee water cooler. Eight employees drank some of the contaminated water. The incident was discovered when employees began leaving bioassay urine samples with elevated tritium levels. The quantity of heavy water involved was far below levels which could induce heavy water toxicity per se, but several employees received elevated radiation doses from tritium and neutron-activated chemicals in the water.^[4] This was not an incident of heavy water poisoning, but rather radiation poisoning from other isotopes in the heavy water. Some news services were not careful to distinguish these points, and some of the public was left with the impression that heavy water is normally radioactive and more severely toxic than it is. Even if pure heavy water had been used in the water cooler indefinitely, it is not likely the incident would have been detected or caused harm, since no employees would be expected to get as much as 25% of their daily drinking water from such a source.^[5]

Production

On Earth, semiheavy water, HDO, occurs naturally in regular water at a proportion of about 1 molecule in 3200. This means that 1 in 6400 hydrogen atoms is deuterium, which is 1 part in 3200 by weight (hydrogen weight). The HDO may be separated from regular water by distillation or electrolysis and also by various chemical exchange processes, all of which exploit a kinetic isotope effect. (For more information about the isotopic distribution of deuterium in water, see Vienna Standard Mean Ocean Water.)

The difference in mass between the two hydrogen isotopes translates into a difference in the zero-point energy and thus into a slight difference in the speed at which the reaction proceeds. Once HDO becomes a significant fraction of the water, heavy water will become more prevalent as water molecules trade hydrogen atoms very frequently. To produce pure heavy water by distillation or electrolysis requires a large cascade of stills or electrolysis chambers, and consumes large amounts of power, so the chemical methods are generally preferred. The most important chemical method is the Girdler sulfide process.

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