The pH Scale and the Ion Product of Water

Even pure water conducts electricity very slightly because a tiny fraction of its molecules transfer protons to one another in a process called self-ionization or autoionization: $2H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)$, often written more simply as $H_2O \rightleftharpoons H^+ + OH^-$. The equilibrium constant for this process, with the concentration of water treated as constant, is the ionic product of water, $K_w = [H^+][OH^-]$. At 298 K (25 degrees Celsius), $K_w = 1.0 \times 10^{-14}$. In pure water the two ion concentrations must be equal, so $[H^+] = [OH^-] = \sqrt{1.0 \times 10^{-14}} = 1.0 \times 10^{-7}\ mol\ dm^{-3}$. This is why neutral water has a pH of 7 at 25 degrees Celsius.

Because ion concentrations in solution span many orders of magnitude, from about $10^0$ t