pH / pOH Calculator
Calculate pH, pOH, [H⁺], and [OH⁻] from any input — includes strong/weak acid and base modes, Henderson-Hasselbalch buffer, and Ka/Kb presets for common compounds
Inputs
pH Scale
Common Substances (click to set pH)
About pH and pOH
pH is the negative base-10 logarithm of the hydrogen ion activity (approximately concentration) in a solution:pH = −log₁₀[H⁺].pOH is analogous for hydroxide: pOH = −log₁₀[OH⁻].
At 25 °C, water's autoionization constant Kw = [H⁺][OH⁻] = 10⁻¹⁴, so pH + pOH = 14. Pure water has pH 7 (neutral). Values below 7 are acidic; above 7 are basic/alkaline.
Formulas Used
pH = −log₁₀[H⁺][H⁺] = 10⁻ᵖᴴ- Strong acid:
[H⁺] = C - Weak acid:
[H⁺] ≈ √(Ka·C) - Buffer:
pH = pKa + log([A⁻]/[HA])
Assumptions
- All values at 25 °C (Kw = 10⁻¹⁴)
- Weak acid/base approximation valid when C/Ka > 100
- Ignores ionic strength / activity corrections
- Strong acids assumed fully dissociated
Educational tool. For analytical chemistry work, use activity coefficients and exact ICE-table solutions where required.
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