pH Calculator

Calculate pH, pOH, hydrogen-ion concentration or hydroxide-ion concentration at 25 °C.

Calculator

Choose one known value. This educational tool uses pH + pOH = 14 at 25 °C.

Known value
pH7
pOH7
[H⁺]1.000e-7 mol/L
[OH⁻]1.000e-7 mol/L

Neutral

Formula and assumptions

pH = −log₁₀[H⁺]; pH + pOH = 14

The tool derives the other displayed values from one entered value using the common aqueous 25 °C relationship. In rigorous chemistry, pH is defined using hydrogen-ion activity rather than concentration.

Assumption: The common aqueous relationship pH + pOH = 14 is used at 25 C.

Variables and units

  • pH — negative base-10 logarithm of hydrogen-ion concentration
  • [H+] — hydrogen-ion concentration (mol/L)
  • [OH-] — hydroxide-ion concentration (mol/L)

Worked example

For [H⁺] = 1 × 10⁻⁷ mol/L, pH = 7.00 in this idealised calculation.

Important limits

Rigorous pH is based on activity, not simply concentration; this is not a buffer or equilibrium model.

Reference and related tools

IUPAC: pH. Return to Chemistry Calculators for the other tools in this group.