Step 1: Understanding the Concept:
The antimicrobial activity of organic acid preservatives (like benzoic acid) depends on the degree of ionization. Only the un-ionized (undissociated) form of the acid can cross the microbial cell membrane to exert its effect.
Step 2: Detailed Explanation:
1. Sodium Benzoate: In water, it dissociates into benzoate ions. In acidic conditions (pH < 4.5), these ions combine with hydrogen ions to form un-ionized benzoic acid.
2. Mechanism: Since benzoic acid has a \( pK_a \) of approximately 4.2, it is most effective as a preservative at pH values below 5. In alkaline pH, it remains ionized and loses its activity.
3. Other options: Benzyl alcohol and Chlorobutanol are less dependent on pH but are not specifically categorized by their high efficacy in acidic ranges like benzoates are.
Step 3: Final Answer:
Sodium benzoate is specifically chosen for acidic preparations (like syrups) because its active form, benzoic acid, predominates in that pH range.