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Citrate Buffer Calculator

Calculate the masses of citric acid and trisodium citrate for a citrate buffer at a target pH and concentration, using all three pKa values of citric acid.

Formula

pKa1=3.128,  pKa2=4.761,  pKa3=6.396(25 ∘C)\mathrm{p}K_{a1} = 3.128,\ \ \mathrm{p}K_{a2} = 4.761,\ \ \mathrm{p}K_{a3} = 6.396\quad(25\,^\circ\mathrm{C})
counter-ion charge=C nˉH+ lost+[OH−]−[H+]\text{counter-ion charge} = C\,\bar{n}_{\text{H}^+\text{ lost}} + [\mathrm{OH}^-] - [\mathrm{H}^+]
m=C×V×MWform usedm = C \times V \times \mathrm{MW}_{\text{form used}}
nˉH+\bar{n}_{\mathrm{H}^+}
average number of protons removed per citrate at the pH, from the three pKa values
CC
total citrate concentration

How it works

Citric acid has three acidic protons, with pKa values of 3.13, 4.76 and 6.40. Because they lie close together, citrate buffers across a wide range, about pH 3 to 6.2. Mixing citric acid (no protons lost) with trisodium citrate (all three lost) gives any pH in that range; the page finds the proportion from the charge balance using all three pKa values, rather than treating it as a single acid-base pair.

Citric acid is supplied as the anhydrous acid or monohydrate, and trisodium citrate as the dihydrate or anhydrous, which have different molecular weights. Citrate chelates calcium and magnesium, which suits some applications, such as antigen retrieval and anticoagulation, and rules it out for others, such as enzymes that need those metals.

Worked example

1 L of 100 mM citrate buffer at pH 5.0 from citric acid monohydrate and trisodium citrate dihydrate.

  1. The charge balance gives 44.66 mM citric acid and 55.34 mM trisodium citrate.
  2. Trisodium citrate dihydrate: 0.05534 mol × 294.10 g/mol = 16.27 g.
  3. Citric acid monohydrate: 0.04466 mol × 210.14 g/mol = 9.385 g.

16.27 g of trisodium citrate dihydrate and 9.385 g of citric acid monohydrate, made up to 1 L.

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

Assumptions

  • An ideal solution at 25 °C; real buffers drift by a few tenths of a pH unit at this ionic strength.
  • The three pKa values entered are accurate for your conditions.
  • The salts are pure, in the hydrate stated.

Common mistakes

  • Using the recipe of a table made for different stock concentrations.
  • Making the buffer by adding acid to citrate and forgetting to check the final pH with a meter.
  • Using citrate in a reaction that needs free calcium or magnesium.

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