pKa Calculator

In short: Calculate pKa from pH and base/acid ratio, pH from pKa, or Henderson-Hasselbalch buffer ratios. It is free, works in your web browser and needs no download.

Calculate pKa from pH and conjugate base/acid ratio, calculate buffer pH from pKa, or solve for the base/acid ratio with Henderson-Hasselbalch.

Henderson-Hasselbalch pKa Formula

• pH: pH = pKa + log10(base / acid).
• pKa: pKa = pH - log10(base / acid).
• base/acid ratio: base / acid = 10^(pH - pKa).

This educational calculator assumes ideal buffer behavior. Activity, ionic strength, temperature, and buffer capacity can shift real measured pH.

Using Henderson–Hasselbalch in practice

The equation pH = pKa + log₁₀([A⁻]/[HA]) links the pH of a buffer to the ratio of conjugate base to acid. When the two are equal, the log term is zero and pH = pKa. A buffer resists pH change best within about one pH unit of its pKa.

Worked example

An acetate buffer contains 0.10 M acetic acid and 0.15 M sodium acetate. Acetic acid has a pKa of about 4.76, so pH = 4.76 + log₁₀(0.15/0.10) = 4.76 + 0.18 = 4.94.

Choosing a buffer

• Pick an acid whose pKa is close to the target pH: acetate (about 4.76) for pH 4–5.5, phosphate (pKa₂ about 7.2) near neutral pH.
• Higher total concentration gives more buffer capacity.

Limitations

The equation uses concentrations in place of activities, so it is an approximation. It becomes less reliable at high ionic strength, for very dilute buffers, and for strong acids or bases. pKa values also change with temperature. For accurate lab work, prepare the buffer and adjust with a calibrated pH meter.