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Acetate Buffer Calculator

Calculate the sodium acetate mass and acetic acid volume for an acetate buffer at a target pH and concentration, with a choice of acetate hydrate.

Formula

pH=pKa+log⁡10[acetate][acetic acid],pKa=4.756\mathrm{pH} = \mathrm{p}K_{\mathrm{a}} + \log_{10}\dfrac{[\text{acetate}]}{[\text{acetic acid}]},\quad \mathrm{p}K_{\mathrm{a}} = 4.756
[Na+]=C αacetate+[OH−]−[H+](charge balance)[\mathrm{Na}^+] = C\,\alpha_{\text{acetate}} + [\mathrm{OH}^-] - [\mathrm{H}^+]\quad(\text{charge balance})
pKa\mathrm{p}K_{\mathrm{a}}
4.756 at 25 °C
CC
total acetate concentration, acid plus base

How it works

Acetate buffers cover about pH 3.8 to 5.8, centred on acetic acid's pKa of 4.76. They are used for nucleic acid precipitation, in some enzyme assays and in protein purification at low pH. The buffer is made from sodium acetate, which is the base form, and acetic acid; the page finds the amounts from the charge balance, which is exact for an ideal solution.

Acetic acid is a liquid, so its amount is given as a volume, assuming glacial acetic acid at about 17.4 M; check the concentration on your bottle. Sodium acetate is supplied as the trihydrate or the anhydrous salt, with different molecular weights.

Worked example

1 L of 100 mM acetate buffer at pH 5.2, from sodium acetate trihydrate and glacial acetic acid.

  1. At pH 5.2 the buffer is 73.54 mM acetate and 26.46 mM acetic acid.
  2. Sodium acetate trihydrate: 0.07354 mol × 136.08 g/mol = 10.01 g.
  3. Acetic acid: 0.02646 mol / 17.4 mol/L = 1.52 mL.

10.01 g of sodium acetate trihydrate and 1.52 mL of glacial acetic acid, in a litre.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • An ideal solution at 25 °C.
  • Glacial acetic acid at 17.4 M.
  • The salts are pure and the hydrate is as stated.

Common mistakes

  • Using the anhydrous weight for sodium acetate trihydrate.
  • Measuring glacial acetic acid without gloves and ventilation; it is corrosive.
  • Using acetate above pH 6, where it has almost no capacity.

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