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Autoionization (Self-ionization) of Water

The autoionization or self-ionization of water is an endothermic ionization reaction where a molecule of pure water gets dissociated into hydrogen ions (H+) and hydroxyl ions (OH-). As water is a very weak electrolyte, it gets ionized minimally.

Autoionization Chemical Equation

When two water molecules collide, a hydrogen ion from one molecule gets transferred to the other, producing a positively charged hydronium ion (H3O+) and a negatively charged hydroxyl ion.

H2O (l) + H2O (l) ⇌ H3O+ (aq.) + OH (aq.)

We commonly use the simplified form of the reaction:

H2O (l) ⇌ H+ (aq.) + OH(aq.)

Autoionization of Water

Acid-Base Properties of Water

As it is amphoteric, water can act as both an acid and a base in solution. When mixed with a base, water donates hydrogen ions and acts as an acid. However, when water is mixed with an acid, it accepts hydrogen ions and acts as a base.

  • Acting as a base:

H2O (l) + CH3COOH (aq.) ⇌ H3O+ (aq.) + CH3COO (aq.)

  • Acting as an acid:

NH3 (aq.) + H2O (l) ⇌ NH4+ (aq.) + OH (aq.)

During autoionization, water can be both an acid and a base to itself.

H2O (l) + H2O (l) ⇌ H3O+ (aq.) + OH (aq.)

Autoionization Constant

As mentioned before, H2O splits into H+ and OH.

Kc = [H+] [OH]/[H2O]

Kc is the dissociation constant of water at equilibrium.

As pure water is a poor electrolyte, it gets slightly ionized. So, the concentration of H2O is much greater than the concentrations of H+ and OH. Therefore, [H2O] can be regarded as constant.

Kc [H2O] = [H+][OH]

As Kc and [H2O] are constants, they are replaced by another constant, ‘Kw’.

Kw = [H+][OH]

Kw is called the autoionization constant of water.

From the above explanation, ‘Kw’ can be regarded as the ionic product of water, i.e., the product of the molar concentrations of H+ and OH ions. As H+ ions exist as H3O+ ions in water, the ionic product can alternatively be stated as the product of molar concentrations of H3O+ and OH ions.

Kw = [H3O+][OH]

Relationship Among pH, pOH, and pKw

The autoionization constant is temperature-dependent and increases with an increase in temperature. It has a fixed value of 1×10-14 at 25 ͦC.

The concentrations of dissociated H+ and OH ions are equal. So, let the [H+] and [OH] be ‘x’.

By substituting the values in the equilibrium expression, we get,

[H+] [OH] = 1 x 10-14

=> x . x = 1 x 10-14

=> x2 = 1 x 10-14

=> x = 1 x 10-7

Therefore, [H+] = [OH] = 1 x 10-7

Taking negative logarithm on both sides of [H+] equation:

-log [H+] = -log (10-7)

=> pH = – (-7)

=> pH = 7 [ Since, pH = -log [H+]]

Similarly, pOH will also be 7. This result is expected since the pH of pure water is 7.

Now, taking the negative logarithm of Kw

-log (Kw) = -log (1×10-14)

=> pKw = – (-14) = 14

Therefore, at 25 ͦC,

pKw = pH + pOH


  1. Water autoionization and Kw –
  2. Autoionization of Water –
  3. Self-Ionization of Water and the pH

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