Henderson-Hasselbalch: Buffer pH
pH = pKa + log₁₀([A⁻]/[HA]). Relates buffer pH to pKa and conjugate pair ratio. Effective range pKa ± 1. Essential for buffer design in biochemistry and pharmaceuticals.
Why This Chemistry Calculation Matters
Why: Henderson-Hasselbalch enables buffer design for labs, pharma, and biology. Predicts pH from ratio; designs buffers for target pH.
How: Enter pKa, [HA], [A⁻]. pH = pKa + log([A⁻]/[HA]). Or enter target pH to find ratio. Buffer range pKa ± 1.
- ●pH = pKa when [A⁻] = [HA].
- ●Buffer range: pKa ± 1.
- ●Higher concentration = greater capacity.
Buffer Examples
🧪 Acetate Buffer pH 5
Common lab buffer - acetic acid/acetate
🩸 Blood Carbonate Buffer
Physiological pH 7.4 buffer system
⚗️ PBS Buffer pH 7.4
Phosphate buffered saline for biology
🧬 Tris Buffer pH 8.0
Common molecular biology buffer
📊 Find Ratio for pH 7.0
Calculate base/acid ratio needed
💨 Ammonia Buffer
High pH buffer system
🧫 HEPES Buffer pH 7.5
Zwitterionic buffer for cell culture
🍊 Citrate Buffer pH 5.0
Citric acid buffer system
🧬 Glycine Buffer pH 10.0
High pH amino acid buffer
⚖️ Half-Equivalence Point
pH = pKa when [A⁻] = [HA]
👁️ Borate Buffer pH 9.0
Eye wash and analytical chemistry
🔬 MOPS Buffer pH 7.2
Biological buffer for electrophoresis
🧪 PIPES Buffer pH 6.8
Piperazine-based buffer
🧂 Carbonate Buffer pH 10
High pH carbonate buffer
📚 Imidazole Buffer pH 7.0
Protein purification buffer
Calculate Buffer pH
⚠️For educational and informational purposes only. Verify with a qualified professional.
🔬 Chemistry Facts
pH = pKa + log([A⁻]/[HA]). Henderson-Hasselbalch.
— IUPAC
At [A⁻]=[HA], pH = pKa. Half-equivalence.
— Buffer
Effective range pKa ± 1. Maximum capacity at pH=pKa.
— Physical
Blood: HCO₃⁻/H₂CO₃. pH 7.4, ratio ~20:1.
— Physiology
The Henderson-Hasselbalch Equation
The Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of the weak acid and the ratio of conjugate base to weak acid concentrations. It's fundamental for buffer design in chemistry and biochemistry.
[A⁻] = conjugate base concentration, [HA] = weak acid concentration
Common Buffer Systems
| Buffer System | Acid/Base | pKa | Optimal pH Range |
|---|---|---|---|
| Acetate | CH₃COOH / CH₃COO⁻ | 4.76 | 3.8-5.8 |
| Phosphate (1st) | H₃PO₄ / H₂PO₄⁻ | 2.15 | 1.2-3.2 |
| Phosphate (2nd) | H₂PO₄⁻ / HPO₄²⁻ | 7.2 | 6.2-8.2 |
| Carbonate/Bicarbonate | H₂CO₃ / HCO₃⁻ | 6.35 | 5.4-7.4 |
| Ammonia/Ammonium | NH₄⁺ / NH₃ | 9.25 | 8.3-10.3 |
| Tris | Tris-H⁺ / Tris | 8.07 | 7.1-9.1 |
| HEPES | HEPES-H⁺ / HEPES | 7.55 | 6.6-8.6 |
| Citrate (2nd) | H₂Cit⁻ / HCit²⁻ | 4.76 | 3.8-5.8 |
Key Concepts
Buffer Range
Effective buffer range is pKa ± 1, where the buffer resists pH changes most effectively.
Half-Equivalence Point
When [A⁻] = [HA], the ratio is 1, log(1) = 0, so pH = pKa exactly.
Buffer Capacity
Higher total concentration = greater buffer capacity. Optimal when pH = pKa.
How Does the Henderson-Hasselbalch Equation Work?
The Henderson-Hasselbalch equation is derived from the acid dissociation equilibrium expression. It provides a convenient way to calculate pH when you know the ratio of conjugate base to weak acid concentrations.
🔬 Derivation from Ka
Starting Point
HA ⇌ H⁺ + A⁻
Ka = [H⁺][A⁻] / [HA]
Solving for [H⁺]:
[H⁺] = Ka × [HA] / [A⁻]
Taking Negative Logarithm
-log[H⁺] = -log(Ka) - log([HA]/[A⁻])
pH = pKa - log([HA]/[A⁻])
pH = pKa + log([A⁻]/[HA])
Henderson-Hasselbalch!
When to Use This Calculator
The Henderson-Hasselbalch equation is essential for anyone working with buffer solutions in laboratories, pharmaceutical development, or biological research.
Molecular Biology
Prepare buffers for DNA/RNA work, enzyme assays, and cell culture.
- Tris buffers (pH 7-9)
- Phosphate buffers (pH 6-8)
- HEPES buffers (pH 6.8-8.2)
Pharmaceuticals
Formulate drug solutions, predict ionization, and optimize bioavailability.
- IV solution preparation
- Drug stability
- Solubility optimization
Clinical Chemistry
Understand blood pH regulation and acid-base disorders.
- Blood gas analysis
- Bicarbonate buffer system
- Acidosis/alkalosis diagnosis
The Bicarbonate Buffer System in Blood
The most important physiological buffer is the bicarbonate/carbonic acid system, which maintains blood pH at 7.35-7.45. The Henderson-Hasselbalch equation explains how this works.
Normal Values
[HCO₃⁻] ≈ 24 mM
[H₂CO₃] ≈ 1.2 mM
Normal Ratio
20:1
Base to acid
Normal Blood pH
7.40
6.1 + log(20) = 7.4
Practical Buffer Preparation Examples
Example: Preparing Acetate Buffer at pH 5.0
Given:
- pKa of acetic acid = 4.76
- Target pH = 5.0
- Total concentration = 0.1 M
Solution:
5.0 = 4.76 + log([A⁻]/[HA])
log([A⁻]/[HA]) = 0.24
[A⁻]/[HA] = 10^0.24 = 1.74
[Acetate] = 0.064 M, [Acetic acid] = 0.036 M
Example: PBS Buffer at pH 7.4
Given:
- pKa of H₂PO₄⁻/HPO₄²⁻ = 7.20
- Target pH = 7.4
Solution:
7.4 = 7.2 + log([HPO₄²⁻]/[H₂PO₄⁻])
[HPO₄²⁻]/[H₂PO₄⁻] = 10^0.2 = 1.58
~61% HPO₄²⁻, ~39% H₂PO₄⁻
Limitations of Henderson-Hasselbalch
⚠️ When It Doesn't Work Well
- • Very dilute solutions (ionic strength effects)
- • Very concentrated solutions (activity ≠ concentration)
- • Polyprotic acids with overlapping pKa values
- • Solutions with strong acids or bases added
- • Extreme pH values (near 0 or 14)
✓ Assumptions Made
- • Activity coefficients ≈ 1 (dilute solutions)
- • Weak acid/base (not fully dissociated)
- • Temperature is constant (affects pKa)
- • No other equilibria interfering
- • Ionic strength is low to moderate