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Heavy water

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Title: Heavy water  
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Heavy water

Heavy water
Spacefill model of heavy water
Identifiers
CAS number  YesY
PubChem
ChemSpider  YesY
UNII  YesY
EC number
KEGG  YesY
MeSH
ChEBI  YesY
ChEMBL  N
RTECS number ZC0230000
Gmelin Reference 97
Jmol-3D images Image 1
Properties
Molecular formula D
2
O
Molar mass 20.0276 g mol−1
Appearance Very pale blue, transparent liquid
Odor Odorless
Density 1.107 g mL−1
Melting point 3.82 °C; 38.88 °F; 276.97 K
Boiling point 101.4 °C (214.5 °F; 374.5 K)
Solubility in water Soluble
log P −1.38
Refractive index (nD) 1.328
Viscosity 1.25 mPa s (at 20 °C)
Dipole moment 1.87 D
Hazards
NFPA 704
0
1
1
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N   YesY/N?)

Heavy water, formally called deuterium oxide (2
H
2
O
or D
2
O
), is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium, (also known as heavy hydrogen, which can be symbolized as 2
H
or D) rather than the common hydrogen-1 isotope (called protium, symbolized as 1
H
) that makes up most of the hydrogen in normal water.[1]

Contents

  • Explanation 1
  • Other heavy forms of water 2
    • Semiheavy water 2.1
    • Heavy-oxygen water 2.2
    • Tritiated water 2.3
  • Physical properties (with comparison to light water) 3
  • History 4
  • Effect on biological systems 5
    • Effect on animals 5.1
    • Toxicity in humans 5.2
  • Name 6
    • Heavy water radiation contamination confusion 6.1

Explanation

Some or most of the hydrogen atoms in heavy water contain a neutron, making those heavy water hydrogen atoms about twice as heavy as normal hydrogen. However, the weight of the heavy water molecule as a whole is not substantially different from that of a normal water molecule because about 89% of the molecular weight of water comes from the single oxygen atom rather than the two hydrogen atoms. The additional neutron in each hydrogen atom does not change the volume of the water noticeably. The increased molecular weight does make the water slightly more dense. The colloquial term heavy water is often also used to refer to a highly enriched water mixture that contains mostly deuterium oxide but also contains some ordinary water molecules as well: for instance heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction, meaning that 99.75% of the hydrogen atoms are of the heavy type. In comparison, in ordinary water, which is the "ordinary water" used for a deuterium standard on Earth, there are only about 156 deuterium atoms per million hydrogen atoms.

Heavy water is not

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 a cafeteria drink dispenser. 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 that could induce heavy water toxicity, but several employees received elevated radiation doses from tritium and neutron-activated chemicals in the water.[33]

Although many people associate heavy water primarily with its use in nuclear reactors, pure heavy water is not radioactive. Commercial-grade heavy water is slightly radioactive due to the presence of minute traces of natural tritium, but the same is true of ordinary water. Heavy water that has been used as a coolant in nuclear power plants contains substantially more tritium as a result of neutron bombardment of the deuterium in the heavy water (tritium is a health risk when ingested in large quantities).

Heavy water radiation contamination confusion

The American patent U.S. Patent 5,223,269 is for the use of heavy water to treat hypertension (high blood pressure). A loss of blood pressure may partially explain the reported incidence of dizziness upon ingestion of heavy water. However, it is more likely that this symptom can be attributed to altered vestibular function.[32]

Oral doses of heavy water in the range of several grams, as well as heavy oxygen 18O, are routinely used in human metabolic experiments. See doubly labeled water testing. Since one in about 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.

Because it would take a very large amount of heavy water to replace 25% to 50% of a human being's body water (water being in turn 50% - 75% of body weight[31]) with heavy water, accidental or intentional poisoning with heavy water is unlikely to the point of practical disregard. Poisoning would require that the victim ingest large amounts of heavy water without significant normal water intake for many days to produce any noticeable toxic effects.

Toxicity in humans

Deuterium oxide is used to enhance boron neutron capture therapy, but this effect does not rely on the biological effects of deuterium per se, but instead on deuterium's ability to moderate (slow) neutrons without capturing them.[28]

Full replacement with heavy atom isotopes can be accomplished in higher organisms with other non-radioactive heavy isotopes (such as carbon-13, nitrogen-15, and oxygen-18), but this cannot be done for the stable heavy isotope of hydrogen.

