Henderson-Hasselbalch Calculator
Calculate buffer pH from pKa and the ratio of conjugate base to acid.
Buffer pH
Calculation Steps
The Henderson-Hasselbalch Equation
The Henderson-Hasselbalch equation estimates the pH of a buffer solution from the acid dissociation constant (expressed as pKa) and the ratio of the conjugate base to the weak acid. It is the cornerstone of buffer chemistry in analytical and biochemical work.
pH = pKa + log₁₀([A⁻] / [HA])
- • pKa = −log₁₀ of the acid dissociation constant Ka
- • [A⁻] = concentration of the conjugate base (mol/L)
- • [HA] = concentration of the weak acid (mol/L)
How Buffers Work
A buffer is a mixture of a weak acid and its conjugate base that resists changes in pH when small amounts of acid or base are added. When the conjugate base and the weak acid are present in equal concentrations, their ratio equals 1, log₁₀(1) = 0, and the pH equals the pKa. This is the point of maximum buffering capacity.
- • If [A⁻] > [HA], the ratio exceeds 1 and the pH rises above the pKa.
- • If [A⁻] < [HA], the ratio is below 1 and the pH falls below the pKa.
- • Buffers work best within ±1 pH unit of the pKa.
Note: The Henderson-Hasselbalch equation assumes ideal dilute solutions and uses equilibrium (not initial) concentrations. It is most accurate when the buffer ratio is between 0.1 and 10 and ionic strength effects are negligible. For precise work, account for activity coefficients and temperature.