Heavy Water

Hydrogen of Class 11

Heavy Water

Heavy water is the oxide of heavy hydrogen (deuterium) and is also called deuterium oxide. It is represented by the formula D₂O or ²H₂O. The credit of discovery of heavy water goes to Urey who first proved that 6000 parts of ordinary water contains about 1 part of heavy water. Lewis and Donald (1933) were able to isolate a few ml of heavy water by the prolonged electrolysis of alkaline water.

Preparation of Heavy Water

The main source of heavy water is ordinary water. From which it is prepared either by prolonged electrolysis or by fractional distillation.

Prolonged Electrolysis of Ordinary Water

This method involves multistage electrolysis of ordinary water containing NaOH. The cell used for electrolysis was designed by Brown, Daggat and Urey. It is a cylindrical vessel made of steel which acts as cathode. The anode is a perforated cylindrical sheet. The electrolysis is carried out in different stage as described below and in actual practice large number of electrolysis cells are used.

Heavy Water

First Stage

In this stage thirty electrolytic cell are used. Each cell is filled with about 3% solution of NaOH. The electrolysis is carried out for about 72 hrs, using a current of 110 volts. The volume reduces to about 1/6th of the original volume taken. The gases evolved ( ¹H₂ and O₂) are discard the volume left contain about 2.5% of heavy water.

Second Stage

This stage involves the electrolysis of residue left from the first stage using 6 electrolytic cells. The gaseous evolved are burnt and water formed is returned to the first stage cell. The residual liquid contains about 12% of heavy water.

Third Stage

This involves the electrolysis of residue of second stage. The content of heavy water is raised to about 60%. The gases evolved are burnt to get water that is fed to 2nd stage cells.

Fourth Stage

This stage involves the electrolysis of residue of third stage and here, nearly 99% of heavy water is obtained. The gases evolved are burnt as usual, and sent to third stage cells.

Fifth stage

The 99% heavy water from fourth stage is made free from alkali and other impurities by distillation and distillate is electrolysis. Here, the gases evolved are D₂ and O₂ which are burnt to get 100% pure water. A flow sheet diagram of the process is shown below in the figure.

Heavy Water

By Fractional Distillation of Ordinary Water

This method involves partial separation of heavy water from ordinary water. Advantage is taken of the small difference in the boiling points of protium oxide (373.2K) and deuterium oxide (374.3K). Since the difference in boiling points is very small, a long fractionating column (about 13 m) is used for distillation and the process is repeated several times. The lighter fraction (H₂O) is distilled first while heavier fraction (D₂O) is left behind. The heavier fraction becomes rich in (D₂O).

Properties of Heavy Water

(i) Heavy water is colourless, tasteless and odourless liquid.

(ii) All physical constants of heavy water are higher than the corresponding values of ordinary water. Some of the physical constants of heavy water are given below in table.

Property	H₂O	D₂O
Molecular mass	18.015	20.028
Melting Point (K)	273.2	276.8
Boiling Point (K)	273.2	374.4
Temperature of maximum Density (K)	277.1	284.4
Maximum Density (g cm^-3)	1.000	1.106
Density (g cm^-3)	0.997	1.104
Heat of Vaporisation (373 K)(kJ//mol^-1)	40.66	41.61
Heat of Fusion (kJ//mol^-1)	6.01	6.28
Specific Heat (Jg^-1K^-1)	4.177	-
Ionization Constant [H⁺][OH^-](mol²L^-2)	1.008 x 10^-14	1.95 x 10^-15
ΔH^°_f(kJmol^-1)	-285.9	- 294.6

Chemical Properties of Heavy Water

Heavy water is chemically similar to ordinary water. The chemical reactions of heavy water are slower than those of ordinary water. Some of the important reactions of heavy water are listed below:

1. Reaction with metals

Alkali metals and alkali earth metals reacts with heavy water to form heavy hydrogen (D₂).

Heavy Water

2. Reaction with metal oxides

(D₂O) reacts slowly with metal oxides to form corresponding deutroxides

Heavy Water

3. Reaction with non-metallic oxides

Non-metallic oxides react with (D₂O) to form corresponding deutro acid,

Heavy Water

4. Reactions with carbides, nitrides, phosphides, arsenide etc.

Heavy Water

5. Electrolysis

A solution of heavy water containing Na₃CO₃, when electrolysed evolve heavy hydrogen at cathode

Heavy Water

Exchange reactions

When compounds having mobile hydrogen react with heavy water, hydrogen is exchanged by deuterium partially or completely.

Heavy Water

Deutero hydrates

Heavy water like ordinary water may be associated with salts as water of crystallization, giving deutro hydrates,

e.g., Heavy Water .

Deuterolysis

Water brings hydrolysis of certain inorganic salts. D₂O gives similar reactions which are termed deuterolysis.

Heavy Water

Biological and Physiological effects

It has been established that heavy water of high concentration retards the growth of plants and animals. It has been confirmed by Lewis that tobacco do not grow in heavy water. Pure heavy water kill small fishes, tadpoles and mice, when fed on with it.

Taylor has shown that heavy water has germicide and bactericide properties. Water containing small quantity of D₂O acts as a tonic and stimulates vegetable growth. Certain moulds have been found to develop better in heavy water in comparison to ordinary water.

Uses of Heavy Water

The following are the important uses of heavy water:

1. As a neutron moderator.

Fission in uranium – 235 is brought in by slow speed neutrons. The substances which are used for slowing down the speed of neutrons are called moderators. Heavy water is used for this purpose in nuclear reactors.

2. For the preparation of deuterium.

Heavy water on electrolysis by its decomposition with metals produces deuterium.

3. As a tracer compound.

Heavy water is commonly used as a tracer compound for studying various reactions mechanisms. It has also used for studying the structure of some oxyacids of phosphorus such as H₃PO₂ and H₃PO₃ as to determine the number of ionisable hydrogen atoms.