pH, pOH, [H₃O⁺] & [OH⁻] calculator

pH expresses acidity on a logarithmic scale. In the concentration approximation commonly used in general chemistry, pH = −log₁₀[H₃O⁺] and pOH = −log₁₀[OH⁻]. At 25 °C the ion-product constant of water is Kᵥ = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴, giving pH + pOH = 14.00. This calculator starts from any one of those four quantities and computes the other three, including unit conversion for the ion concentrations.

When pH is known, hydronium concentration is obtained from [H₃O⁺] = 10⁻ᵖᴴ. Conversely, a known [H₃O⁺] in mol/L gives pH = −log₁₀[H₃O⁺]. The same pair of inverse relationships applies to pOH and hydroxide concentration.

At 25 °C, OpenStax gives Kᵥ = 1.0 × 10⁻¹⁴ and pKᵥ = 14.00. Therefore pOH = 14.00 − pH, and [OH⁻] follows from 10⁻ᵖᴼᴴ. The number 14 is temperature-dependent rather than universal.

The rigorous thermodynamic definition from IUPAC uses hydrogen-ion activity. Replacing activity by concentration is a useful approximation for sufficiently dilute solutions. High ionic strength or very concentrated solutions can produce meaningful deviations.

The calculator does not impose an artificial 0–14 limit. Values below 0 or above 14 can occur, particularly in concentrated systems, although the simple concentration model becomes less reliable there.

Concentrations can be entered as µmol/L, mmol/L, mol/L or mol/m³. The shared unit engine converts them to mol/L for the logarithmic expressions and converts output back to the selected unit.

At 25 °C a solution is acidic below pH 7, neutral at pH 7 and basic above pH 7. Neutral pH is actually pKᵥ/2, so it changes when water temperature changes.

This tool converts among equilibrium quantities that are already known. It does not solve weak-acid, weak-base or buffer equilibria from analytical concentration alone; those calculations need equilibrium constants such as Ka or Kb.