Question

From the list below, pick an acid, along with its conjugate base, that can be used to make a pH 7.40 buffer.

H3PO4 Ka = 6.9 x 10-3 H2PO4- Ka = 6.4 x 10-8 HPO42- Ka = 4.8 x 10-13

Answer 1

H3PO4 Ka values :

Ka1 = 6.9 x 10^-3 ; pKa1 = 2.16

H3PO4 <---------> H+ +   H2PO4-

H2PO4-   <----------> H+ + HPO42-

HPO42- <-------------> H+ + PO43-

Ka2 = 6.4 x 10^-8 ; pKa2 = 7.19

Ka3 = 4.8 x 10^-13 ; pKa3 = 12.32

here the given pH of buffer = 7.40

for best buffer : pH = pKa +/- 1

here pKa2 value is close the given pH .

so for prepare pH of 7.40 buffer use

buffer pair : H2PO4- and HPO42-