[30][28] Notwithstanding the problems of plants and animals in living with too much deuterium,

Experiments in mice, rats, and dogs[28] 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 kill 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.[29] 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. It is more toxic to malignant cells than normal cells but the concentrations needed are too high for regular use.[28] As in chemotherapy, deuterium-poisoned mammals die of a failure of bone marrow (bleeding and infection) and intestinal-barrier functions (diarrhea and fluid loss).

Effect on animals

However, all concentrations over 50% of deuterium in the water molecules were found to kill plants. [27] Experiments showed that bacteria can live in 98% heavy water.[26][25][24] Particularly hard-hit by heavy water are the delicate assemblies of

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[18] than one involving ordinary hydrogen, in a highly deuterated environment, some normal reactions in cells are disrupted.

[17] Heavy water is the only known chemical substance that affects the period of

Different isotopes of chemical elements have slightly different chemical behaviors, but for most elements the differences are far too small to use, or even detect. For hydrogen, however, this is not true. The larger chemical isotope-effects seen between protium (light hydrogen) versus 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) 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.

Effect on biological systems

[16] studied the autodissociation of heavy water in 1934.Otto Redlich and Emilian Bratu [15]

History

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 ionised 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 water. However, if it were made in the gas phase and directly frozen into a solid, semiheavy water (in the form of ice) could be stable.

An early experiment[11] reported not the "slightest difference" in taste between ordinary and heavy water. On the other hand, rats given a choice between distilled normal water and heavy water were able to avoid the heavy water based on smell, and it may be possible that it has a different taste.[12]

Physical properties obvious by inspection: Heavy water is 10.6% denser than ordinary water, a difference not immediately obvious. 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 (it sinks). 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 endures very well in ice-cold normal water.[10]

Properties[7] D2O (Heavy water) HDO (Semiheavy water) H2O (Light water)
Freezing point 3.82 °C (38.9 °F) 2.04 °C (35.7 °F) 0.0 °C (32 °F)
Boiling point 101.4 °C (214.5 °F) 100.7 °C (213.3 °F) 100.0 °C (212 °F)
Density at STP (g/mL) 1.1056 1.054 0.9982
Temp. of maximum density 11.6 °C 3.98 °C[8]
Dynamic viscosity (at 20 °C, mPa·s) 1.2467 1.1248 1.0016
Surface tension (at 25 °C, N/m) 0.07187 0.07193 0.07198
Heat of fusion (kJ/mol) 6.132 6.227 6.00678
Heat of vaporisation (kJ/mol) 41.521 40.657
pH (at 25 °C) 7.43 (sometimes "pD") 7.266 (sometimes "pHD") 6.9996
Refractive index (at 20 °C, 0.5893 μm)[9] 1.32844 1.33335

Physical properties (with comparison to light water)

Tritiated water contains tritium in place of protium or deuterium.

Tritiated water

Water enriched in the heavier oxygen isotopes 17O and 18O is also commercially available, e.g., for use as a non-radioactive isotopic tracer. It is "heavy water" as it is denser than normal water (H218O is approximately as dense as D2O, H217O is about halfway between H2O and D2O)—but is rarely called heavy water, since it does not contain the deuterium that gives D2O its unusual nuclear and biological properties. It is more expensive than D2O due to the more difficult separation of 17O and 18O.[6]

Heavy-oxygen water

Semiheavy water, HDO, exists whenever there is water with light hydrogen (protium, 1H) and deuterium (D or 2H) in the mix. 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. In normal water, about 1 molecule in 3,200 is HDO (one hydrogen in 6,400 is in the form of D), and heavy water molecules (D2O) only occur in a proportion of about 1 molecule in 41 million (i.e. one in 6,4002). Thus semiheavy water molecules are far more common than "pure" (homoisotopic) heavy water molecules.

Semiheavy water

Other heavy forms of water

Heavy water was first produced in 1932, a few months after the discovery of deuterium.[3] 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 can pose radiological[4] and dust explosion[5] hazards in the decommissioning phase. Most modern reactors use enriched uranium with normal "light water" (H2O) as the moderator.

[2]

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