Heavy Water - Uses, Preparation, Physical and Chemical Properties of Heavy Water
Heavy water (D2O) is basically water composed of deuterium. It is also known as deuterium oxide. Deuterium is the hydrogen isotope with a mass double that of ordinary hydrogen. (Ordinary water is represented by H2O.) Heavy water has a molecular weight of about 20 (the sum of twice the atomic weight of deuterium, which is 2, in addition to the atomic weight of oxygen that 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 can be found mainly from normal resources which include about one deuterium atom for every 6,760 ordinary hydrogen atoms and the leftover water is improved in deuterium content. Therefore to get deuterium oxide, continued electrolysis of hundreds of liters of water until only a few milliliters remain is done. Only then, D2O can be considered as almost pure. Until 1943, the only large-scale method used to produce heavy water was continuous electrolysis. It has been superseded by less expensive processes, such as fractional distillation. Fractional distillation is effective because D2O becomes concentrated in the liquid residue since it is less volatile than H2O. One major use of heavy water is that it is used as a moderator of neutrons in nuclear power plants. Heavy water is employed as an isotopic tracer in studies of biochemical and chemical processes, in the laboratory.
Uses of Heavy Water
India is one of the world’s major manufacturers of heavy water. It exports D2O to countries like The United States and The
Republic of Korea through its Heavy water board. The diverse applications and uses of heavy water are as follows:
• Nuclear magnetic resonance: Since the signal from H2O solvent molecules interact and blocks the signal from the molecule of interest, D2O is used. D2O has a different magnetic moment and hence makes it convenient to be used in nuclear magnetic resonance spectroscopy.
• In Organic chemistry: Specifically labeled isotopologues of organic compounds require deuterium for their preparation. D2O is often used as a source for Deuterium.
• Neutron moderator: D2O is used as a neutron moderator in certain types of nuclear reactors. It helps in slowing down neutrons so that the probability of them reacting with the fissile uranium-235 than with uranium-238 increases. This, in turn, helps in capturing neutrons without fissioning.
• Neutrino detector: Neutrinos react with heavy water to produce flashes of lights.
• Metabolic rate testing in physiology and biology: Conducting tests to check the safety of mean metabolic rate in humans and animals requires heavy water as part of a mixture with H218O.
• Tritium production: Tritium is created in small amounts in heavy water moderated reactors.
1) By prolonged electrolysis
• When ordinary water is electrolyzed, protium is formed. The liberation of protium happens readily in the case of H2O because H+ ions have greater mobility as compared to D+ ions. This factor owes to its smaller size. They also possess lower discharge potential and hence H+ ions are discharged at the cathode more easily. In addition to that, hydrogen atoms form molecular hydrogen more readily than deuterium atoms.
• The concentration of heavy water in ordinary water increases as the electrolysis continues. Eventually, almost pure heavy water can be obtained when very little volume remains.
• Brown, Degget and Urey designed an electrolytic cell for the preparation of heavy water. It consists of a steel cell 45 cm long and 10 cm in diameter. This is the cathode. A large number of these cells are used for the electrolysis of water in different stages.
2) By fractional distillation
There is a small difference in the boiling points of normal and heavy water. This is the basis of using fractional distillation for the preparation of heavy water. The boiling point of normal water is 373K whereas that of heavy water is 374.42K (at normal atmospheric pressure).
• The molecular mass of heavy water is higher than ordinary water which makes their properties different from each other.
• It is colorless, odorless and tasteless but heavier than normal water.
• Ionic compounds are less soluble in heavy water because its dielectric constant is lower than that of H2O.
The reactions involving D2O proceed at a slower rate compared to H2O.
• Electrolysis: When heavy water is electrolyzed, deuterium can be obtained at the cathode.
• The action of alkali and alkaline metal: The reaction produces di-deuterium but it takes place at a very slow pace.
• The action of metallic oxides: The reaction which takes place at a slow pace produces respective deuteroxides.
• The action of non-metallic oxides: Corresponding deuteroacids are formed. Non-metallic oxides like phosphorus pentoxide and sulfur trioxide readily dissolve in heavy water.
• Action with metallic nitrides, phosphides, and arsenides: when heavy water reacts with metallic nitrides, phosphides and arsenides, deuteroammonia, deuterophosphine, and deuteroarsine is liberated (respectively).
• Formation of deuterates: Heavy water is known to combine with many compounds as heavy water of crystallization. Heavy hydrates thus obtained are called as deuterates.
• Exchange reaction: Active hydrogen atoms are exchanged either partially or fully when compounds are treated with heavy water.
Since heavy water slows down many reactions, it is harmful to human beings, plants and animals as it also slows down the rate of reactions occurring to them. Hence,
• Tobacco seeds do not grow in heavy water.
• Pure heavy water kills tadpoles, small fish, and mice when fed upon it.
• Heavy water retards the growth of living organisms like plants and animal.