Hydronium - Determination of PH

Determination of PH

It is the presence of hydronium ion relative to hydroxide that determines a solution's pH. Water molecules auto-dissociate into hydronium and hydroxide ions in the following equilibrium:

2 H₂O OH− + H₃O+

In pure water, there is an equal number of hydroxide and hydronium ions. At 25 °C and atmospheric pressure their concentrations are approximately equal to 1.0 × 10−7 mol∙dm−3 (or mol/L). For these conditions, = 10−pH so pH = 7 is defined as neutral. A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution.