What it is and when to use it
A buffer is a solution containing a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists changes in pH when small amounts of acid or base are added. The acid neutralises added base, and the conjugate base neutralises added acid, so the pH stays close to the acid's pKa. Buffers matter in biochemistry, blood chemistry, cell culture, analytical chemistry and anywhere a stable pH is needed.
Use this calculator to predict the pH of a buffer from its pKa and the concentrations of the acid and conjugate base, or to see how changing their ratio moves the pH. It is most accurate when both concentrations are reasonably large compared with the acid dissociation, and when the ratio of base to acid stays between about 0.1 and 10.
The Henderson-Hasselbalch equation
The calculator uses pH = pKa + log₁₀( [A⁻] / [HA] ).
- pKa: the negative log of the weak acid's dissociation constant, a fixed property of the acid at a given temperature.
- [A⁻]: concentration of the conjugate base, in mol/L (the "Conjugate Base" field).
- [HA]: concentration of the weak acid, in mol/L (the "Acid Concentration" field).
Only the ratio matters, so 0.1 M and 0.2 M give the same pH as 0.01 M and 0.02 M. Concentration determines buffer capacity, not the pH itself.
Worked example: an acetate buffer
Acetic acid has a pKa of about 4.76. Mix 0.10 M acetic acid with 0.20 M acetate.
Ratio = 0.20 / 0.10 = 2. log₁₀(2) = 0.30103. So pH = 4.76 + 0.30103 = 5.06103, which the calculator shows as pH 5.06.
With equal concentrations the ratio is 1, log₁₀(1) = 0, and the pH equals the pKa exactly. That is the point of maximum buffering.
Common mistakes and how to interpret the result
- Swapping the acid and base fields. The base goes in the numerator, so reversing them flips the sign of the log term and gives a pH on the wrong side of the pKa.
- Using a buffer far from its pKa. Once the ratio is beyond about 10:1 either way, the buffer has little capacity, so treat the result as unreliable for practical use.
- Forgetting temperature. Published pKa values are for a stated temperature (often 25 °C), and some buffers such as Tris shift noticeably as temperature changes.
- Confusing the equation's ideal result with a measured pH. Activity effects at high ionic strength mean a pH meter reading can differ from the calculated value by a few hundredths to tenths of a unit.