Ionic Product of Water:

Pure water is a very weak electrolyte and ionises according to the equation

H₂O ↔ H⁺ + OH^-

Applying law of mass action at equilibrium, the value of dissociation constant, K comes to

K = [H⁺] [OH^-]/[H₂O]

or [H⁺][OH^-] = K[H₂0]

Since dissociation takes place to a very small extent, the concentration of undissociated water molecules, [H₂0], may be regarded as constant. Thus, the product #[H₂0] gives another constant which is designated as K_w. So,

[H⁺][OH^-] = K_w

The constant, K_w, is termed as ionic product of water.

The product of concentrations of H1 and OH ions in water at a particular temperature is known as ionic product of water. The value of K_w increases with the increase of temperature, i.e., the concentration of H⁺ and OH^- ions increases with increase in temperature.

Temperature (°C) Value of K_w

0 0.11 x 10^-14

10 0.31 x 10^-14

25 1.00 x 10^-14

100 7.50 x 10^-14

The value of K_w at 25°C is 1 x 10^-14. Since pure water is neutral in nature, H⁺ ion concentration must be equal to OH^- ion concentration.

[H⁺] = [OH˜] = x

or [H⁺][OH^-]=x²= 1 x 10^-14

or x = 1 x 10^-7 M

or [H⁺] = [OH^-] = 1 × 10^-7 mol litre^-1

This shows that at 25°C, in 1 litre only 10^-7 mole of water is in ionic form out of a total of approximately 55.5 moles.

When an acid or a base is added to water, the ionic concentration product, [H⁺][OH^-], remains constant, i.e., equal to K_w but concentrations of H⁺ and OH^- ions do not remain equal. The addition of acid increases the hydrogen ion concentration while that of hydroxyl ion concentration decreases, i.e.,

[H⁺] > [OH^-]; (Acidic solution)

Similarly, when a base is added, the OH^- ion concentration increases while H⁺ ion concentration decreases,

i.e., [OH^-] > [H⁺]; (Alkaline or basic solution)

In neutral solution, [H⁺] = [OH^-] = 1 x 10^-7 M

In acidic solution, [H⁺] > [OH^-]

or [H⁺] > 1 x 10^-7 M

and [OH^-] < 1 x 10^-7 M

In alkaline solution, [OH^-] > [H⁺]

or [OH^-] > 1 × 10^-7 M

and [H⁺] < 1 x 10^-7 M