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 renewable natural resources which include about one deuterium atom for every 6,760 ordinary hydrogen atoms. Therefore, the residual water contains a good quantity of deuterium content. To get deuterium oxide, continued electrolysis of hundreds of litres of water until only a few millilitres remain. 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 It has been superseded by less expensive processes, such as fractional distillation.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 (HWB). The diverse applications and uses of heavy water are as follows:

• Nuclear Magnetic Resonance: Since the signal from H₂O solvent molecules interact and block 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 labelled 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.

Preparation of Heavy Water

1) By Prolonged Electrolysis

When ordinary water is electrolysed, 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).

Physical Properties of Heavy Water

The molecular mass of heavy water is higher than ordinary water which makes their properties different from each other.

It is colourless, odourless 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.

Chemical Properties of Heavy Water

• 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.

• Action of Alkali and Alkaline Metal: The reaction produces di-deuterium but it takes place at a very slow pace.

• Action of Metallic Oxides: The reaction which takes place at a slow pace produces respective deuteroxides.

• Action of Non-Metallic Oxides: Corresponding deutero acids are formed. Non-metallic oxides like phosphorus pentoxide and sulphur 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 crystallisation. Heavy hydrates thus obtained are called deuterates.

• Exchange Reaction: Active hydrogen atoms are exchanged either partially or fully when compounds are treated with heavy water.

Biological Properties of 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. Some of their effects are given below:

• 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.

Physical and  Chemical Properties of Heavy Water