Buffer pH Calculator

The Henderson–Hasselbalch equation is pH = pKa + log₁₀([A⁻]/[HA]) for an acid buffer, where [A⁻] is the conjugate base concentration and [HA] is the weak acid concentration. For a base buffer, pOH = pKb + log₁₀([BH⁺]/[B]), and then pH = 14 − pOH. It is the fundamental formula used to calculate buffer pH at given component concentrations.

A buffer solution resists changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base (acid buffer), or a weak base and its conjugate acid (base buffer). Buffers are essential in biological, chemical, and pharmaceutical applications where stable pH is critical.

An acid buffer consists of a weak acid (HA) and its conjugate base (A⁻), usually supplied as the acid and a salt of the acid. A base buffer consists of a weak base (B) and its conjugate acid (BH⁺). The ratio of these two components, together with the pKa or pKb, determines the resulting pH.

A buffer works best when the pH is close to the pKa of the weak acid used (or 14 − pKb for base buffers). The effective buffering range is generally pKa ± 1 pH unit. Outside this range the buffer capacity drops significantly and the solution becomes less resistant to pH changes.

For a carbonate buffer, use the second dissociation of carbonic acid with pKa₂ ≈ 10.33 at 25 °C. Enter this pKa value along with the concentrations of bicarbonate (HCO₃⁻) as the acid and carbonate (CO₃²⁻) as the conjugate base into the Henderson–Hasselbalch equation: pH = 10.33 + log([CO₃²⁻]/[HCO₃⁻]).

Human blood is primarily buffered by the bicarbonate buffer system (H₂CO₃ / HCO₃⁻, pKa ≈ 6.1), the phosphate buffer system (H₂PO₄⁻ / HPO₄²⁻, pKa ≈ 7.2), and proteins such as hemoglobin. These systems together maintain blood pH in the narrow range of 7.35–7.45.

Yes. If you select 'Ka / Kb' as the constant type, enter the Ka (or Kb) value and the calculator automatically converts it using pKa = −log₁₀(Ka). This is useful when your reference source provides Ka directly rather than pKa.

When [A⁻] = [HA], the log term log₁₀(1) equals zero, so pH = pKa exactly. This is a useful reference point: equal concentrations of the weak acid and its conjugate base always produce a buffer pH equal to the pKa. The same principle applies to base buffers at equal concentrations.