In liquid-liquid extraction, if the selectivity is unity, then
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- Separation of the components is most effective
- No separation is possible
- Amount of solvent required is minimum
- Solvent flow rate should be very high
The Correct Option is B
Solution and Explanation
In liquid-liquid extraction, selectivity (\( \beta \)) measures the ability of a solvent to preferentially extract one component over another from a feed mixture. For two components A and B in the feed, with a solvent S:
Let \( K_A \): Distribution coefficient of A (ratio of concentration of A in the extract phase to the raffinate phase),
Let \( K_B \): Distribution coefficient of B.
Selectivity is defined as: \[ \beta = \frac{K_A}{K_B}. \] If \( \beta>1 \), the solvent prefers A over B, enabling separation.
If \( \beta = 1 \), the solvent extracts A and B equally (\( K_A = K_B \)), meaning it has no preference.
Step 2: Analyze the effect of selectivity = 1.
Selectivity \( \beta = 1 \) means \( K_A = K_B \), so the solvent distributes both components A and B equally between the extract and raffinate phases.
In liquid-liquid extraction, separation relies on the solvent preferentially extracting one component over the other. If \( \beta = 1 \), there is no preferential extraction, and the compositions of A and B in the extract and raffinate phases will be the same as in the feed (adjusted for the amount of solvent). Thus, no separation is achieved. Step 3: Evaluate the options.
(1) Separation of the components is most effective: Incorrect, as \( \beta = 1 \) means no separation occurs. Incorrect.
(2) No separation is possible: Correct, as equal distribution coefficients prevent preferential extraction, resulting in no separation. Correct.
(3) Amount of solvent required is minimum: Incorrect, as the amount of solvent does not address the lack of separation; separation is still impossible. Incorrect.
(4) Solvent flow rate should be very high: Incorrect, as increasing the solvent flow rate does not overcome the lack of selectivity (\( \beta = 1 \)). Incorrect.
Step 4: Select the correct answer.
In liquid-liquid extraction, if the selectivity is unity, no separation is possible, matching option (2).
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