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PBS Recipe Calculator

Calculate the masses of NaCl, KCl, Na₂HPO₄ and KH₂PO₄ for phosphate-buffered saline (137 mM NaCl, pH 7.4) as a 1×, 10× or 20× stock, with a choice of hydrate.

Formula

1× PBS: 137 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4, 1.8 mM KH2PO41\times\ \mathrm{PBS}:\ 137\ \mathrm{mM\ NaCl},\ 2.7\ \mathrm{mM\ KCl},\ 10\ \mathrm{mM\ Na_2HPO_4},\ 1.8\ \mathrm{mM\ KH_2PO_4}
m=C×s×V×MW,s=strengthm = C \times s \times V \times \mathrm{MW},\quad s = \text{strength}
strength\text{strength}
1 for working PBS, 10 or 20 for a concentrated stock
MW\mathrm{MW}
molecular weight of the salt as supplied, hydrate included

How it works

PBS is the most common isotonic buffer in biology. The standard recipe has 137 mM NaCl, 2.7 mM KCl, 10 mM disodium hydrogen phosphate and 1.8 mM potassium dihydrogen phosphate at pH 7.4. It is derived from the balanced salt solution of Dulbecco and Vogt.

The calculator gives the mass of each salt for the volume and strength you choose, and accounts for the hydrate of the dibasic salt. The pH is set by measurement, not by calculation: as weighed, the ideal pH from the phosphate ratio is about 7.9, but at the ionic strength of PBS (about 0.17 M) phosphate's apparent pKa₂ is much lower, and the solution reads close to 7.4. Check it with a meter and adjust with HCl.

Worked example

1 L of 1× PBS from anhydrous salts.

  1. NaCl: 0.137 mol × 58.44 g/mol = 8.006 g.
  2. KCl: 0.0027 mol × 74.55 g/mol = 0.2013 g.
  3. Na₂HPO₄: 0.010 mol × 141.96 g/mol = 1.42 g.
  4. KH₂PO₄: 0.0018 mol × 136.08 g/mol = 0.245 g.

8.006 g NaCl, 0.2013 g KCl, 1.42 g Na₂HPO₄ and 0.245 g KH₂PO₄ per litre: the familiar 8 g, 0.2 g, 1.4 g and 0.24 g recipe.

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

Assumptions

  • The standard PBS composition without calcium or magnesium.
  • The pH is finished by measurement and adjusted with HCl.
  • Ionic strength and pKa effects are not included in the masses, which are straightforward.

Common mistakes

  • Using the heptahydrate's molecular weight for the anhydrous salt, or the reverse.
  • Checking the pH of a 10× stock instead of the diluted 1× solution. They differ.
  • Adding calcium or magnesium to a concentrated phosphate stock, which precipitates.

Related tools

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Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.