Chapter 19, Problem 19.1P

Interpretation Introduction

Interpretation:

The purpose of an acid-base buffer has to be given.

Explanation of Solution

Buffer solution:

An aqueous solution consisting mixture of weak base and its conjugate acid or weak acid and its conjugate base.  Its $\text{pH}$ value changes very small when a small amount of strong base or an acid added to it.

Basis of buffer action:

Take acetic acid and sodium acetate buffer and its dissociation reactions are given as follows,

Assume a small amount of strong acid is added to the buffer solution.  The ${\text{H}}^{\text{+}}$ ions of the added acid will react with the basic component, ${\text{CH}}_{\text{3}}{\text{COO}}^{\text{-}}$ in the buffer.

${\text{CH}}_{\text{3}}{\text{COO}}^{\text{-}}{}_{\left(\text{aq}\right)}\text{+}{\text{H}}^{\text{+}}{}_{\left(\text{aq}\right)}\stackrel{}{\to }{\text{CH}}_{\text{3}}{\text{COOH}}_{\left(\text{aq}\right)}$

Maximum amount of the ${\text{H}}^{\text{+}}$ ions from added acid are consumed because concentration of ${\text{CH}}_{\text{3}}{\text{COO}}^{\text{-}}$ in the buffer is high.  The reaction nearly goes to completion because ${\text{CH}}_{\text{3}}\text{COOH}$ is a weak acid and ${\text{CH}}_{\text{3}}{\text{COO}}^{\text{-}}$ ions have a strong tendency to form non-ionized ${\text{CH}}_{\text{3}}\text{COOH}$ molecules.  Due to most of the added ${\text{H}}^{\text{+}}$ ions are removed, there is no appreciable decrease in $\text{pH}$.

Therefore, an acid-base buffer is used to maintain a relatively constant $\text{pH}$ in a solution.