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Tris hydroxyethyl aminoethane (THAE) has a pk, of 8.4. The buffering capacity of a buffer made with this compound will be greatest at which pH?

(1) pH 9.4

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(2) pH 7.4

(3) pH 8.4

(4) Both at pH 7.4 and 9.4

Concept: Buffering Capacity and pKa

The buffering capacity of a buffer is highest when the pH is equal to its pKa, because at this point, the concentrations of the weak acid (HA) and its conjugate base (A⁻) are equal.

Given Data:

  • Tris hydroxyethyl aminoethane (THAE) has a pKa of 8.4
  • The buffering range is typically ±1 pH unit around the pKa (7.4 to 9.4).
  • The best buffering occurs at pH = pKa, which is 8.4.

Analyzing the Options:

  1. pH 9.4 → Within the buffering range but not at maximum capacity. ❌
  2. pH 7.4 → Within the buffering range but not at maximum capacity. ❌
  3. pH 8.4 → Best buffering capacity ✅
  4. Both at pH 7.4 and 9.4 → Not the strongest buffering point ❌

Final Answer:

(3) pH 8.4

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