Buffer pH calculator, Henderson-Hasselbalch equation
Buffer pH, base/acid ratio for a target pH, and the amounts to weigh for a given volume, with the buffer zone checked.
Result
4.76
- Calculation
- pH = pKa + log([A⁻]/[AH]) = 4.76 + log(1) = 4.76
Buffering power peaks at pH = pKa; the pair to choose is the one whose pKa is closest to the target pH.
- Base/acid ratio
- 1
- Share of the basic form
- 50 %
Notes (2)
Shaded band: pKa ± 1, ratio from 1:10 to 10:1. Near its corners, the point is already on the slope, although classed inside the zone.
This buffer has a pH of 4.76.
Calculation, assumptions and sources
OverviewThe relation pH = pKa + log([A⁻]/[AH]) for three bench uses: the pH of a buffer of known concentrations, the base/acid ratio for a target pH, and the full preparation (moles of each species for a given volume and concentration).
The relation pH = pKa + log([A⁻]/[AH]) for three bench uses: the pH of a buffer of known concentrations, the base/acid ratio for a target pH, and the full preparation (moles of each species for a given volume and concentration). The pH is compared with the pair’s buffer zone, pKa ± 1. Outside that zone, the minority species runs out at the first drop.
Method & formulaspH = pKa + log([A⁻]/[AH])
The relation follows from the acidity constant written in logarithms. The pH depends only on the ratio of the two forms: a 1 M and a 0.1 M buffer at the same ratio have the same pH, but the former absorbs ten times more acid.
- pKa
- the pH at which both forms of the pair are equal: acetate 4.76, phosphate 7.21, ammonium 9.25 at 25 °C.
- AH / A⁻
- the acid form of the pair and its conjugate base.
- Buffer zone
- pKa ± 1, where the ratio stays between 1:10 and 10:1 and the solution resists additions of acid or base.
- Capacity
- the amount of acid or base the buffer can absorb: it grows with the total concentration of the pair.
Validity domainThe relation takes the introduced concentrations as the equilibrium ones.
The relation takes the introduced concentrations as the equilibrium ones. It holds when they largely dominate water autoionisation and the pair’s own dissociation, that is roughly 0.01 to 1 mol/L and a pH between 3 and 11. A very dilute, very acidic or very basic solution requires the exact balance of the pH tool. pKa values vary with temperature and ionic strength. That of Tris drops by nearly 0.03 units per degree: a Tris buffer prepared at 25 °C no longer has the same pH at 37 °C.