MolDraw › Chemistry tools › Buffer Preparation Calculator
Buffer Preparation Calculator
In short: Get a lab recipe for a buffer of any pH, volume and concentration: grams of each salt (with the right hydrate), volumes of two stock solutions, or one reagent plus NaOH or HCl. It is free, works in your web browser and needs no download.
Choose from 12 common buffers with pKa values at 25 °C. The calculator corrects pKa for temperature, can correct for ionic strength (Davies equation), and checks the recipe against a full equilibrium calculation. For the bare pH ↔ ratio equation, use the Henderson–Hasselbalch calculator.
Buffer pKa values at 25 °C
| Buffer (acid form / base form) | pKa, 25 °C | dpKa/dT per °C | Useful pH | Acid-form reagents, g/mol | Base-form reagents, g/mol |
|---|---|---|---|---|---|
| Acetate (acetic acid / sodium acetate) | 4.76 | −0.0002 | 3.8–5.8 | C2H4O2 60.05 | C2H3NaO2 82.03; C2H3NaO2·3H2O 136.08 |
| Citrate pKa₁ (citric acid / monosodium citrate) | 3.13 | −0.0024 | 2.1–4.1 | C6H8O7 192.12; C6H8O7·H2O 210.14 | C6H7NaO7 214.11 |
| Citrate pKa₂ (monosodium / disodium citrate) | 4.76 | −0.0013 | 3.8–5.8 | C6H7NaO7 214.11 | C6H6Na2O7·1.5H2O 263.11 |
| Citrate pKa₃ (disodium / trisodium citrate) | 6.40 | +0.002 | 5.4–7.4 | C6H6Na2O7·1.5H2O 263.11 | C6H5Na3O7·2H2O 294.10; C6H5Na3O7 258.07 |
| MES (MES free acid / MES sodium salt) | 6.27 | −0.011 | 5.5–6.7 | C6H13NO4S 195.23; C6H13NO4S·H2O 213.25 | C6H12NNaO4S 217.22 |
| Sodium phosphate pKa₂ (NaH₂PO₄ / Na₂HPO₄) | 7.20 | −0.0028 | 6.2–8.2 | NaH2PO4·H2O 137.99; NaH2PO4·2H2O 156.01; NaH2PO4 119.98 | Na2HPO4 141.96; Na2HPO4·2H2O 177.99; Na2HPO4·7H2O 268.06; Na2HPO4·12H2O 358.14 |
| Potassium phosphate pKa₂ (KH₂PO₄ / K₂HPO₄) | 7.20 | −0.0028 | 6.2–8.2 | KH2PO4 136.08 | K2HPO4 174.17; K2HPO4·3H2O 228.22 |
| MOPS (MOPS free acid / MOPS sodium salt) | 7.18 | −0.011 | 6.5–7.9 | C7H15NO4S 209.26 | C7H14NNaO4S 231.24 |
| HEPES (HEPES free acid / HEPES sodium salt) | 7.56 | −0.014 | 6.8–8.2 | C8H18N2O4S 238.30 | C8H17N2NaO4S 260.28 |
| Tris (Tris-HCl / Tris base) | 8.07 | −0.028 | 7.1–9.1 | C4H12ClNO3 157.59 | C4H11NO3 121.14 |
| Glycine pKa₂ (glycine / sodium glycinate) | 9.78 | −0.025 | 8.8–10.8 | C2H5NO2 75.07 | C2H4NNaO2 97.05 |
| Carbonate pKa₂ (NaHCO₃ / Na₂CO₃) | 10.33 | −0.009 | 9.3–11.3 | NaHCO3 84.01 | Na2CO3 105.99; Na2CO3·10H2O 286.14 |
pKa: thermodynamic values (ionic strength 0) at 25 °C from Goldberg, Kishore & Lennen, J. Phys. Chem. Ref. Data 2002, 31, 231, rounded to two decimals. Other pKa values used in the full-equilibrium check: phosphoric acid 2.15 and 12.35, citric acid 3.13, 4.76 and 6.40, glycine 2.35, carbonic acid 6.35 (apparent, includes dissolved CO₂). dpKa/dT: Good et al., Biochemistry 1966, 5, 467 and the same review; approximate. Molar masses from IUPAC standard atomic weights.
