Heavy Water

Heavy Water:

Heavy water (D₂O) is the oxide of heavy hydrogen. One part of heavy water is present in 5000 parts of ordinary water. It is mainly obtained as the product of electrolysis of water, as D₂O does not undergo electrolysis as easily as H₂O.

D₂O is a colorless, odorless and tasteless liquid. However, there is a marked difference between physical properties of water and heavy water as shown in Table 4.2.

Hard  water  produces less foam with detergents. Do you know why?

The cleaning capacity of soap is reduced when used in hard water. Soaps are sodium or potassium salts of long chain fatty acids (e.g., coconut oil). When soap is added to hard water, the divalent magnesium and calcium ions present in hard water react with soap. The sodium salts present in soaps are converted to their corresponding magnesium and calcium salts which are precipitated as scum/precipitate.

Chemical properties of heavy water:

When compounds containing hydrogen are treated with D₂O, hydrogen undergoes an exchange for deuterium

2NaOH + D₂O → 2NaOD + HOD

HCl + D₂O → DCl + HOD

NH₄Cl + 4D₂O → ND₄Cl + 4HOD

These exchange reactions are useful in determining the number of ionic hydrogens present in a given compound.

For example, when D₂O is treated with of hypo-phosphorus acid only one hydrogen atom is exchanged with deuterium. It indicates that, it is a monobasic acid.

H₃PO₂ + D₂O → H₂DPO₂ + HDO

It is also used to prepare some deuterium compounds:

Al₄C₃ + 12D₂O → 4Al(OD)₃ + 3CD₄

CaC₂ + 2 D₂O → Ca(OD)₂ + C₂D₂

Mg₃N₂ + 6D₂O → 3Mg(OD)₂ + 2 ND₃

Ca₃P₂ + 6D₂O → 3Ca(OD)₂ + 2PD₃

Uses of heavy water:

1. Heavy water is widely used as moderator in nuclear reactors as it can lower the energies of fast neutrons

2. It is commonly used as a tracer to study organic reaction mechanisms and mechanism of metabolic reactions

3. It is also used as a coolant in nuclear reactors as it absorbs the heat generated.