Henderson-Hasselbalch Calculator

The Henderson-Hasselbalch equation calculates the pH of buffer solutions. A buffer resists pH change by containing both a weak acid (HA) and its conjugate base (A⁻). Most effective within ±1 pH unit of pKa.

🧪 Acid-Base📐 pH = pKa + log([A⁻]/[HA])🔬 Buffers

pKa of weak acid

[A⁻] Conjugate base (M)

[HA] Weak acid (M)

Please enter valid values.

Formula & Reference

Variable	Symbol	Formula	Units
pH	pH	pKa + log([A⁻]/[HA])	pH units
pKa	pKa	pH - log([A⁻]/[HA])	pH units
Ratio	[A⁻]/[HA]	10^(pH - pKa)	—
Buffer Range	—	pKa ± 1 pH unit	pH units
Acetic acid	pKa = 4.76	Best buffer: pH 3.76-5.76	—

Step-by-Step Examples

Example 1

Acetate Buffer

pKa(CH₃COOH)=4.76, [CH₃COO⁻]=0.10 M, [CH₃COOH]=0.10 M.

pH = 4.76 + log(0.10/0.10) = 4.76 + log(1)
pH = 4.76 + 0 = 4.76

✓ pH = 4.76 (equal concentrations: pH = pKa)

Example 2

Phosphate Buffer pH 7.4

pKa(H₂PO₄⁻/HPO₄²⁻)=7.21. Need pH 7.40.

7.40 = 7.21 + log([HPO₄²⁻]/[H₂PO₄⁻])
log(ratio) = 0.19, ratio = 10^0.19 = 1.55
Need 1.55x more Na₂HPO₄ than NaH₂PO₄

✓ Ratio = 1.55:1 (HPO₄²⁻:H₂PO₄⁻)

Example 3

Blood Buffer

Bicarbonate buffer: pKa=6.10, [HCO₃⁻]=24 mM, [H₂CO₃]=1.2 mM.

pH = 6.10 + log(24/1.2) = 6.10 + log(20)
pH = 6.10 + 1.301 = 7.40

✓ pH = 7.40 (normal blood pH)

Real-World Applications

🧬

Biochemistry

Biological buffers (phosphate, bicarbonate, HEPES) maintain pH for enzyme activity.

🏥

Medicine

Blood bicarbonate buffer maintains pH 7.35-7.45 for protein function.

🧪

Analytical Chemistry

HPLC mobile phases use carefully buffered pH for reproducible separations.

💊

Pharmaceuticals

Drug formulations use buffers to maintain pH for stability and bioavailability.

Common Mistakes to Avoid

⚠️

Using concentrations not ratio

The equation uses the RATIO [A⁻]/[HA], not individual concentrations. The ratio is what matters.

⚠️

Using Ka instead of pKa

pKa = -log(Ka). Convert: pKa(acetic acid) = -log(1.8×10⁻⁵) = 4.74. Use pKa in the equation.

⚠️

Buffer works best near pKa

Henderson-Hasselbalch is most accurate when pH is within ±1 of pKa. Extreme ratios (>10:1) give poor buffering capacity.

Frequently Asked Questions

What is a buffer solution? ▾

A solution that resists pH changes when small amounts of acid or base are added. Contains both a weak acid (HA) and its conjugate base (A⁻).

Why is maximum buffering at pH = pKa? ▾

When [A⁻] = [HA], the buffer has equal capacity to absorb added acid or base. Ratio = 1, log(1) = 0, pH = pKa.

What is buffer capacity? ▾

Moles of acid or base a buffer can absorb per unit pH change. Maximum at pH = pKa. Increases with buffer concentration.

How do I prepare a buffer of a specific pH? ▾

Choose acid with pKa within ±1 of target pH. Use H-H equation to find ratio. Mix appropriate volumes of acid and conjugate base.

What buffers are used in biology? ▾

PBS (pH 7.4), HEPES (pKa 7.5), TRIS (pKa 8.1), acetate (pKa 4.76), citrate (pKa 3.13, 4.76, 6.40).

Related Chemistry Calculators

Formula Explorer connections

Interpretation: This relationship connects hydrogen-ion activity, dissociation, conjugate ratios or titration stoichiometry to acid–base behavior. Assumption: Concentration approximates activity mainly in dilute solutions. Temperature, ionic strength, acid strength and equilibrium approximations affect accuracy.

Ka Calculator →Kb Calculator →Buffer Capacity Calculator →Chemistry Formula Explorer →