Buffer pH (Henderson-Hasselbalch) Calculator

How it is calculated

The tool divides the base concentration by the acid concentration to get the ratio, then applies pH = pKa + log10([A⁻] ÷ [HA]). [A⁻] is the conjugate base (the salt form) and [HA] the weak acid, entered in the same units, for example mol/L, because only their ratio matters. The pKa is the value of the specific acid at your temperature. Results are shown to two decimals, along with the ratio as x : 1 and the pKa you entered. Both concentrations must be above zero; zero or negative values return no result. The equation is an approximation that works best when both concentrations are much larger than the hydrogen ion concentration, and it ignores activity effects.

Common mistakes

• Swapping base and acid. The base goes on top, so the wrong order flips the sign of the log term and gives a pH that is as far below pKa as it should be above.
• Entering the amounts of the original acid and strong base added, rather than the concentrations after they have reacted. The formula needs the resulting [A⁻] and [HA].
• Using a pKa for the wrong proton of a polyprotic acid, such as phosphoric acid, which has several pKa values. Choose the one closest to your target pH.
• Using very dilute buffers or ratios beyond about 10 : 1 or 1 : 10, where the approximation and the buffering capacity break down.
• Mixing units between the two concentration fields, such as mmol/L for one and mol/L for the other, which distorts the ratio.