Ion product of water

K_w = 1.0×10⁻¹⁴ at 25 °C

unitless (activity-based)

The ion product of water is the equilibrium constant for water's autoionization: K_w = [H+][OH−] = 1.0×10⁻¹⁴ at 25 °C. Taking negative logarithms gives pH + pOH = 14.00, the basis of the pH scale. K_w is strongly temperature-dependent: it increases as temperature rises, so neutral pH is exactly 7.00 only at 25 °C.

Used in

pH calculations
Acid-base equilibria

Source: Standard thermodynamic data at 25 °C