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pH and hydrogen ion concentration

Convert between pH, pOH, [H⁺] and [OH⁻]. Leave the unknown blank.

Work it out

Enter any one of the four and the rest follow.

A worked example

A solution has [H⁺] = 3.2 × 10⁻⁵ M.

pH 4.49

  1. 1pH = −log₁₀(3.2 × 10⁻⁵) = 4.49.
  2. 2pOH = 14 − 4.49 = 9.51.
  3. 3[OH⁻] = 10⁻⁹·⁵¹ = 3.1 × 10⁻¹⁰ M.

What this assumes

  • pH + pOH = 14 holds at 25 °C. Kw rises with temperature, so at 50 °C the sum is about 13.3 — neutral water is then pH 6.6 and still neutral.
  • This works with concentrations. Strictly pH is defined by activity, which differs noticeably in concentrated or high-salt solutions.
  • For a weak acid, [H⁺] is not the acid's concentration. Use the buffer calculator or an ICE table.

The terms used here

pH
The negative base-10 logarithm of the hydrogen ion concentration.
Kw
The ion product of water, [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C.
Neutral
[H⁺] = [OH⁻]. That is pH 7 only at 25 °C.

Where to next

Buffer pH calculator

A weak acid and its conjugate base, by Henderson–Hasselbalch.

Molarity calculator

Prepare a solution at a known concentration.

The other calculators

Molar mass calculator

Enter a formula and get its molar mass, broken down by element.

Molarity calculator

Relate concentration, volume and moles. Leave the unknown blank.

Dilution calculator (C₁V₁ = C₂V₂)

C₁V₁ = C₂V₂. Leave the unknown blank.

Percent yield calculator

Actual over theoretical, as a percentage. Leave the unknown blank.

Beer–Lambert law calculator

A = εlc. Leave the unknown blank.

Buffer pH calculator (Henderson–Hasselbalch)

pH = pKa + log([A⁻]/[HA]). Leave the unknown blank.

This calculator runs in your browser. Nothing you type is sent anywhere, and no account is needed. If you want to go further than arithmetic — look a structure up, see what a reaction has been recorded as giving, or follow a mechanism — the library is open too.