A measure of Concentration of Hydrogen ions in a solution. It is defined as:

$\text{pH} = -\log{[\text{H}^+]}$

where $[\text{H}^+]$ is in units of Molarity. Thus pH = 7, refers to $[\text{H}^+] = 10^{-7}$ M.

The pH also indirectly expresses the concentration of Hydroxyde ions, in an aqueous solution. This is because Water molecules dissociate to form H⁺ and OH^- in a process:

$H_2 O \rightleftharpoons H^+ + OH^-$

The Equilibrium constant for this dissociation is

$K = \frac{[H^+][OH^-]}{[H_2 O]}$

and has a value of $K = 1.8 \times 10^{-16}$, where the concentrations are in molarity units.

The concentration of water in pure water, $[H_2 O]$, is 55.5 M, and is constant under most conditions. We can then define a new constant:

$K_W = K [H_2 O] = [H^+][OH^-]$ $= 10^{-14}$,

so

$[OH^-] = \frac{10^{-14}}{ [H^+]}$

See also Acid-base equilibria

• Acid
• Base

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