Solvent extraction performance: The effectiveness of a solvent in liquid–liquid extraction is quantified primarily by which measures?

Introduction:
Choosing an extraction solvent in downstream processing requires assessing how strongly the solute partitions into the solvent and how well the solvent discriminates the solute from impurities. Two complementary metrics summarize this capability.

Given Data / Assumptions:

• Two-phase liquid system (raffinate and extract).
• Target solute competes with other species.
• Equilibrium partitioning approximates stagewise operation.

Concept / Approach:
The distribution coefficient, D = C_extract/C_raffinate (for a defined basis), captures how much solute moves into the solvent—higher D means fewer stages or less solvent are needed. Selectivity compares the distribution coefficient of the solute to that of a key impurity (or matrix species). High selectivity enables purification, not just transfer.

Step-by-Step Solution:
Define D for the target: D_target = C_E/C_R at equilibrium.Define selectivity: S = D_target/D_impurity (or ratio for two components).High D_target reduces solvent usage and stage count; high S ensures purity.Therefore, both D and S are required to judge solvent effectiveness.

Verification / Alternative check:
McCabe–Thiele stage analyses show that increasing D shifts operating lines favorably, while selectivity determines achievable purity through countercurrent staging, validating the dual-metric assessment.

Why Other Options Are Wrong:
D only: ignores separation from impurities.

Selectivity only: ignores capacity/phase ratio implications.

Diffusivity: affects kinetics but not the fundamental equilibrium that sets stage requirements.

Common Pitfalls:

• Comparing D values measured at different phase ratios or temperatures.
• Overlooking solvent losses, toxicity, and miscibility limits that also impact selection.

Final Answer:
Both distribution coefficient(s) and selectivity