Water ion product constant???

[ldv1452]

Conceptually, why does this constant exist so that the products of hydronium and hydroxide ions are always in a constant ration? Why does this not fluctuate?

[Borek]

It is not different from any other equilibrium constant, just water concentration is assumed to be constant at 55.55M.

Water ion product (http://www.chembuddy.com/?left=pH-calculation&right=water-ion-product).