Calculate buffer pH from pKa and conjugate base/acid ratio, or find the ratio needed for a target pH.
To find the pH of a buffer, enter the pKa (or click a preset) and the concentrations of conjugate base [A⁻] and weak acid [HA]. To find what ratio you need for a target pH, select "Ratio" and enter pKa and your desired pH. The units for [A⁻] and [HA] don't matter as long as they're the same — only the ratio matters.
The Henderson-Hasselbalch equation pH = pKa + log₁₀([A⁻]/[HA]) relates the pH of a buffer solution to the pKa of the weak acid and the ratio of conjugate base to weak acid concentrations.
Worked example: You want to make a Tris buffer at pH 7.4 (pKa of Tris = 8.07 at 25°C). The ratio needed is: [A⁻]/[HA] = 10^(pH − pKa) = 10^(7.4 − 8.07) = 10^(−0.67) = 0.214. This means you need about 1 part Tris base to 4.7 parts Tris-HCl.
Buffers work best within ±1 pH unit of their pKa. Outside this range, the buffering capacity drops sharply. When choosing a buffer for your experiment, pick one whose pKa is close to your target pH.
Temperature matters: pKa values change with temperature. Tris is particularly temperature-sensitive (ΔpKa ≈ −0.03/°C), so a Tris buffer prepared at room temperature will be ~0.5 pH units lower at 4°C. Always adjust pH at the temperature you'll use the buffer.