Heavy Water
The oxide of heavy hydrogen is called heavy water. It was discovered by Urey (1932). He showed that ordinary water contains small proportion (about 1 part in 5000) of the oxide of heavy hydrogen, i.e. D2O.
1. Prolonged Electrolysis of Water
The apparatus designed by Brown, Dagget and Urey for electrolysis of water consists of a steel cell 18 inches long and 4 inches in diameter (Figure). The cell itself serves as the cathode while the anode consists of a cylindrical sheet of nickel with a number of holes punched in it. A large number of such cells are used for electrolysis of water in several stages. The gases obtained from each stage are separately burnt and the water thus formed is returned to the previous stage. The heavy water gradually concentrates in the residue left behind. The process usually consists of five stages, as described below.
First stage Thirty electrolytic cells, described above, are used in the first stage. Each cell is charged with three litres of a 3 per cent solution of sodium hydroxide in ordinary water. An electric current of 110 volts is used for electrolysis. The evolved gases containing light hydrogen (1H1) and oxygen are discarded. Whea the electrolysis has been going on for about 72 hours, the total charge gets reduced to 1/6th of its volume and contains about 2.5% heavy water.
Second stage In the second stage, the residue from the first stage is electrolysed using 6 cells till the content of heavy water increases to about 12%. The gases evolved are burnt and the water formed is returned to the first stage cells.
Third stage The residue from the second stage is further electro lysed and the content of heavy water is raised to nearly 60%. The gases evolved are burnt and the water formed is returned to the second stage cells.
Fourth stage The residue from the third stage is further subjected to electrolysis and nearly 99.5% heavy water is obtained. The gases are burnt, as usual, and sent to the third stage cells.
Fifth stage The nearly 99.5% heavy water obtained from the fourth stage is freed from alkali and other impurities by distillation and the distillate is then electrolysed. The hydrogen evolved is pure deuterium This, when burnt in oxygen, yields 100% pure D2O.
A flow sheet diagram of the process is shown below-
A highly efficient plant has been set up at Nangal where cheap electric power is available.
2. Fractional Distillation of Water
A partial separation of heavy water from ordinary water can be affected by fractional distillation. Advantage is taken of small difference in boiling points of protium oxide and deuterium oxide. Since the boiling points do not differ much, It is necessary to use a very long column and to repeat the process several times.
3. By Exchange Reaction
Heavy water can also be prepared by the exchange reaction. This reaction is catalysed by finely divided nickel.
H2O + D2 ⇌ D2O + H2
In one of the methods, hydrogen sulphide, H2S is first passed through hot water. In this way, hydrogen of H2S gets exchanged to a small extent by deuterium of heavy water present in traces in water. H2S thereby gets associated with some D2S as well. On passing this gas through cold water, deuterium (D2) of the D2S exchanges with hydrogen of H2O yielding D2O. On repeating the process, the concentration of D2O in water is increasingly raised.
Properties of Heavy Water
Physical Properties
Heavy water, like ordinary water, is a colourless, odourless and tasteless mobile liquid. However, there is a marked difference between physical properties of ordinary water and heavy water.
Higher viscosity of heavy water is responsible for lower solubility of ionic solids like NaCl and smaller mobilities of ions. Density of heavy water is expected to be higher on account of higher molar mass of this compound.
Chemical Properties
Heavy water differs only slightly from ordinary water in its chemical properties. However, the reactions of D2O are slower than those of H2O. Some of the important chemical reactions of heavy water are given below. All these reactions are given by ordinary water with a much greater vigour.
1. Action with metals: D2O reacts slowly with alkali and alkaline earth metals liberating heavy hydrogen.
2Na + 2D2O → 2NaOD + D2
Ca + 2D2O → Ca(OD)2 + D2
2. Action with Metallic Oxides: Basic oxides of metals like sodium and calcium dissolve in D2O and form heavy alkalies.
2Na2O + D2O → 2NaOD
CaO + D2O → Ca(OD)2
3. Action with Acid Anhydrides: D2O dissolves acid anhydrides or acidic oxides of non-metals like P2O5, SO3, etc., to form the corresponding acids containing heavy hydrogen.
SO3 + D2O → D2SO4
P2O5 + 3D2O → 2D3PO4
4. Action with Metallic Carbides: Like H2O, heavy water reacts with metallic carbides and forms deutero-hydrocarbons.
Al4C3 +12D2O → 4Al(OD)3 + 3CD4
CaC2 + 2D2O → Ca(OD)2 + C2D2
5. Action with metallic nitrides, phosphides and arsenides: Heavy water decomposes metallic nitrides, phosphides, etc., forming deutero ammonia, deutero phosphine, etc.
Mg3N2 + 6D2O → 3Mg(OD)2 + 2ND3
Ca3P2 + 6D2O → 3Ca(OD)2 + 2PD3
Na3As + 3D2O → 3NaOD + AsD3
The melting and boiling points of deutero-compounds are higher than those of the corresponding compounds of hydrogen. Thus, the melting and boiling points of ND3 are higher than those of NH3.
6. Electrolysis: Heavy water containing dissolved P2O, (to make it a good conductor) on electrolysis decomposes into deuterium and oxygen which are liberated at the cathode and anode, respectively.
2D2O → 2D2 + O2
7. Formation of deuterates: Like ordinary water, D2O may be associated with salts and other compounds as water of crystallization. The hydrates so formed are termed as deuterates, e.g., CuSO4.5D2O, Na2SO4.10D2O, NiCl2.6D2O, etc.
8. Exchange reactions: When compounds containing hydrogen are treated with D2O, hydrogen undergoes an exchange for deuterium. If the compounds contain ionically bound hydrogen, the exchange reactions take place almost instantaneously, e.g.,
NaOH + D2O → NaOD + HOD
HCl(aq) + D2O → DCl + HOD
NH4Cl + 4D2O → ND4CI + 4HOD
In the case of compounds containing non-ionic hydrogen, the exchange reactions take place slowly and require the presence of a catalyst.
Biological Properties:
The biological properties of heavy water are still in dispute although it is established that heavy water retards the growth of living organisms, plants and animals. Lewis has shown that tobacco seeds do not grow in heavy water. Also, pure heavy water kills small fish, tadpoles and mice when fed upon it. Certain moulds have, however, been found to develop better in heavy water.
Uses of Heavy Water:
1. As a neutron moderator: Heavy water is used for slowing down the fast moving neutrons.
Neutrons are used for bringing about fission of uranium atoms. But, for this purpose their speed should be slowed down. This is done by passing them through heavy water.
2. As a tracer compound: Heavy water is commonly employed as a tracer compound for studying the mechanism of many chemical reactions for confirming the structures of compounds. For example, when hypophosphorous acid, H3PO2, is treated with D2O, only one ionic hydrogen of the acid is found to exchange for deuterium. Since deuterium is known to exchange only with ionic hydrogen, it can be safely inferred that H3PO2 contains only one ionizable hydrogen and therefore, it is a monobasic acid.
3. For the production of deuterium: Heavy hydrogen or deuterium is produced by the electrolysis of heavy water or by its decomposition by sodium meta.
Source: Principles of Inorganic Chemistry By P.S.K.