Question

Suppose you were designing a buffer system for imitation blood and wanted the buffer to maintain the blood at the realistic pHof 7.40. All other things being equal, which buffer system would be preferable: ${\mathbf{\text{H}}}_{\mathbf{2}}{\mathbf{\text{CO}}}_{\mathbf{3}}\mathbf{/}{\mathbf{\text{HCO}}}_{\mathbf{3}}^{\mathbf{-}}$or ${\mathbf{\text{H}}}_{\mathbf{2}}{\mathbf{\text{PO}}}_{\mathbf{4}}^{\mathbf{-}}\mathbf{/}{\mathbf{\text{HPO}}}_{\mathbf{4}}^{\mathbf{2}\mathbf{-}}$? Explain

Short answer

The closer value to the 7.40 (blood) has the buffer system ${\text{H}}_2{\text{PO}}_4-/{{\text{HPO}}_4}^{2-}$ which is pKa=7.21

Step-by-step solution

Step 1:To determine the buffer system would be preferable

To determine which buffer system would be preferable, we must compare it's pKa values with the pH of the blood.

value for   pKa  of both buffer system

Check out the table 15.2 where we are given the value for pKa of both buffer systems:

$\begin{array}{l}{\text{pK}}_{-}a\left({\text{H}}_{-}{\text{CO}}_{\text{3}}{\text{/HCO}}_{\text{3}}{\text{3}}^{\text{-}}\right)=6.37\\ {\text{pK}}_{-}a\left({{\text{H}}_{\text{2}}}_{\text{-}}{\text{PO}}_{\text{-}}{\text{4}}^{\text{-}}{\text{/HPO}}_{\text{-}}{\text{4}}^{\text{2-}}\right)=7.21\end{array}$

The closer value of the buffer system

Knowing that the pH of the blood is 7.40 and to prepare the buffer system for imitation blood, we see that the closest value to the 7.40 (blood) has the buffer system ${\text{H}}_2{\text{PO}}_4-/{{\text{HPO}}_4}^{2-}$which is ${\text{pK}}_a=\mathrm{7.21.}$