Henderson-Hasselbalch Calculator

About the Henderson-Hasselbalch Calculator

The Henderson-Hasselbalch equation is the cornerstone of buffer chemistry. It relates the pH of a buffer solution to the pKa of the weak acid and the ratio of conjugate base to acid concentrations: pH = pKa + log([A⁻]/[HA]). This simple relationship makes it possible to design buffers at any desired pH.

Buffers resist pH changes when small amounts of acid or base are added. They work because the weak acid can neutralize added base, while the conjugate base neutralizes added acid. The buffering capacity is greatest when [A⁻] = [HA] (i.e., pH = pKa), and the effective buffer range is typically pKa ± 1.

This calculator solves the Henderson-Hasselbalch equation for any variable: pH, pKa, [A⁻], or [HA]. It calculates buffer capacity, shows how pH changes when strong acid or base is added, provides common buffer recipes for laboratory use, and visualizes the buffer region on a titration-like curve.

When This Page Helps

Design buffer solutions for any desired pH, calculate buffer capacity, and understand how buffers respond to added acid or base. Essential for biochemistry, analytical chemistry, and laboratory work.

How to Use the Inputs

1. Enter the pKa of the weak acid component.
2. Enter the concentrations of conjugate base [A⁻] and weak acid [HA].
3. View the calculated buffer pH.
4. Switch solve mode to find any unknown from the other three values.
5. Explore buffer capacity and effective buffer range.
6. Select from common buffer recipes for laboratory buffers.
7. See how the pH shifts when acid or base is added.

Example Calculation

Tips & Best Practices

• For best buffering, keep [A⁻]/[HA] between 0.1 and 10 (pH within ± 1 of pKa).
• Total buffer concentration matters: 1 M buffer resists pH change 10× better than 0.1 M.
• Common biological buffers: phosphate (pKa 7.2), HEPES (pKa 7.5), Tris (pKa 8.1), carbonate (pKa 6.4, 10.3).
• Temperature affects pKa: Tris buffers shift ~-0.03 pH per °C increase.
• When preparing buffers, adjust to final pH with a pH meter after mixing, as activity coefficients affect actual pH.
• The equation also works for base buffers: pOH = pKb + log([BH⁺]/[B]).

Buffer Design in Practice

Laboratory buffers are typically prepared at 20-200 mM total concentration. The choice of buffer depends on the target pH, compatibility with the experiment (some buffers chelate metals or interfere with assays), temperature stability, and ionic strength requirements. Good's buffers (HEPES, MES, MOPS, etc.) were specifically designed for biological research.

The Buffer Region of Titration Curves

During the titration of a weak acid with a strong base, the buffer region spans the half-equivalence point. At half-equivalence, exactly half the acid is converted to its conjugate base, so [A⁻] = [HA] and pH = pKa. The Henderson-Hasselbalch equation describes the entire buffer region between 10% and 90% neutralization.

Polyprotic Buffers

Polyprotic acids (H₃PO₄, H₂CO₃, citric acid) can buffer at multiple pH values — one for each ionization. Phosphate buffer works near pH 2.1, 7.2, and 12.4. This makes phosphate versatile but requires choosing the right conjugate pair for your target pH.

Frequently Asked Questions

• When is the Henderson-Hasselbalch equation valid?

  It's valid when (1) the acid is weak (pKa 2-12), (2) concentrations are >> Ka, and (3) the solution doesn't dilute too much. It breaks down for very dilute buffers, strong acids/bases, or polyprotic systems without simplification.

• What happens at pH = pKa?

  At pH = pKa, [A⁻] = [HA] (the ratio is 1, and log(1) = 0). This is where the buffer has maximum capacity — equal ability to neutralize added acid or base.

• What is buffer capacity?

  Buffer capacity (β) measures resistance to pH change. It equals the moles of strong acid or base needed to change pH by one unit per liter. It depends on total buffer concentration and how close pH is to pKa.

• Can I use this for basic buffers?

  Yes, use pKb of the base equivalently, or use the conjugate acid's pKa with pH = pKa + log([B]/[BH⁺]). For NH₃/NH₄⁺ buffer: pKa(NH₄⁺) = 9.25.

• What is the effective buffer range?

  Typically pKa ± 1 (where [A⁻]/[HA] is between 0.1 and 10). Outside this range, the buffer has insufficient capacity in one direction.

• How do I choose a buffer for a specific pH?

  Choose a weak acid whose pKa is close to your target pH. The buffer is most effective when the desired pH equals the pKa. Adjust the [A⁻]/[HA] ratio for fine-tuning.