Introduction 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 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 It has been superseded by less expensive processes, such as fractional distillation.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
FAQs on Heavy Water
1. What is heavy water and what is its chemical formula?
Heavy water is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium, rather than the common hydrogen-1 isotope (protium) that makes up most of the hydrogen in ordinary water. Its chemical formula is D₂O or ²H₂O. It is also known as deuterium oxide.
2. What are the main physical and chemical differences between heavy water (D₂O) and ordinary water (H₂O)?
Heavy water differs from ordinary water in several key properties due to the higher mass of deuterium compared to protium:
- Molecular Mass: The molecular mass of D₂O is approximately 20.03 g/mol, while H₂O is about 18.01 g/mol.
- Density: Heavy water is about 11% denser than ordinary water. Consequently, an ice cube made from D₂O will sink in normal water.
- Boiling and Freezing Points: The boiling point of D₂O is slightly higher at 101.4 °C (374.4 K), and its freezing point is 3.82 °C.
- Solvent Properties: Ionic compounds are generally less soluble in heavy water because its dielectric constant is lower than that of H₂O.
3. How is heavy water prepared on a large scale?
Heavy water is prepared commercially using two primary methods that exploit the slight physical differences between D₂O and H₂O:
- Prolonged Electrolysis: When an aqueous solution is electrolysed, ordinary hydrogen (H₂) is liberated at the cathode more readily than deuterium (D₂) because of its lower mass and greater mobility. This enriches the remaining water with D₂O.
- Fractional Distillation: This method relies on the small difference in boiling points between H₂O (100 °C) and D₂O (101.4 °C). By repeatedly distilling water, the less volatile D₂O becomes concentrated in the liquid residue.
4. What are the most important applications or uses of heavy water?
Heavy water has several specialised applications, particularly in the field of nuclear science and chemistry:
- Neutron Moderator: Its primary use is in certain types of nuclear reactors, like CANDU reactors, to slow down fast neutrons, making them more likely to cause fission with uranium-235.
- NMR Spectroscopy: In Nuclear Magnetic Resonance (NMR), D₂O is used as a solvent because it does not produce a signal in the proton NMR spectrum, thus preventing interference with the signals from the molecule being studied.
- Isotopic Tracer: It is used to study the mechanisms of chemical and biochemical reactions by replacing hydrogen with deuterium, a process known as isotopic labelling.
- Production of Tritium: Heavy water is used in reactors for the production of tritium, an important isotope for nuclear fusion research.
5. Why can't humans or animals survive on pure heavy water?
Living organisms cannot survive on pure heavy water because the rates of biochemical reactions are finely tuned to the properties of ordinary water (H₂O). The stronger chemical bonds and greater mass of deuterium in D₂O cause these vital reactions, such as enzyme functions and cell division, to proceed at a much slower rate. This slowdown disrupts the delicate balance of metabolic processes, which is ultimately lethal to most organisms if a significant percentage of their body water is replaced.
6. Why is the production of heavy water a difficult and expensive process?
The production of heavy water is expensive primarily due to the low natural abundance of deuterium. Only about 1 in every 6,760 hydrogen atoms in natural water is deuterium. Separating D₂O from H₂O is challenging because their physical properties, such as boiling point and density, are very similar. Therefore, the separation requires large-scale, energy-intensive industrial processes like prolonged electrolysis or multi-stage fractional distillation, making the final product costly.
7. How exactly does heavy water work as a neutron moderator in a nuclear reactor?
In a nuclear reactor, heavy water acts as a moderator by slowing down the high-energy (fast) neutrons produced during nuclear fission. When these fast neutrons collide with the deuterium nuclei in D₂O, they transfer some of their energy, becoming 'slow' or 'thermal' neutrons. These slow neutrons have a much higher probability of being captured by fissile uranium-235 nuclei to sustain the nuclear chain reaction. A key advantage of heavy water is that it has a very low tendency to absorb neutrons itself, making it a highly efficient moderator.
