pH is a measure of the activity of H₃O⁺ (H+) ions in a solution, expressing its acidity or basicity. In aqueous systems, if a pH value < 7 is considered acidic, a pH value > 7 is considered alkaline.

The pH value is determined according to the following formula:

• Calculate the pH of an acidic solution: determine the molar concentration of H+ ions in the solution at equilibrium.
  pH = -log [ H+ ]
• Calculate the pH of a basic solution: determine the molar concentration of OH- ions in the solution at equilibrium
  pH = 14 – pOH = 14 + lg([OH-])
• Formula to remember more:
  [H+][OH−] = 10-14

In there:
H+ represents the activity of H+ ions (hidronium ions) measured in units of mol/l. In dilute solutions such as river water, lake water, tap water, swimming pool water, the index will be approximately equal to the concentration of H+ ions.
OH- is an expression of the activity of OH- ions (hydroxide ions) measured in units of mol/l.
Log is logarithm base 10