Question

pKa values of phosphoric acid are 2.2, 7.2 and 12.7. A phosphate buffer of pH = 7.4 can be prepared using ________.

Answer 1

Answer:

$H_2PO_4^-$

Explanation:

All buffer solutions have an effective range in which the buffer has a good regulation of pH.

This range is calculated by: pKa-1 < pH < pKa+1

The pH we need to control should be in that range. So for the phosphoric acid:

1) $H_3PO_4$ => 2.2-1< pH < 2.2+1 => 1.2<pH<3.2

2) $H_2PO_4^-$ => 7.2-1< pH < 7.2+1 => 6.2<pH<8.2

3) $HPO_4{{-2}$ => 12.7-1< pH < 12.7+1 => 11.7<pH<13.7

For a pH=7.4 using a phosphate buffer we need to use diacid phosphate ($H_2PO_4^-$)