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Buffer pH calculator

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

Work it out

Fill in all but one. 4 boxes are empty, so there is more than one answer.

A worked example

An acetate buffer, pKa 4.76, with 0.10 M acetate and 0.050 M acetic acid.

pH 5.06

  1. 1The ratio [A⁻]/[HA] is 0.10 / 0.050 = 2.0.
  2. 2log₁₀(2.0) = 0.30.
  3. 3pH = 4.76 + 0.30 = 5.06.

What this assumes

  • Only the ratio matters for pH, but the absolute concentrations decide the buffer capacity — how much acid or base it can absorb before the pH moves.
  • The approximation holds when both concentrations are much larger than the amount that dissociates, which fails for very dilute buffers and for a pH far from the pKa.
  • It ignores activity coefficients, so a high ionic strength shifts the real pH from the calculated one. Check a buffer with a meter before using it.
  • pKa is temperature dependent. Tris is the well-known case: it moves about −0.03 per °C, so a buffer set at room temperature is not the same pH in a cold room.

The terms used here

Buffer
A mixture of a weak acid and its conjugate base that resists pH change when acid or base is added.
pKa
The pH at which an acid is half dissociated. A buffer works best within about one unit of it.
Buffer capacity
How much added acid or base the buffer can absorb. It rises with total concentration and is greatest at the pKa.

Where to next

pH and [H⁺] calculator

Convert between pH, pOH and the ion concentrations.

Molarity calculator

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The other calculators

Molar mass calculator

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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.

pH, pOH and [H⁺] calculator

Convert between pH, pOH, [H⁺] and [OH⁻]. 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.