Liquid–liquid extraction — For a useful separation, the distribution selectivity should be...

Introduction:In extraction, selectivity quantifies preferential partitioning of a solute between phases, typically defined as a ratio of distribution coefficients or partition coefficients for desired vs undesired species. A favorable selectivity is critical for stage efficiency and solvent economy.

Given Data / Assumptions:

• Standard definition of selectivity S as a ratio exceeding unity for favorable separation.
• Isothermal operation without chemical reaction.
• Steady partitioning behavior.

Concept / Approach:When S > 1, the target component prefers the extract phase relative to the raffinate or relative to impurities, enabling effective separation with a reasonable number of stages. S ≤ 1 implies poor or no selectivity, demanding impractical solvent flows or excessive stages.

Step-by-Step Solution:Recall definition: S = K_A / K_B for components A and B or equivalent selectivity expression.Assess requirement: For enrichment, S must exceed unity.Conclude that S > 1 is desired.

Verification / Alternative check:McCabe–Thiele style constructions for extraction show rapidly converging stage counts when S > 1 and divergence when S ≤ 1.

Why Other Options Are Wrong:

• < 1 or = 1 provide little to no driving force.
• Zero or negative are nonphysical in this context.

Common Pitfalls:Confusing distribution ratio D with selectivity S; assuming constant S when complexation or pH effects change partitioning.

Final Answer:> 1 (greater than one)