Chapter 19, Problem 90A

Interpretation Introduction

Interpretation : The buffer system of phosphate buffer has to be explained.

Concept Introduction : A buffer is a solution that retains the pH of a solution irrespective of the addition of small amounts of acids or bases.

Answer to Problem 90A

The buffer system of phosphate buffer is given as

  ${\text{H}}_{\text{2}}{\text{PO}}_4^{-}\left(aq\right)+{\text{OH}}^{-}\left(aq\right)\to {\text{H}}_{\text{2}}\text{O}\left(aq\right)+{\text{HPO}}_4^2{}^{-}\left(aq\right)$

  ${\text{HPO}}_4^2{}^{-}\left(aq\right)+{\text{H}}^{+}\left(aq\right)\to {\text{H}}_{\text{2}}{\text{PO}}_4^{-}\left(aq\right)$

Explanation of Solution

A weak acid and one of its salts or a weak base and one of its salts are solutions that are used as buffers.

Buffers are formed by the combination of weak acids or bases with their respective salts.

The phosphate buffer is ${\text{H}}_{\text{2}}{\text{PO}}_4^{-}{\text{/HPO}}_4^2{}^{-}$ .

The base when added to the buffer reacts with ${\text{H}}_{\text{2}}{\text{PO}}_4^{-}$ . But since it is a weak acid, there is only minimal change in pH.

  ${\text{H}}_{\text{2}}{\text{PO}}_4^{-}\left(aq\right)+{\text{OH}}^{-}\left(aq\right)\to {\text{H}}_{\text{2}}\text{O}\left(aq\right)+{\text{HPO}}_4^2{}^{-}\left(aq\right)$

The acid when added to the buffer reacts with ${\text{HPO}}_4^2{}^{-}$ . Since it is a weak base, it cannot accept many hydrogen ions in order to change the pH.

  ${\text{HPO}}_4^2{}^{-}\left(aq\right)+{\text{H}}^{+}\left(aq\right)\to {\text{H}}_{\text{2}}{\text{PO}}_4^{-}\left(aq\right)$

Since the buffer is taken for weak acid and weak base, there is only minimal change in pH.