How the recipe is calculated
r = [A⁻]/[HA] = 10^(pH − pKa) [A⁻] = C·r/(1 + r) [HA] = C/(1 + r) grams = mol × M(form incl. water of crystallisation)
pKa(T) = pKa(25 °C) + (dpKa/dT)·(T − 25) Davies: pKa′ = pKa − A·(z_A² − z_HA²)·(√I/(1 + √I) − 0.3·I), A ≈ 0.51 at 25 °C
Henderson–Hasselbalch splits the total concentration C between the acid and base forms of the chosen pair. Full equilibrium solves the charge balance with every pKa of the acid plus H⁺ and OH⁻, so it stays right for dilute buffers, far from the pKa, and for citrate, whose three pKa values overlap. Whichever method you pick, the result panel shows the pH the full equilibrium predicts for the recipe and warns when the two differ by more than 0.05.
Ionic strength lowers the apparent pKa of acids whose base form carries more charge. The effect is largest for phosphate (HPO₄²⁻): about −0.4 at 0.1 M. Tris works the other way (TrisH⁺ → Tris), so its apparent pKa rises slightly. The Davies equation is a fair estimate up to about I = 0.5 M; above that, trust only the pH meter.
Temperature matters most for amine buffers. Tris changes by −0.028 per °C, so a Tris buffer set to pH 8.00 at 25 °C is about pH 7.66 at 37 °C and 8.59 at 4 °C. Acetate and phosphate barely change.
How to prepare a buffer
- Pick a buffer with pKa within 1 unit of your target pH. The calculator warns if it is not.
- Enter the target pH, final volume, total concentration and the temperature you will use the buffer at.
- Choose Weigh two solids (pick the hydrates on your bottles), Mix two stocks, or One reagent + NaOH / HCl.
- Dissolve in about 80% of the final volume, check the pH with a calibrated meter at the working temperature, adjust with a little acid or base, then make up to volume.
Limitations
- The recipe is a starting point. Real pH depends on reagent purity, water content of hygroscopic salts, CO₂ uptake and the electrode, so always finish with a pH meter.
- Temperature correction is linear in dpKa/dT and applies only to the pKa of the chosen pair. It is reasonable between about 4 and 50 °C.
- Activity correction uses the Davies equation and counts zwitterionic buffers (MES, MOPS, HEPES, glycine) as uncharged. Added salt (NaCl, KCl) is not included; at high salt the pH shifts more.
- Volumes are not corrected for the volume the solids take up: always make up to the mark in a volumetric flask.
- For simple pH, pKa or ratio questions without a recipe, the Henderson–Hasselbalch calculator is quicker. To weigh a single compound, use the solution prep calculator.
Diluting a stock buffer? Use C1V1 = C2V2 for one step, or plan a whole series of tubes.
Serial dilution calculatorFrequently asked questions
How many grams do I need for 1 L of 0.1 M acetate buffer at pH 4.76?
At pH = pKa = 4.76 the acid and base are equimolar: 0.050 mol each. That is 3.00 g acetic acid (2.86 mL glacial) and 4.10 g anhydrous sodium acetate, or 6.80 g sodium acetate trihydrate, made up to 1 L.
Why does the hydrate form matter?
Water of crystallisation adds mass. Na2HPO4 is 141.96 g/mol but Na2HPO4·7H2O is 268.06 g/mol, so weighing the wrong form gives a buffer almost half as concentrated and at the wrong pH. Pick the form printed on your bottle.
Why is my Tris buffer pH different at 37 °C?
The pKa of Tris falls by about 0.028 per °C. A Tris buffer set to pH 8.00 at 25 °C is about pH 7.66 at 37 °C and pH 8.59 at 4 °C. Set the pH at the temperature you will use it.
Why does my phosphate buffer come out at a lower pH than calculated?
Tabulated pKa values are for zero ionic strength. In 0.1 M phosphate the apparent pKa2 is about 6.8, not 7.2, because the doubly charged HPO4 2− ion is strongly affected by ionic strength. Tick the Davies correction for a better first estimate, then adjust with a pH meter.
What pH range can a buffer cover?
About pKa ± 1. Outside that range one form is more than 90% of the total and the buffer capacity is low. The calculator warns you and suggests a buffer with a closer pKa.
Last updated: