The equation and what each term means
[HA] is the weak acid, [A−] is its conjugate base, and pKa is the negative log of the acid's dissociation constant. The whole result turns on the ratio of the two forms. If they are equal the log term is zero, so pH equals pKa. More base pushes pH above pKa; more acid pulls it below.
How the ratio moves the pH
| [A−] / [HA] | log10 of ratio | Resulting pH |
|---|---|---|
| 0.1 | −1 | pKa − 1 |
| 0.5 | −0.30 | pKa − 0.30 |
| 1 | 0 | pKa |
| 2 | +0.30 | pKa + 0.30 |
| 10 | +1 | pKa + 1 |
A tenfold change in the ratio shifts pH by exactly one unit. The useful working range is pKa ± 1; beyond it the buffer barely holds.
Pick a buffer whose pKa sits near your target pH
To hold a given pH, choose an acid with a pKa close to it, then set the base-to-acid ratio. Common lab buffers and where they work:
| Buffer | pKa | Useful pH range |
|---|---|---|
| Acetate | 4.74 | 3.6–5.6 |
| MES | 6.15 | 5.5–6.7 |
| Phosphate (pKa2) | 7.21 | 6.2–8.2 |
| HEPES | 7.48 | 6.8–8.2 |
| Tris | 8.06 | 7.0–9.0 |
Where the equation stops being exact
- It assumes a weak acid. The formula treats the acid as barely dissociating, so it drifts for strong acids or very dilute buffers where water's own H+ and OH− matter.
- pKa moves with temperature. It shifts roughly 0.01 to 0.03 units per degree, so mix and measure the buffer at the temperature you will use it.
- Always confirm on a meter. The calculation gets you close; a calibrated pH meter is what you trust for the final adjustment.
Common questions
What is the Henderson-Hasselbalch equation?
It gives the pH of a buffer from the acid strength and the ratio of the two forms present: pH = pKa + log10 of the conjugate base over the weak acid. When the two are equal, the log is zero and pH equals pKa.
When is a buffer most effective?
When the pH is within about one unit of the pKa, which is where the base-to-acid ratio stays between 0.1 and 10. Outside that window one form runs low and the buffer stops resisting pH change well.
What is the difference between pKa and pH?
pKa is a fixed property of the acid, describing how readily it gives up a proton. pH is the current state of the solution. A buffer sits at its best when pH equals pKa.
