Sodium Bicarbonate Test: Identifying Carboxylic Acids
The Sodium Bicarbonate Test (or Sodium Hydrogen Carbonate test) is the primary qualitative test used to detect the presence of Carboxylic Acids ($R-COOH$). It relies on the acidity difference between carboxylic acids and carbonic acid.
1. General Reaction
When a carboxylic acid reacts with aqueous Sodium Bicarbonate ($NaHCO_3$), it displaces the weaker carbonic acid, which decomposes to release Carbon Dioxide ($CO_2$) gas.
Observation: The rapid evolution of colorless, odorless gas bubbles (Brisk Effervescence) indicates a positive test.
2. The Principle: Acid Strength
Rule of Displacement
A stronger acid displaces a weaker acid from its salt.
Reaction occurs if: $\text{Acidity of Reactant Acid} > \text{Acidity of } H_2CO_3$.
Acidity Order ($pK_a$)
- Sulfonic Acids ($R-SO_3H$): $pK_a \approx -2$ (Very Strong)
- Carboxylic Acids ($R-COOH$): $pK_a \approx 4-5$ (Stronger)
- Carbonic Acid ($H_2CO_3$): $pK_a \approx 6.4$ (Weak)
- Phenols ($Ph-OH$): $pK_a \approx 10$ (Weaker)
- Alcohols ($R-OH$): $pK_a \approx 16$ (Weakest)
3. Distinguishing Phenols from Acids
This is the most common application in exams.
Carboxylic Acids
Since $RCOOH$ is stronger than $H_2CO_3$, it protonates the bicarbonate ion.
Result: $CO_2$ Evolved.
Phenols
Simple phenols are weaker acids than $H_2CO_3$. They cannot protonate the bicarbonate ion.
Result: No Reaction.
Important Exception: Picric Acid
Picric Acid (2,4,6-Trinitrophenol) and 2,4-Dinitrophenol are phenols, but due to strong electron-withdrawing $-NO_2$ groups, they are stronger acids than Carbonic acid.
Observation: They give a Positive Sodium Bicarbonate test despite being phenols.
4. Mechanism
It is a simple Brønsted-Lowry acid-base reaction.
5. Solved Examples
| Compound | Reaction with $NaHCO_3$ | Reason |
|---|---|---|
| Acetic Acid | Positive ($CO_2 \uparrow$) | $pK_a (4.7) < 6.4$ |
| Benzoic Acid | Positive ($CO_2 \uparrow$) | $pK_a (4.2) < 6.4$ |
| Phenol | Negative | $pK_a (10) > 6.4$ |
| Picric Acid | Positive ($CO_2 \uparrow$) | $pK_a (0.38) \ll 6.4$ |
| Benzenesulfonic Acid | Positive ($CO_2 \uparrow$) | Very Strong Acid |
Bicarbonate Test Quiz
Test your concepts on Acidity. 10 MCQs with explanations.
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