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Acids & bases · Chemistry

pH, pOH & Ion Concentration Calculator

Convert among pH, pOH, hydrogen-ion concentration, and hydroxide-ion concentration at 25 °C.

Formula shown Runs locally Reviewed Aug 11, 2026
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Equation guide

Dilute aqueous solutions at 25 °C

Hydrogen ion definition

pH = −log₁₀[H⁺]

Hydroxide ion definition

pOH = −log₁₀[OH⁻]

Water relation

pH + pOH = 14
Acid–base calculator

Start with one known value

The relation pH + pOH = 14 and Kw = 1.0 × 10⁻¹⁴ apply to dilute aqueous solutions at 25 °C.

pH ⇄ pOH

All four acid–base values will appear here.

Using the model

Symbols, assumptions and limitations

Interpretation notes

The pH + pOH = 14 relationship assumes dilute aqueous solution at 25 °C.

Worked examples

See the method in practice

01

Acidic solution

pH 3 corresponds to [H⁺] = 1 × 10⁻³ mol/L.

02

Basic solution

pOH 2 corresponds to pH 12 and [OH⁻] = 1 × 10⁻² mol/L.

Questions

Frequently asked

Can pH be below 0 or above 14?

Yes in concentrated solutions, although the simple concentration model becomes less accurate because activities matter.

Is neutral pH always 7?

No. Neutrality means [H⁺] = [OH⁻]; the corresponding pH changes with temperature.

Sources & review

Equations and examples are checked against the references below. Results are educational and should be independently verified for safety-critical work.

OpenStax Chemistry — pH and pOH

Written by the STEM Hub editorial team · Reviewed August 11, 2026 · Review methodology