Henderson-Hasselbalch Equation
pH = pKa + log([A-]/[HA])
The Henderson-Hasselbalch equation calculates buffer pH from the ratio of conjugate base to weak acid. It's essential for buffer preparation and understanding biological pH control.
Variables
pH of the buffer solution
-log(Ka) of the weak acid
Concentration of conjugate base
Concentration of weak acid
Example Calculation
Scenario
Calculate the pH of a buffer containing 0.15 M acetic acid and 0.20 M sodium acetate. Ka = 1.8 x 10^-5.
Given Data
Calculation
pH = pKa + log([A-]/[HA]) = 4.74 + log(0.20/0.15) = 4.74 + log(1.33) = 4.74 + 0.12
Result
pH = 4.86
Interpretation
The buffer pH is 4.86, which is slightly above the pKa. When [A-] > [HA], pH > pKa. This buffer would resist pH changes when small amounts of acid or base are added.
When to Use This Formula
- ✓Calculating buffer pH
- ✓Preparing buffers at specific pH
- ✓Finding the ratio needed for a target pH
- ✓Understanding the half-equivalence point (pH = pKa)
Common Mistakes
- ✗Confusing Ka with pKa (need to take -log)
- ✗Reversing the ratio in the log term
- ✗Forgetting that at half-equivalence point, [A-] = [HA], so pH = pKa
- ✗Using this equation for strong acids/bases
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Common questions about this formula
Buffers work best when pH is within 1 unit of pKa (i.e., pKa - 1 to pKa + 1). This corresponds to [A-]/[HA] ratios between 0.1 and 10. Outside this range, buffer capacity is low.
Choose a weak acid with pKa close to your target pH. The buffer is most effective when pH equals pKa, so select an acid with pKa within 1 unit of your desired pH.