Ion Concentrations in Pure Water

Water is perhaps the most common amphoteric substance, and is present as the solvent in most acid or base solutions. Molecules of an amphiprotic substance – like water – can react with one another:

The process in which two molecules of the same substance react to yield ions is called autoionization. Liquid water undergoes autoionization to a very slight extent; at 25 °C, approximately two out of every billion water molecules are ionized. The extent of the water autoionization process is reflected in the value of its equilibrium constant, the ion-product constant for water, Kw:

$$H_2O\;(l)+H_2O\;(l)⇌H_3O^+\;(aq)+OH^-\;(aq)\qquad K_w=[H_3O^+][OH^-]$$

The slight ionization of pure water is reflected in the small value of the equilibrium constant; at 25 °C, Kw has a value of 1.0 × 10−14. The process is endothermic, and so the extent of ionization and the resulting concentrations of hydronium ion and hydroxide ion increase with temperature. For example, at 100 °C, the value for Kw is about 5.6 × 10−13, roughly 50 times larger than the value at 25 °C.

What are the hydronium ion concentration and the hydroxide ion concentration in pure water at 25 °C?

The autoionization of water yields the same number of hydronium and hydroxide ions. Therefore, in pure water, [H3O+] = [OH] = x. At 25 °C:




The hydronium ion concentration and the hydroxide ion concentration are the same, $1.0×10^{-7}\;M$.

Check Your Learning

The ion product of water at 80 °C is $2.4×10^{-13}$. What are the concentrations of hydronium and hydroxide ions in pure water at 80 °C?


[H3O+] = [OH] = $4.9×10^{−7}\;M$

And yet, although the $[H^+]$ of this solution is not $1\times 10^{-7}$, it is still pH neutral. Neutrality is defined as whether there is an excess of either $H^+$ or $OH^-$ in solution, not the actual concentration of either.