Penicillin solvent extraction — distribution of penicillin between phases as a function of pH During liquid–liquid extraction of penicillin, at acidic pH the antibiotic preferentially partitions into which phase?

Introduction / Context:
Downstream processing of penicillin relies on pH-dependent partitioning between aqueous broth and an organic solvent. Understanding ionization behavior guides efficient extraction and back-extraction steps to achieve purity and yield.

Given Data / Assumptions:

• Penicillin has ionizable groups with pKa values that determine charge state.
• Organic solvents such as amyl acetate or butyl acetate are used industrially.
• Process involves alternating acidic and basic pH setpoints.

Concept / Approach:

At low (acidic) pH, penicillin is predominantly in the un-ionized (free acid) form, which is more soluble in nonpolar solvents. At higher (basic) pH, the penicillinate salt form is charged and prefers the aqueous phase. Thus, extraction at acidic pH moves penicillin into the organic phase; back-extraction at higher pH recovers it into water for further crystallization as a stable salt.

Step-by-Step Solution:

Verification / Alternative check:

Process flow diagrams in antibiotic recovery consistently show acid extraction into solvent followed by base back-extraction, confirming phase preferences.

Why Other Options Are Wrong:

Aqueous phase (option a) is favored at basic pH, not acidic. Precipitation (option d) is not the standard primary step. Foam fraction (option e) is unrelated to solvent extraction.

Common Pitfalls:

Reversing the pH logic: remember “acid into organic; base back into aqueous.”

Final Answer:

Solvent (organic) phase