Heavy water and uses

Preparation of heavy water

Heavy water is prepared either by prolonged electrolysis or by fractional distillation of ordinary water.

Electrolysis of ordinary water

Multistage electrolysis of ordinary water containing NaOH gives heavy water. The cell used for electrolysis, contains a cylindrical vessel made of steel as cathode while a perforated cylindrical sheet acts as the anode. The electrolysis is carried out in different stages.

Different stages

First stage

Thirty electrolytic cells are used are used in the first stage. Each cell is filled with about 3% solution of NaOH. Electrolysis is carried out for about 72 hrs using a current of 110 volts till the volume reduces to about l/6th of the original volume taken. Gases like ¹H₂ and O₂ are evolved and discarded. The volume left contains about 2.5% of heavy water.

Second stage

The residue left from the first stage is electrolyzed using six electrolytic cells. The gases evolved are burnt and water formed is returned to the first stage cell. The residual liquid contains about 12% of heavy water.

Fig: 11.6 - Electrolytic cell of heavy water

Third stage

Electrolysis of residue of second stage is carried out in this stage. The gases evolved are burnt to get water that is fed to 2^nd stage cells. The content of heavy water is raised to about 60%.

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 is made free from alkali and other impurities by distillation. The distillate is further electrolysed in the fourth stage. Here, the gases evolved are D₂ and O₂, which are burnt to get 100% pure heavy water.

Fractional distillation of ordinary water

In the partial separation of heavy water from ordinary water, advantage is taken of the small difference in the boiling points of protium oxide (373.2 K) and deuterium oxide (374.3 K). Since the difference in boiling points is very small, a long fractionating column (about 13m) 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

Physical properties

• Heavy water is colourless, tasteless and odourless liquid.

• It has all higher values for physical constants than the corresponding values of ordinary water.

Physical Properties of Water and Heavy Water at 298 K

Chemical Properties of Heavy Water

Although heavy water is chemically similar to ordinary water, chemical reactions of heavy water are slower than those of ordinary water.

Reaction with metals

Alkali metals and alkaline Earth metals react with heavy water to form heavy hydrogen (D₂).

Reaction with metal oxides

D₂O reacts slowly with metal oxides to form corresponding deuteroxides

Reaction with non-metallic oxides

Non-metallic oxides react with D₂O to form corresponding deutero acid,

Reaction with carbides, nitrides, phosphides, arsenides etc.

Electrolysis

A solution of heavy water containing Na₂CO₃, when electrolysed evolve heavy hydrogen at cathode

Exchange reactions

Compounds having labile hydrogen react with heavy water when hydrogen is exchanged by deuterium partially or completely.

Deutero-hydrates

Heavy water like ordinary water may be associated with salts as water of crystallization, giving deutero hydrates, e.g., NaSO₄.10D₂O, CuSO₄.5D₂O, MgSO₄.D₂O, etc.

Deuterolysis

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

Biological and physiological effects

Heavy water of high concentration retards the growth of plants and animals. For example, tobacco seeds do not grow in heavy water. Pure heavy water kills small fishes, tadpoles and mice when fed. Heavy water has germicide and bactericide properties too. 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:

As a neutron moderator

Fission in uranium-235 is brought by slow speed neutron. Heavy water is used for this purpose in nuclear reactors as moderators.

For the preparation of deuterium

Heavy water produces deuterium on electrylosis or by its decomposition with metals.

As a tracer compound

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