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

Difficulty: Easy

Correct Answer: > 1 (greater than one)

Explanation:


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)

Discussion & Comments

No comments yet. Be the first to comment!
Join Discussion