A Graduate Student at SDSU Wants to Measure the Activity

A graduate student at SDSU wants to measure the activity of a particular enzyme at pH 4.0.To buffer her reaction,she will use a buffer system based on one of the acids listed below,which acid is most appropriate for the experiment?

A) Formic acid (K_a 1.78 × 10⁻⁴)
B) MES (K_a 8.13 × 10⁻⁷)
C) PIPES (K_a 1.74 × 10⁻⁷)
D) Tris (K_a 8.32 × 10⁻⁹)
E) Piperidine (K_a 7.58 × 10⁻¹²)