pKa Calculators

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pKa is the negative base-10 logarithm of the acid dissociation constant Ka: pKa = −log₁₀(Ka). It measures the strength of an acid in solution — lower pKa indicates a stronger acid (dissociates more completely at any given pH). The Henderson-Hasselbalch equation (pH = pKa + log([A⁻]/[HA])) uses pKa to calculate buffer pH or to determine the ionization state of an acid at any pH. In biochemistry, pKa values of amino acid side chains, coenzymes, and active site residues are critical for understanding enzyme mechanisms, protein folding, and drug binding.

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pKa Definition

pKa = −log₁₀(Ka)

Ka = [H⁺][A⁻]/[HA] (acid dissociation constant). Lower pKa = stronger acid = dissociates more at any pH. At pH = pKa: [A⁻] = [HA] (50% dissociated). Below pKa: predominantly HA (protonated form). Above pKa: predominantly A⁻ (deprotonated form).

Henderson-Hasselbalch Equation

pH = pKa + log([A⁻]/[HA])

Use to: design buffers (select pKa ≈ target pH ± 1); calculate ionization state ([A⁻]/[HA] = 10^(pH−pKa)); determine pH from known ratio.

Effective buffer range: pKa ± 1 pH unit (10:1 to 1:10 ratio of conjugate base to acid).

Amino Acid pKa Values

  • α-COOH: pKa ≈ 2.0–2.4 (charged at physiological pH)
  • α-NH₃⁺: pKa ≈ 9.0–9.8
  • Side chains: Asp/Glu pKa ≈ 3.9–4.1 (acidic); His pKa ≈ 6.0 (unique — switches near physiological pH); Cys pKa ≈ 8.3; Lys pKa ≈ 10.5; Arg pKa ≈ 12.5; Tyr pKa ≈ 10.5

Common Buffer pKa Values

  • Phosphate: H₂PO₄⁻/HPO₄²⁻ pKa = 7.20 → useful buffer at pH 6.2–8.2
  • HEPES: pKa = 7.48 → biological buffer pH 6.8–8.2
  • Tris: pKa = 8.06 (25°C) — temperature-sensitive (ΔpKa/°C = −0.028)
  • Acetate: pKa = 4.75 → buffer pH 3.8–5.8
  • Carbonic acid: pKa₁ = 6.37; pKa₂ = 10.32 (blood buffering system)

Glossary

pKa
−log₁₀(Ka); lower pKa = stronger acid; at pH = pKa, exactly 50% of the acid is dissociated; used in Henderson-Hasselbalch equation to design buffers and calculate ionization states.
Henderson-Hasselbalch Equation
pH = pKa + log([A⁻]/[HA]); calculates buffer pH from the ratio of conjugate base to weak acid; effective buffer range is pKa ± 1 pH unit.
Effective Buffer Range
pKa ± 1 pH unit; the range where a weak acid/base pair provides significant buffering capacity; outside this range, buffering drops below practical effectiveness.

Frequently Asked Questions

pKa = −log₁₀(Ka), where Ka = [H⁺][A⁻]/[HA]. Lower pKa = stronger acid (higher Ka = more dissociation). Key interpretation: at pH = pKa, exactly half the acid is dissociated ([A⁻] = [HA]). Below pKa: protonated (HA) form dominates. Above pKa: deprotonated (A⁻) form dominates. Example: acetic acid pKa = 4.75. At pH 4.75: 50% CH₃COOH, 50% CH₃COO⁻. At pH 7.4: log([A⁻]/[HA]) = 7.4 − 4.75 = 2.65; [A⁻]/[HA] = 10^2.65 ≈ 450; so 99.8% is in the deprotonated (acetate) form.

Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]). Applications: (1) Calculate buffer pH: if [A⁻]/[HA] = 2 and pKa = 7.2: pH = 7.2 + log(2) = 7.2 + 0.301 = 7.50. (2) Find [A⁻]/[HA] at a given pH: for pH 6.5 and pKa 7.2: log([A⁻]/[HA]) = 6.5 − 7.2 = −0.7; [A⁻]/[HA] = 10⁻⁰·⁷ = 0.20; so 83% is in the HA form. (3) Design buffer: choose a weak acid with pKa within ±1 pH unit of your target pH; mix the acid and its conjugate base in the calculated ratio.

Histidine's imidazole side chain has pKa ≈ 6.0 in free amino acid — uniquely positioned to switch between protonated (His⁺H, charged) and neutral (His, uncharged) forms near physiological pH (7.4). At pH 7.4: His is ~97% neutral. The pKa of His residues in protein active sites is often perturbed by the local environment to 5–8, allowing them to act as proton donors/acceptors at physiological pH — a key mechanism in enzyme catalysis (serine proteases, histidine kinases, carbonic anhydrase). No other standard amino acid has a side chain pKa near physiological pH, making His uniquely suited for acid-base catalysis at the pH of cellular processes.

Choose a buffer with pKa close to your target pH (within ±1 unit for effective buffering): for pH 7.4 → phosphate (pKa 7.2), HEPES (pKa 7.5), MOPS (pKa 7.2). Consider: temperature dependence (Tris pKa changes −0.028 pH units/°C — prepare at experimental temperature); biological compatibility (phosphate inhibits many kinases; HEPES does not); interactions with metal ions (EDTA chelates metals; phosphate can precipitate with Ca²⁺ or Mg²⁺). Final pH adjustment: use strong acid (HCl) or base (NaOH) after mixing components; always check pH with a calibrated electrode at the temperature of use.