Which one of the following amino-containing buffering molecules will be the most able to resist change in pH when in a pH 7.0 solution at a total (ionized and non-ionic) concentration of 100mM?
(1) Tris (pKa = 8.1)
(3) MOPS {pKa = 7.3}
(2) HEPES (pKa = 7.7)
(4) PIPES (pKa = 7.1)
Concept: Best Buffer at pH 7.0
A buffer is most effective when its pKa is close to the desired pH because buffering capacity is strongest within ±1 pH unit of its pKa.
Comparing the Given Buffers:
| Buffer | pKa | Buffering Range (±1 pH unit) |
|---|---|---|
| Tris | 8.1 | 7.1 – 9.1 |
| HEPES | 7.7 | 6.7 – 8.7 |
| MOPS | 7.3 | 6.3 – 8.3 |
| PIPES | 7.1 | 6.1 – 8.1 |
Best Choice for pH 7.0
- The buffer with a pKa closest to 7.0 will best resist pH changes.
- PIPES (pKa = 7.1) is the closest to 7.0, making it the best buffer in this case.
Final Answer:
(4) PIPES (pKa = 7.1)