Henderson-Hasselbalch Calculator
Connecting pKa to pH Through a Ratio
The Henderson-Hasselbalch equation is the algebraic bridge between a weak acid's intrinsic strength — its pKa — and the actual pH of a solution containing that acid alongside its conjugate base. Rather than measuring pH directly, this equation lets you predict it from concentrations alone, or work backward to find the ratio needed to hit a target pH.
The Formula
[A−] is the concentration of the conjugate base (the deprotonated form), and [HA] is the concentration of the undissociated weak acid. When the two are equal, log10(1) = 0, so pH simply equals pKa.
Where This Calculation Matters
- Designing a buffer at a target pH — choosing a weak acid whose pKa is close to the desired pH, then calculating the ratio of base to acid needed.
- Predicting drug ionization state — a drug's ionization at physiological pH, calculated from its pKa, affects how well it crosses cell membranes.
- Protein and amino acid chemistry — side chain ionization states at a given pH follow the same relationship using each residue's pKa.
- Interpreting titration curves — the inflection point of a titration curve is where pH equals the acid's pKa.
pH as the Base-to-Acid Ratio Shifts (pKa = 4.76, Acetic Acid)
| [A−] : [HA] | log10 Ratio | Resulting pH |
|---|---|---|
| 1 : 10 | −1.00 | 3.76 |
| 1 : 2 | −0.30 | 4.46 |
| 1 : 1 | 0.00 | 4.76 |
| 2 : 1 | 0.30 | 5.06 |
| 5 : 1 | 0.70 | 5.46 |
How to Use This Calculator
- Enter the pKa of Weak Acid for the acid-base pair in question.
- Enter the Conjugate Base Concentration [A-] (mol/L).
- Enter the Weak Acid Concentration [HA] (mol/L).
- Select Calculate to get the resulting pH.
Related Calculations
Looking to prepare an actual buffer solution from this result? The Buffer pH Calculator applies the same equation with a practical buffer-prep framing. For strong acid/base titrations rather than weak-acid equilibria, see the Acid-Base Titration Calculator.