Henderson-Hasselbalch Calculator
This finds the pH of a buffer solution directly from its acid dissociation constant and the ratio of base to acid present — the standard calculation behind designing and understanding buffer chemistry.
Inputs
Result
pH 4.76
How it works
pH = pKa + log([A⁻]/[HA])
How the henderson-hasselbalch calculator works
pH = pKa + log₁₀([A⁻] ÷ [HA]), where [A⁻] is the conjugate base concentration and [HA] is the weak acid concentration.
Worked example: pKa 4.76, base and acid both at 0.1 M
- log₁₀(0.1 ÷ 0.1) = log₁₀(1) = 0.
- pH = 4.76 + 0 = 4.76.
- This confirms a key property of buffers: when base and acid concentrations are exactly equal, pH equals pKa exactly, since the log term vanishes to zero.
Common mistakes to avoid
Swapping the base and acid concentrations in the ratio
[A⁻] (base) divided by [HA] (acid) is not the same as the reverse ratio — swapping them flips the sign of the log term, shifting the calculated pH away from pKa in the wrong direction entirely.
Applying this formula outside a buffer's effective range
The Henderson-Hasselbalch equation works best when both the acid and base forms are present in meaningfully comparable amounts (typically within about a factor of 10 of each other) — far outside that range, the buffer's actual behavior deviates more from this simplified equation's prediction.
Frequently asked questions
Why does pH equal pKa exactly when base and acid concentrations are equal?
Because log₁₀ of a ratio of 1 (equal concentrations) is exactly 0, leaving pH = pKa + 0 = pKa — this is precisely the point of maximum buffering capacity, where the solution best resists pH changes from added acid or base.
What does pKa actually represent?
It's the pH at which a weak acid is exactly half dissociated into its conjugate base form — a fixed property of each specific acid, analogous to how molar mass is a fixed property tied to a substance's chemical identity.
Why are buffers useful in chemistry and biology?
They resist significant pH changes when small amounts of acid or base are added, which is essential for many biological systems (like blood, which must maintain a very narrow pH range) and many controlled laboratory reactions sensitive to pH.
How is this related to the plain pH Calculator?
The plain pH Calculator finds pH directly from hydrogen ion concentration for any solution; this tool is specifically for buffer systems, finding pH from the buffer's characteristic pKa and its base-to-acid ratio instead.
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