Buffer pH Calculator
Calculate the pH of a buffer from the pKa and the concentrations of the weak acid and its conjugate base, see what adding strong acid or base does, or get the ratio and amounts you need to make a buffer at a target pH.
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Use a negative number for strong acid (e.g. −0.01 for 0.01 M HCl)
[HA] + [A⁻]
How to use the Buffer pH Calculator
- Choose the mode: pH from concentrations, or a recipe for a target pH.
- Pick the buffer system or enter a custom pKa.
- Enter the acid and conjugate base concentrations, and optionally strong acid or base added.
- For a recipe, enter the target pH, total concentration, volume and molar masses.
- Read the pH or amounts, the buffer capacity and the ratio table.
Buffer formulas
| Quantity | Formula |
|---|---|
| Buffer pH | pH = pKa + log₁₀([A⁻] ÷ [HA]) |
| Ratio for a target pH | [A⁻] ÷ [HA] = 10(pH − pKa) |
| Split a total concentration C | [A⁻] = C · r ÷ (1 + r), [HA] = C ÷ (1 + r) |
| Adding strong base x | [HA] → [HA] − x, [A⁻] → [A⁻] + x |
| Buffer capacity | β = 2.303 · C · Ka[H⁺] ÷ (Ka + [H⁺])² |
Common buffer systems (25 °C)
| Buffer | pKa | Useful pH range |
|---|---|---|
| Phosphoric acid / dihydrogen phosphate | 2.15 | 1.2 – 3.2 |
| Citric acid (pKa1, pKa2, pKa3) | 3.13, 4.76, 6.40 | 2.2 – 7.4 |
| Formic acid / formate | 3.75 | 2.8 – 4.8 |
| Acetic acid / acetate | 4.76 | 3.8 – 5.8 |
| MES | 6.15 | 5.5 – 6.7 |
| Carbonic acid / bicarbonate | 6.35 | 5.4 – 7.4 |
| Phosphate H₂PO₄⁻ / HPO₄²⁻ | 7.20 | 6.2 – 8.2 |
| HEPES | 7.48 | 6.8 – 8.2 |
| Tris | 8.07 | 7.1 – 9.0 |
| Ammonium / ammonia | 9.25 | 8.3 – 10.3 |
| Bicarbonate / carbonate | 10.33 | 9.3 – 11.3 |
Literature pKa values vary slightly with temperature and ionic strength. The Henderson–Hasselbalch equation uses concentrations rather than activities, so a prepared buffer is usually adjusted to the exact pH with a meter.
Frequently asked questions
What is the Henderson–Hasselbalch equation?
pH = pKa + log₁₀([A⁻]/[HA]), where [A⁻] is the conjugate base and [HA] the weak acid concentration. It follows from the acid dissociation constant and works well for buffers between about 1 mM and 1 M.
What is the pH of a buffer with equal acid and base?
When [A⁻] = [HA] the log term is zero, so pH = pKa. An equimolar acetic acid/acetate buffer has pH 4.76.
How do I make a buffer of a specific pH?
Pick a weak acid with pKa within 1 unit of the target pH, compute the ratio [A⁻]/[HA] = 10^(pH − pKa), split the total concentration in that ratio and weigh out each component. For phosphate (pKa 7.20) at pH 7.4 the ratio is 1.58, so a 0.1 M buffer has 0.0613 M HPO₄²⁻ and 0.0387 M H₂PO₄⁻. Always check the final pH with a meter.
What happens when strong acid or base is added?
Strong base converts some HA into A⁻ and strong acid converts A⁻ into HA, mole for mole. Adding 0.01 M NaOH to a 0.1 M/0.1 M acetate buffer changes the ratio to 0.11/0.09 and the pH only from 4.76 to 4.85, while the same amount in pure water would give pH 12.
What is buffer capacity?
Buffer capacity β is the amount of strong acid or base (mol/L) needed to change the pH by one unit. It is greatest when pH = pKa and grows with the total buffer concentration.
Why should the pH be within pKa ± 1?
Outside that range one component is more than 10 times the other, so the buffer has little of one form left to neutralize added acid or base, and its capacity falls quickly.
Do pKa values depend on temperature?
Yes. Tris in particular changes by about −0.028 pH units per °C. The preset values are for about 25 °C and low ionic strength; real buffers at other temperatures or high salt can differ by 0.1 or more.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.