What is the Henderson-Hasselbalch equation?

It relates the pH of a buffer to the pKa of the weak acid and the ratio of conjugate base to acid: pH = pKa + log([A-]/[HA]). It lets you predict the pH of any buffer or work out the ratio needed to hit a target pH.

What happens when acid and base are equal?

When [A-] equals [HA] the ratio is 1, its logarithm is 0, and the pH equals the pKa exactly. This is the point of maximum buffering capacity, which is why buffers are chosen so their pKa is close to the desired working pH.

What is the useful buffering range?

A buffer works well within about plus or minus one pH unit of its pKa, where the base-to-acid ratio runs from roughly 1:10 to 10:1. Outside that window one species is nearly exhausted and the buffer can no longer resist pH change.

Does this account for ionic strength or temperature?

No. The equation uses concentrations rather than activities and assumes a fixed pKa, so it ignores ionic-strength and temperature effects. For precise work calibrate the final buffer with a pH meter and adjust at the working temperature.

Can I solve for the ratio I need?

Yes, indirectly. Rearranged, the base-to-acid ratio equals 10 raised to the power (pH minus pKa). Adjust the two concentrations until the displayed pH matches your target, and the shown ratio is what you must mix.

Henderson-Hasselbalch Buffer pH Calculator

The Henderson-Hasselbalch equation is the workhorse for buffer preparation. Given a weak acid’s pKa and the concentrations of its acid and conjugate-base forms, this calculator returns the buffer pH and tells you whether you are inside the effective buffering window.

How it works

Buffer pH depends on the pKa and the logarithm of the base-to-acid ratio:

pH = pKa + log10( [A-] / [HA] )

where [A-] is the conjugate base concentration and [HA] is the weak acid concentration. When the two are equal the log term is zero and pH equals pKa. Rearranged, the ratio you must mix is [A-]/[HA] = 10^(pH − pKa).

Tips and example

For an acetate buffer (pKa 4.76) mixed at 0.1 M acid and 0.1 M acetate, the pH is 4.76 — equal amounts give pH = pKa. To reach pH 5.76 you would need a ten-fold excess of base over acid. Keep both species within roughly a factor of ten of each other so the buffer stays in its strong-capacity range, and always fine-tune the finished buffer on a calibrated pH meter, since the equation ignores ionic strength and temperature.