pKa Calculator — Ka to pKa and Buffer pH

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How this is calculated

The acid dissociation constant Ka describes how completely a weak acid HA donates a proton in water: HA ⇌ H⁺ + A⁻. Because Ka spans many orders of magnitude — roughly 10⁻¹⁵ to 10⁻¹ for common weak acids — chemists use pKa = −log₁₀(Ka), which compresses the scale to a convenient single-digit range. A lower pKa means a stronger weak acid; acetic acid has pKa ≈ 4.74, while carbonic acid (first dissociation) has pKa ≈ 6.35.

The Henderson-Hasselbalch equation pH = pKa + log₁₀([A⁻]/[HA]) predicts the pH of a buffer made from the weak acid and its conjugate base. When both concentrations are equal the log term vanishes and pH = pKa — this is the half-equivalence point of a titration and the point of maximum buffer capacity. The calculator lets you set any [A⁻]/[HA] ratio to model any buffer composition.

For % ionisation the calculator solves the exact quadratic Ka = x²/([HA] − x) rather than the common approximation x ≈ √(Ka · [HA]), so it stays accurate even when Ka/[HA] is not negligible. All results assume an ideal dilute aqueous solution at 25 °C; temperature shifts pKa (typically ±0.01–0.02 per °C for small acids), and high ionic strength changes effective Ka through activity coefficients.