General Chemistry
Henderson-Hasselbalch Equation
Calculates pH of buffers — pH = pKa + log([A⁻]/[HA])
The Henderson-Hasselbalch equation calculates the pH of a buffer solution from the ratio of conjugate base to weak acid: pH = pKa + log([A⁻]/[HA]). Derived from acid dissociation equilibrium. Critical for: making buffers, understanding biology (blood pH, drug ionization), titration analysis. When [A⁻] = [HA]: pH = pKa (half-equivalence). Each unit shift in log ratio = 1 pH unit shift. Limitations: assumes weak acid, valid for moderately concentrated buffers, doesn't account for activity at high concentrations.
- EquationpH = pKa + log([A⁻]/[HA])
- At [A⁻] = [HA]pH = pKa
- Derived fromKa = [H⁺][A⁻]/[HA]
- UseBuffer pH calculations, pKa determination
- DiscoveredLawrence Henderson (1908) + Karl Hasselbalch (1916)
- Buffer optimal±1 pH unit of pKa
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Why H-H matters
- Buffer design. Calculate composition for desired pH.
- Biology. Drug behavior, protein ionization.
- Pharmacology. Predicting drug absorption.
- pKa determination. Read from titration curve.
- Protein chemistry. Isoelectric point.
- Lab work. Day-to-day buffer preparation.
- Education. Foundation of acid-base chemistry.
Common misconceptions
- Always pH = pKa. Only when [A⁻] = [HA].
- Equation works for strong acids. Weak acids only.
- Ratio determines absolute pH. ratio with pKa.
- Higher pKa = stronger acid. Lower pKa = stronger acid.
- Equation accounts for activity. Approximation; concentrations.
- Polyprotic acids one equation. Multiple equations for multiple ionizations.
Frequently asked questions
How is the equation derived?
From acid dissociation. HA ⇌ H⁺ + A⁻. Ka = [H⁺][A⁻]/[HA]. Solve for [H⁺]: [H⁺] = Ka × [HA]/[A⁻]. Take negative log: -log[H⁺] = -log(Ka) - log([HA]/[A⁻]). pH = pKa + log([A⁻]/[HA]). Direct rearrangement of Ka expression. Convenient form for buffer calculations.
When is it most useful?
(1) Calculating buffer pH from concentrations of HA and A⁻. (2) Designing buffer at desired pH. (3) Calculating ratio of forms at any pH (drug ionization). (4) Determining pKa from titration curve (at half-equivalence). (5) Predicting protein behavior in biology.
Why is buffer best near pKa?
Maximum capacity. At pH = pKa, [A⁻] = [HA]. Adding small acid: ratio shifts slightly; pH changes minimally. Adding small base: same. Far from pKa: ratio extreme; small amount converts most of one species to other; pH jumps. Practical buffer range: pH ± 1 from pKa.
How is it used in pharmacology?
Drug ionization affects absorption. Acidic drugs (e.g., aspirin pKa 3.5): in stomach (pH 2): mostly protonated → uncharged → absorbed. In blood (pH 7.4): mostly deprotonated → charged → poorly absorbed. Equation predicts ratio at any pH. Determines: bioavailability, distribution, excretion, food/drink interactions.
How does it apply to amino acids?
Amino acids have multiple ionizable groups. Carboxylic acid (pKa ≈ 2): always deprotonated at physiological pH (7.4). Amine (pKa ≈ 9): always protonated. Side chains vary. Henderson-Hasselbalch predicts charge state at given pH. Critical for: protein structure, enzyme function, isoelectric point (where charges balance).
What are limitations?
(1) Assumes Ka is constant — valid for dilute solutions. (2) Activity coefficient corrections at high ionic strength. (3) Polyprotic acids: separate equation for each ionization. (4) Very weak acids: approximation breaks down near pH 7. (5) Strong acids: can't use (Ka → ∞).
What's the half-equivalence point relation?
At titration of weak acid with strong base, halfway to equivalence: half acid converted to base. So [HA] = [A⁻]; ratio = 1; log = 0; pH = pKa. This means: read pH at half-equivalence on titration curve → directly gives pKa. Practical method for measuring pKa in lab.