Henderson–Hasselbalch Equation
The equation giving the pH of a buffer from the pKa of its acid and the ratio of conjugate base to acid.
Formula / equation
pH = pKa + log([A⁻]/[HA])
Unit
Dimensionless
In depth
The equation shows that pH depends on the ratio of the two components rather than their absolute concentrations, which is why dilution barely changes a buffer's pH. It is derived from the Ka expression by taking logarithms and is accurate when both components are present in reasonable amounts.
Examples in the real world
A buffer with twice as much acetate as acetic acid has pH = 4.76 + log 2 = 5.06.