Most biological reactions take place at a pH between 6 and 8, so ideal buffers have pKa values in this range to provide maximum buffering capacity there. Most buffers work best at a pH within 1 unit of their pKa at 20°C. If you think your experiment will lower the environmental pH, then select a buffer with a pKa that’s just a little lower than your working pH. Likewise, select a buffer with a pKa slightly higher if you expect your experiment to raise your working pH.