pH and pOH calculator

A logarithmic concentration scale

pH describes hydronium and pOH describes hydroxide on a logarithmic scale. In the dilute ideal solution approximation, each is the negative base ten logarithm of the ion concentration expressed in mol/L. For example, 0.001 mol/L hydronium gives pH 3. If pKw is 14, pOH is 11 and hydroxide concentration is 0.00000000001 mol/L.

Equilibrium and temperature

pH and pOH add up to pKw. Do not assume that their sum is always 14 away from 25 °C. The total acid concentration on a bottle label is not the free hydronium concentration. Weak acid equilibria, buffers, ionic strength and activity coefficients are outside this conversion. Neutral pH equals half of pKw.

Follow the powers in both directions

Choose example pH 5 and user supplied pKw 14. Hydronium concentration is 10⁻⁵ = 0.00001 mol/L. The pOH is 14 − 5 = 9 and hydroxide concentration is 10⁻⁹ = 0.000000001 mol/L. Entering the same ion concentration in the corresponding mode recovers the initial pH. The pKw here is an example input, not a constant for every temperature.

On this logarithmic scale, lowering pH by one corresponds to a tenfold increase in hydronium concentration under the stated approximation. Adding pH values and dividing by volume therefore does not calculate mixture pH. Mixing requires a mole balance and chemical equilibrium where applicable.

Ion concentration cannot be zero because the logarithm of zero is not finite. A measurement below detection is not a measured zero concentration. If the input is total acid concentration, first establish free ion concentration with an appropriate equilibrium calculation. Acidic or basic classification depends on the chosen pKw. This tool does not assess corrosiveness, toxicity or safe use from pH alone.

Frequently asked questions

How do you calculate pH from hydrogen ion concentration?

For an example hydronium concentration of 0.001 mol/L, taking the negative base ten logarithm of concentration gives a pH of 3. This preliminary calculation assumes a dilute ideal solution, and total acid concentration is not necessarily free ion concentration, while activity coefficients and weak acid equilibria are excluded.

Calculate with these values