📐 Henderson-Hasselbalch Calculator
pH = pKa + log([A⁻]/[HA]) — Buffer pH calculation.
How to Use This Calculator
This tool solves the Henderson-Hasselbalch equation for any one of its four variables: pH, pKa, [A⁻], or [HA]. Select the variable you want to find, enter the other three, and click Calculate. Concentrations should be in mol/L and must be positive numbers.
Use the "Solve for" dropdown to choose which quantity you want: pH, pKa, the conjugate base concentration [A⁻], or the acid concentration [HA].
Enter the three known values. If solving for pH, you need pKa, [A⁻], and [HA]. If solving for the concentration ratio needed to reach a target pH, enter your target pH and the pKa.
Click Calculate. The ratio [A⁻]/[HA] appears in the result panel so you can verify your buffer composition at a glance.
Check that the ratio falls between 0.1 and 10 (pH within ±1 of pKa). Outside that range, buffer capacity drops steeply and the equation becomes less reliable.
The Henderson-Hasselbalch Equation
pKa is the negative log of the acid dissociation constant Ka. [A⁻] is the molar concentration of the conjugate base (often a dissolved salt) and [HA] is the molar concentration of the weak acid. When the two are equal, the log term is zero and pH equals pKa.
Common Buffer Systems
Where This Calculation Comes Up
The Henderson-Hasselbalch equation sits at the centre of buffer preparation in biochemistry. When a protocol tells you to prepare 50 mM phosphate buffer at pH 7.4, you use this equation to find the exact ratio of Na₂HPO₄ (the base form) to NaH₂PO₄ (the acid form). Using pKa = 7.21, the equation gives 10^(7.4 - 7.21) = 1.55, so you need a [HPO₄²⁻]/[H₂PO₄⁻] ratio of 1.55. That means 1.55 parts dibasic phosphate for every 1 part monobasic phosphate.
The equation also comes up in pharmacology. A drug's pKa determines how much of it is in the ionised versus unionised form at physiological pH. For a weak acid with pKa 4.5 at stomach pH 2.0, the ratio [A⁻]/[HA] = 10^(2.0 - 4.5) = 0.003, meaning over 99.7% of the drug is in the unionised form and can cross the gastric mucosa. This concept is tested in every pharmacy and medical school entrance examination that covers acid-base chemistry.
Frequently Asked Questions
What is the Henderson-Hasselbalch equation?
pH = pKa + log([A⁻]/[HA]), where [A⁻] is the concentration of the conjugate base and [HA] is the concentration of the weak acid.
When is this equation valid?
It is most accurate when the ratio [A⁻]/[HA] is between 0.1 and 10 (pH within ±1 unit of pKa) and concentrations are > 0.001 M.
How do I prepare a pH 7.4 phosphate buffer?
For phosphate buffer (pKa ≈ 7.2), use H-H equation: 7.4 = 7.2 + log([HPO₄²⁻]/[H₂PO₄⁻]) → ratio ≈ 1.58.
What is the best buffering range?
A buffer works best within ±1 pH unit of the acid's pKa. Outside this range, buffer capacity drops significantly.
Can I solve for [A⁻] or [HA]?
Yes. This calculator can solve for pH, [A⁻], [HA], or pKa given the other three values.