Buffer pH Calculator (Henderson-Hasselbalch)
Calculate the pH of a buffer solution from pKa and acid/conjugate base concentrations using the Henderson-Hasselbalch equation.
How to use this buffer ph calculator
- Look up the pKa of your weak acid.
- Enter concentrations of conjugate base [A⁻] (e.g., sodium acetate) and weak acid [HA] (e.g., acetic acid).
- For maximum buffer capacity, keep [A⁻]/[HA] between 0.1 and 10 (pH = pKa ± 1).
- Example: acetic acid/acetate buffer at pKa 4.76, equal concentrations → pH 4.76.
Formula
pH = pKa + log₁₀([A⁻]/[HA]). At [A⁻]=[HA]: pH = pKa. Effective buffering: pH = pKa ± 1.
About the Buffer pH Calculator
Buffer solutions are critical in biochemistry, medicine, and analytical chemistry. Human blood maintains pH 7.35–7.45 via the bicarbonate buffer system. pH 7.0 or 7.8 — just 0.35 units away — is medically critical.
The Henderson-Hasselbalch equation is derived from Ka = [H⁺][A⁻]/[HA]. Taking −log of both sides: pKa = pH − log([A⁻]/[HA]), rearranged to pH = pKa + log([A⁻]/[HA]).
Frequently asked questions
+What is a buffer?
A buffer resists pH change when acid or base is added. It consists of a weak acid and its conjugate base. Blood uses bicarbonate (pKa 6.35) to hold pH at 7.4.
+When is buffer capacity greatest?
When [A⁻] = [HA], giving pH = pKa. Buffer is effective within pKa ± 1. Outside this range, the buffer has little resistance to pH change.
+How do I choose a buffer for my target pH?
Choose a weak acid with pKa within 1 unit of your target. For pH 7.4: phosphate (pKa 7.2) is ideal. For pH 5: acetate (pKa 4.76). For pH 9: borate (pKa 9.24).