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Question
Explain buffer action of sodium acetate-acetic acid buffer.
Solution
- Sodium acetate-acetic acid buffer is an example of an acidic buffer in which weak acid \[\ce{(CH3COOH)}\] and its salt with strong base are present.
Here sodium acetate \[\ce{(CH3COONa)}\] salt is a strong electrolyte, which dissociates completely in water.
Acetic acid is a weak electrolyte, hence does not dissociate completely.
Sodium acetate dissociates as given below:
\[\ce{CH3COONa_{(aq)} -> CH3COO{_{(aq)}^-} + Na{_{(aq)}^+}}\]
Due to the presence of the common ion \[\ce{(CH3COO^-)}\], dissociation of acetic acid is further suppressed. - Reserve acidity: When a small quantity of a strong base \[\ce{OH-}\] is added, the hydroxide ions react with the acid as follows:
\[\ce{CH3COOH_{(aq)} + OH{_{(aq)}^-} <=>CH3COO{_{(aq)}^-} + H2O_{(l)}}\]
This removal of added \[\ce{OH-}\] ions is called reversed acidity. - Reserve basicity: When a small quantity of strong acid is added to the solution, the added \[\ce{H+}\] ions will be consumed by the conjugate base \[\ce{CH3COO-}\] present in large concentration as given below:
\[\ce{CH3COO{_{(aq)}^-} + H{_{(aq)}^{+}} <=> CH3COOH_{(aq)}}\]
The removal of added H+ ions is called reversed basicity. - The acid or base added thus cannot change the \[\ce{[H+] or [OH-]}\] concentration, and the pH of the buffer remains unchanged. Dilution does not have any effect on pH of buffer. This is because the concentration ratio term in the following equation remains the same.
`"pH" = "pK"_"a" + log_10 "[Salt]"/"[acid]"`
`"pOH" = "pK"_"a" + log_10 "[Salt]"/"[base]"`
Hence, dilution does not change this ratio.
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