Henderson-Hasselbalch (Buffer pH) Calculator
Find the pH of a buffer from its pKa and the concentrations of the conjugate base and weak acid.
How it works
pH = pKa + log₁₀([A⁻] ÷ [HA]) · when [A⁻] = [HA], pH = pKa
The Henderson-Hasselbalch equation gives the pH of a buffer from the ratio of conjugate base to weak acid. When the two concentrations are equal the log term is zero and the pH equals the pKa — the point of maximum buffering. Because it depends only on the ratio, diluting the buffer barely shifts the pH, which is what makes buffers useful.
Worked example
An acetate buffer with equal 0.1 M acetic acid and 0.1 M acetate (pKa 4.76) has pH = 4.76 + log(0.1 ÷ 0.1) = 4.76 + 0 = 4.76.
Frequently asked questions
What happens when base and acid are equal?
The ratio is 1, its logarithm is 0, and the pH equals the pKa. This is the centre of the buffer's effective range and where it resists pH change best.
Does the equation work for strong acids?
No. It assumes a weak acid and its conjugate base in comparable amounts. Strong acids dissociate fully and need a direct pH-from-concentration calculation instead.
What is the useful buffer range?
Roughly pKa ± 1, where the base-to-acid ratio stays between about 1:10 and 10:1. Outside that window the buffer capacity drops sharply.
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