Citric acid (H3C6H5O7) is a weak triprotic acid with pKa1 = 3.13, pKa2 = 4.77, and pKa3 = 6.40. Which of the following combinations of conjugate acid and base would be most appropriate as a buffer whose purpose is to maintain the pH around 5.8?
A) Na2HC6H5O7 and Na3C6H5O7
B) NaH2C6H5O7 and Na2HC6H5O7
C) H3C6H5O7 and NaH2C6H5O7
D) H3C6H5O7 and Na2HC6H5O7
E) H3C6H5O7 and Na3C6H5O7
The Correct Answer and Explanation is :
The correct answer is B) NaH2C6H5O7 and Na2HC6H5O7.
Explanation:
Citric acid (H₃C₆H₅O₇) is a triprotic weak acid with three dissociation steps, and its conjugate bases form a series of buffer pairs. To create a buffer solution with a pH around 5.8, we must choose a combination of an acid and its conjugate base that will allow us to maintain that pH. This can be determined by the pKa values of citric acid and the desired pH.
The pKa values for citric acid are:
- pKa1 = 3.13
- pKa2 = 4.77
- pKa3 = 6.40
A buffer is most effective when the pH is close to the pKa of the weak acid in the buffer pair. The pH of 5.8 is closest to pKa2 (4.77), indicating that the second dissociation step (from H₂C₆H₅O₇⁻ to HC₆H₅O₇²⁻) is the most relevant for buffering around this pH.
Looking at the options:
- Option A: Na₂HC₆H₅O₇ and Na₃C₆H₅O₇: This pair is based on the second and third dissociation steps. The pKa3 (6.40) is higher than 5.8, so this pair would not be effective for buffering at this pH.
- Option B: NaH₂C₆H₅O₇ and Na₂HC₆H₅O₇: This pair corresponds to the second dissociation step (pKa2 = 4.77), where H₂C₆H₅O₇⁻ is the acid and HC₆H₅O₇²⁻ is the base. This combination is appropriate for buffering near pH 5.8 because the pKa2 value is closest to the desired pH.
- Option C: H₃C₆H₅O₇ and NaH₂C₆H₅O₇: This pair involves the first dissociation (pKa1 = 3.13), which is too low for a pH of 5.8.
- Option D: H₃C₆H₅O₇ and Na₂HC₆H₅O₇: This pair is related to the first dissociation step, which is also not appropriate for a pH of 5.8.
- Option E: H₃C₆H₅O₇ and Na₃C₆H₅O₇: This involves the first and third dissociation steps, both of which are not effective for buffering at pH 5.8.
Thus, Option B (NaH₂C₆H₅O₇ and Na₂HC₆H₅O₇) is the most appropriate choice to buffer at a pH around 5.8.