Henderson-Hasselbalch Calculator
What is Henderson-Hasselbalch?
Henderson-Hasselbalch is a simplified equation for calculating the pH of buffer solutions.
A buffer consists of a weak acid and its conjugate base (or a weak base and its conjugate acid).
Key Concepts:
- pKa: Acid strength (smaller = stronger)
- [A⁻]: Concentration of salt form (conjugate base)
- [HA]: Concentration of acid form
- Buffer Capacity: Ability to resist pH changes
Practical Applications:
Buffers are essential in:
- Blood buffer (pH ≈ 7.4)
- Laboratory and biochemistry
- Pharmacy and medicine
- Food industry
Tip: When pH = pKa, then [A⁻] = [HA] and the buffer has maximum capacity.
Formulas
Henderson-Hasselbalch Equation:
pH = pKa + log([A⁻]/[HA])
pH = hydrogen exponent, pKa = acid constant, [A⁻] = salt form, [HA] = acid form
pH = pKa + log([A⁻]/[HA])
pH = hydrogen exponent, pKa = acid constant, [A⁻] = salt form, [HA] = acid form
Calculate pKa:
pKa = -log(Ka)
Ka = acid dissociation constant
pKa = -log(Ka)
Ka = acid dissociation constant
Calculate Ratio:
[A⁻]/[HA] = 10^(pH - pKa)
Ratio of salt form to acid form
[A⁻]/[HA] = 10^(pH - pKa)
Ratio of salt form to acid form
Maximum Buffer Capacity:
pH ≈ pKa (when [A⁻] = [HA])
Buffer works best within pKa ± 1 pH unit
pH ≈ pKa (when [A⁻] = [HA])
Buffer works best within pKa ± 1 pH unit
Examples
Example 1: Acetic Acid Buffer
• pKa = 4.75
• [CH₃COO⁻]/[CH₃COOH] = 1
• pH = 4.75 + log(1) = 4.75
• pKa = 4.75
• [CH₃COO⁻]/[CH₃COOH] = 1
• pH = 4.75 + log(1) = 4.75
Example 2: Phosphate Buffer
• pKa = 7.2
• [HPO₄²⁻]/[H₂PO₄⁻] = 1
• pH = 7.2 + log(1) = 7.2
• pKa = 7.2
• [HPO₄²⁻]/[H₂PO₄⁻] = 1
• pH = 7.2 + log(1) = 7.2
Example 3: More Salt Form
• pKa = 4.75
• [A⁻]/[HA] = 10
• pH = 4.75 + log(10) = 5.75
(pH increases by 1 unit)
• pKa = 4.75
• [A⁻]/[HA] = 10
• pH = 4.75 + log(10) = 5.75
(pH increases by 1 unit)
Example 4: More Acid Form
• pKa = 4.75
• [A⁻]/[HA] = 0.1
• pH = 4.75 + log(0.1) = 3.75
(pH decreases by 1 unit)
• pKa = 4.75
• [A⁻]/[HA] = 0.1
• pH = 4.75 + log(0.1) = 3.75
(pH decreases by 1 unit)
Technical Background
The Henderson-Hasselbalch Equation
This equation is an approximation of the exact acid dissociation equation. It works well when:
- The buffer concentration is relatively high
- The ratio [A⁻]/[HA] is not more extreme than 100:1
- Water autoionization is negligible
Important pKa Values (25°C)
| Acid | Formula | pKa |
|---|---|---|
| Acetic Acid | CH₃COOH | 4.76 |
| Phosphoric Acid (1.) | H₃PO₄ | 2.12 |
| Phosphoric Acid (2.) | H₂PO₄⁻ | 7.21 |
| Carbonic Acid | H₂CO₃ | 6.35 |
| Ammonium | NH₄⁺ | 9.25 |
Buffer Ranges
A buffer with pKa = 7.0 works best at pH 6–8 (pKa ± 1).
Practical Applications
- Blood Buffer: Carbonate/Carbonic acid buffer (pH 7.4)
- Laboratory Buffers: Phosphate, citrate, Tris, acetate buffers
- Biology: Plant and animal cells require stable pH values
- Analytics: pH control in titrations
Common Mistakes
- Confusing pKa with Ka (pKa = -log Ka)
- Using common logarithm (log₁₀) instead of natural logarithm
- Confusing concentration with amount (equation needs concentration!)
- Oversimplifying (Henderson-Hasselbalch is an approximation)
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