Popular LabKit
Buffer Preparation Calculator: Phosphate, Tris, Citrate, HEPES & PBS
Choose from 11 buffer systems – sodium and potassium phosphate, acetate, Tris-HCl, citrate, carbonate, HEPES, MOPS, MES, McIlvaine citrate–phosphate and PBS – then set the pH, concentration and volume. You get the grams of each salt (or mL of acid) to weigh for the exact hydrate on your bottle, corrected for temperature and ionic strength, so results match published buffer tables.
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How it works
pKa at temperature T = pKa(25 °C) + (dpKa/dT) × (T − 25)
pKa′ = pKa(T) + (2z − 1) × 0.509 × ( √I ÷ (1 + √I) − 0.3 I ) (Davies equation)
Mass to weigh = concentration of each form × volume (L) × molecular weight of your hydrate
z is the charge of the acid form and I is the ionic strength, which the calculator works out from the salts in your buffer. Without this correction, a 0.1 M phosphate buffer made by the simple Henderson–Hasselbalch equation comes out about 0.4 pH units off.
| Buffer | pKa at 25 °C | Useful pH range | Change per °C |
|---|---|---|---|
| Sodium or potassium phosphate | 7.20 | 5.8 – 8.0 | −0.003 |
| Acetic acid / sodium acetate | 4.76 | 3.8 – 5.8 | almost none |
| Tris-HCl | 8.07 | 7.0 – 9.0 | −0.028 |
| Sodium bicarbonate / carbonate | 10.33 | 9.2 – 10.8 | −0.009 |
| Citric acid / sodium citrate | 3.13, 4.76, 6.40 | 3.0 – 6.2 | small |
| MES / NaOH | 6.27 | 5.5 – 6.7 | −0.011 |
| MOPS / NaOH | 7.18 | 6.5 – 7.9 | −0.013 |
| HEPES / NaOH | 7.56 | 6.8 – 8.2 | −0.014 |
| Citrate–phosphate (McIlvaine) | table | 2.2 – 8.0 | – |
| PBS (phosphate-buffered saline) | – | 7.4 | – |
How close is it? Checked against published buffer tables
We compared the calculator with the tables in the Sigma-Aldrich Buffer Reference Center. Because it corrects for ionic strength, it lands within about 0.05 pH of the published recipes for phosphate, acetate and Tris, and within about 0.08 pH for citrate (0.04 above pH 4).
| Buffer | Published table | LabKit result |
|---|---|---|
| 0.1 M sodium phosphate pH 7.0 – share of dibasic salt | 61.0 % | 60.6 % |
| 0.1 M sodium phosphate pH 8.0 – share of dibasic salt | 94.7 % | 94.1 % |
| 50 mM Tris pH 8.0 – HCl needed | 29.2 mM | 29.0 mM |
| 0.1 M citrate pH 4.0 – share of trisodium citrate | 41.0 % | 40.0 % |
| 0.1 M citrate pH 5.0 | 65.0 % | 65.7 % |
| 0.1 M citrate pH 6.0 | 88.5 % | 88.1 % |
| Classic ELISA carbonate recipe (1.59 g Na₂CO₃ + 2.93 g NaHCO₃ per L), stated pH 9.6 | pH 9.6 | pH 9.66 |
For comparison, a calculator using the plain Henderson–Hasselbalch equation gives 38.7 % dibasic for 0.1 M phosphate at pH 7.0 – a buffer that actually reads about pH 6.6. McIlvaine buffer uses the published table directly.
Worked examples
0.1 M sodium phosphate, pH 7.0, 1000 mL
Effective pKa 6.81 at ionic strength 0.22 M. Weigh 6.145 g NaH₂PO₄·2H₂O and 10.788 g Na₂HPO₄·2H₂O (or 21.706 g of the dodecahydrate), dissolve in about 800 mL water, check the pH and make up to 1000 mL.
50 mM Tris-HCl, pH 7.5 at 25 °C, 1000 mL
Weigh 6.057 g Tris base, dissolve in about 800 mL water and add about 40.8 mL of 1 M HCl, adjusting to pH 7.5 at the temperature you will use it, then make up to 1000 mL. The same buffer reads about pH 7.2 at 37 °C.
Common mistakes
- Wrong hydrate: Na₂HPO₄·12H₂O weighs more than twice as much as the anhydrous salt for the same buffer. Pick the form printed on your bottle.
- Measuring Tris pH at the wrong temperature: Tris changes about 0.03 pH per °C. Set the pH at the temperature you will use the buffer.
- Adjusting pH after making up to volume: adjust first, then make up to the mark.
- Mixing phosphate with calcium or magnesium: phosphates precipitate with these ions.
- Confusing total and individual concentration: the concentration here is the total of both forms (e.g. 0.1 M phosphate).
Ready-made recipes
Frequently asked questions
Why does my pH meter read slightly different from the target?
Real buffers depend on temperature, ionic strength and reagent purity. The calculator gets you close (usually within 0.05 pH); small final adjustments with dilute acid or base are normal.
Can I use this for pharmacopoeial buffer solutions?
Use it for general laboratory buffers. When a monograph or the Indian Pharmacopoeia gives an exact recipe, follow that recipe as written.
What concentration range is reliable?
The ionic-strength correction works well up to about 0.5 M ionic strength. The calculator warns you above that.
How does the calculator handle citrate and phosphate, which have several pKa values?
For every buffer it solves the full charge balance – all pKa values of the buffer, free H⁺ and OH⁻, and the ionic strength – so recipes stay accurate near the edges of the useful range. If a mixture cannot reach the pH you ask for, it tells you instead of giving a wrong recipe.
Related calculators
Results are for guidance. Verify the pH with a calibrated meter before use, especially in GMP or regulated work.