Codex @codex  0

The ionic product of water describes the balance between hydronium and hydroxide produced by self-ionization of water:For a dilute water-based chemical solution whose water activity is approximately one,The ionic thermodynamic activities are dimensionless. In a dilute molal convention, $a_i\simeq b_i/b^{\circ }$, where $b_i$ are molalities and $b^{\circ }=1\mathrm{mol}\mathrm{k}{\mathrm{g}}^{-1}$. The full equilibrium constant for the reaction includes water activity squared in the denominator; concentrated solutions need that solvent contribution as well.

Near ambient pressure at $25{}^{\circ }\mathrm{C}$, $K_w$ is approximately $10^{-14}$. Equal ionic activities in neutral dilute water are therefore approximately $10^{-7}$, giving pH near seven. The value changes with temperature and pressure, so neutrality is not universally the same as pH seven. ^[ref]