AP Chemistry - 8.10: Buffer Capacity
What is Buffer Capacity?
Buffer capacity is a measure of how well a buffer solution can resist changes in pH upon the addition of a strong acid or a strong base. It quantifies the amount of strong acid or base that can be added before a significant change in pH occurs. A buffer with a high capacity can neutralize more added acid or base than a buffer with a low capacity.
Factors Affecting Buffer Capacity
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Concentration of the Buffer Components: The higher the concentrations of the weak acid and its conjugate base (or weak base and its conjugate acid), the greater the buffer capacity. A more concentrated buffer can neutralize more added acid or base.
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Ratio of the Buffer Components: Buffer capacity is optimal when the concentrations of the weak acid (HA) and its conjugate base (A-) are equal, i.e., [HA] = [A-]. The further the ratio deviates from 1, the lower the buffer capacity. The buffer is most effective within approximately ±1 pH unit of its pKa.
Quantifying Buffer Capacity
Buffer capacity isn’t typically expressed with a specific numerical value with units in AP chem. Instead, it’s assessed qualitatively by comparing different buffer systems and considering the factors described above. However, one can think of it as the moles of acid/base needed to cause a certain pH change, but that’s not tested in AP chem.
Henderson-Hasselbalch Equation and Buffer Range
The Henderson-Hasselbalch equation is useful for understanding the effective pH range of a buffer:
$$ pH = pK_a + log \frac{[A^-]}{[HA]} $$
Where:
- $ pH $ is the pH of the buffer solution.
- $ pK_a $ is the negative logarithm of the Acid Dissociation Constant ( $ K_a $ ) of the weak acid. ( $ pK_a = -log(K_a) $ ).
- $ [A^-] $ is the concentration of the conjugate base.
- $ [HA] $ is the concentration of the weak acid.
The effective buffer range is generally considered to be $ pH = pK_a \pm 1 $ . Outside this range, the buffer’s ability to resist pH changes diminishes significantly.
Buffer Capacity and Titration
During a Titration, the buffering region is where the pH changes slowly as the titrant (strong acid or base) is added. Buffer capacity is related to the slope of the titration curve; a flatter slope indicates a higher buffer capacity. The buffer capacity is at its maximum at the half-equivalence point, where $ [HA] = [A^-] $ and $ pH = pK_a $ . Past the equivalence point the buffer is exhausted.
Limiting Reactant
When adding a strong acid/base to a buffer, it is important to remember the concept of a limiting reactant. For example, if you add a strong acid to a buffer made from a weak acid and its conjugate base, the strong acid will react with the conjugate base. To determine the new concentrations of the weak acid and conjugate base, it’s best to use an ICE Tables table with the strong acid/base acting as the “change” in the ICE table. Then using the new concentrations to calculate the new pH with the henderson hasselbalch equation.
Examples
- A buffer solution containing 1.0 M acetic acid and 1.0 M sodium acetate has a greater capacity than a buffer solution containing 0.1 M acetic acid and 0.1 M sodium acetate.
- A buffer solution containing equal concentrations of a weak acid and its conjugate base has a higher buffer capacity than a buffer solution where the concentration of the weak acid is significantly higher than the concentration of its conjugate base.
- Buffer Solutions