Buffer Preparation Calculator (Henderson-Hasselbalch)

The pH of a buffer is determined by the ratio of the conjugate acid-base pair, not by the total concentration:

pH = pKa + log₁₀( [A⁻] / [HA] )

Conversely, given a target pH the ratio follows directly: [A⁻]/[HA] = 10^(pH − pKa). Combined with total concentration and volume, this gives the moles and grams needed for each component.

Buffer pair selection rule: the target pH should fall within pKa ± 1. Outside this range, buffering capacity drops sharply — at pH = pKa buffering capacity is at its maximum; at 1 unit away the ratio is already 10:1, and beyond that there is essentially no buffering.

Key considerations:

• pKa shifts with temperature. Tris is particularly sensitive, approximately −0.028 pH/°C: a Tris buffer prepared at 4 °C will drop ~0.9 pH units by 37 °C. Always calibrate at the working temperature.
• Ionic strength affects the effective pKa; at high concentrations, calculated values can deviate from measured ones — always verify with a pH meter after preparation.
• This tool calculates under ideal conditions and provides a starting point, not a final answer.