Henderson–Hasselbalch Equation (Weak Acid Buffer)
Also known as buffer equation · buffer pH
Worked example: Acetate buffer 0.200 M / 0.100 M at pKa 4.76 → pH 5.0610 — press Try an example to run it live, then adjust anything.
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Grade 12Grade 12 Chemistry
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Henderson–Hasselbalch Equation (Weak Acid Buffer) explained
A buffer resists pH change because it holds a reservoir of both a weak acid and its conjugate base: add acid and the base mops it up, add base and the acid neutralises it. The Henderson–Hasselbalch equation says the pH depends only on the acid's pKa and the ratio of the two forms — not on how concentrated they are. Equal amounts give pH = pKa exactly, which is why you choose a buffer whose pKa sits within about one unit of your target. Acetic acid has pKa 4.76, so an acetate buffer holding 0.200 M acetate against 0.100 M acetic acid sits at pH = 4.76 + log₁₀(2) = 4.76 + 0.301 = 5.06.
Lawrence Joseph Henderson, a Harvard physiologist, wrote the mass-action version in 1908 while working out how blood keeps its pH steady; Karl Albert Hasselbalch, a Dane, recast it in logarithms in 1917 using Sørensen's brand-new pH scale, and the joint name stuck. Their subject remains the textbook example: blood's bicarbonate system has an effective pKa of 6.10 and runs at a 20:1 ratio of HCO₃⁻ to dissolved CO₂, giving 6.10 + log₁₀(20) = 7.40 — the pH your body defends to within about 0.05 units. The classic trap is that the equation is an approximation built on assuming the acid and base concentrations are their formal (added) values; it fails for very dilute buffers, for pH far from pKa, and it never applies to a strong acid, which has no meaningful pKa to work with.
Henderson–Hasselbalch Equation (Weak Acid Buffer)
- = pH of the buffer
- = pKa of the weak acid
- = Conjugate base concentration (M)
- = Weak acid concentration (M)
Missing one of these? Work it out first, then come back
- pH of the buffer — Henderson–Hasselbalch Equation (Weak Base Buffer)
- Conjugate base concentration — Henderson–Hasselbalch Equation (Weak Base Buffer), pH of a Weak Base from Kb
- Weak acid concentration — Henderson–Hasselbalch Equation (Weak Base Buffer), pH of a Weak Acid from Ka