Effect of ionic strength on pKa
Effect of ionic strength on pKa
| Buffer ion | z | pKa | pKa@
I=0.01 | pKa@
I=0.1
| Acetate | -1 | 4.76 | 4.72 | 4.66
| Phosphate | -2 | 7.20 | 7.08 | 6.89
| Citrate | -3 | 6.40 | 6.29 | 5.88
| Tris | +1 | 8.06 | 8.10 | 8.16 | | | | |
From
Protein Purification, Scopes 2nd ed. p 329
The Henderson-Hasselbalch equation holds true for actviites, not concentrations. At finite salt concentrations activities depart from concentrations particularly for oligovalent species. For phosphate and citrate buffers ionic strength corrections are essential for Henderson-Hasselbalch to yield useful predictions.