IONIC PRODUCT OF WATER
Although pure water is often considered as a non-conductor of electricity, precise measurements show that it conducts electricity to a little extent. This conductivity of water has resulted from the self-ionisation of water. Self-ionisation or auto ionisation is a reaction in which two like molecules react to give ions. In the process of ionisation of water, a proton from one water molecule is transferred to another water molecule leaving behind an OH— ion. The proton gets dissolved in water forming the hydronium ion as shown in the following equation:
The hydronium ion formed is a strong acid and the hydroxyl ion is a strong base. So as fast as they are formed, they react again to produce water. Thus, it is a reversible reaction and attains equilibrium very quickly. So, the extent of ionisation is very little and the concentration of the ions produced is also very less. The product of the concentration of the hydronium ion and the hydroxyl ion is called ‘ionic product of water’. It is denoted as ‘Kw’. It is mathematically expressed as follows:
Kw = [H3O+] [OH−]
[H3O+] may be simply written as [H+]. Thus the ionic product of water may also be expressed as
Kw = [H+] [OH−]
Its unit is mol2 dm−6. At 25° C, its value is 1.00 × 10−14.