Unit 3: Separation Techniques-I
Course Code: CHM-DSC-352
Paper Name: Analytical Chemistry
Solvent extraction (also known as liquid-liquid extraction) is a method to separate a compound based on its relative solubilities in two different immiscible liquids, usually water and an organic solvent.
Key Requirements
- The two solvents must be immiscible (e.g., Water and Chloroform).
- The solute should have a significant difference in solubility between the two phases.
- The process is physically driven by the transfer of mass from one phase to the other.
The efficiency of an extraction is governed by the Nernst Distribution Law.
Distribution Coefficient (Kd)
For a solute that exists in the same molecular form in both phases:
Kd = [Solute]_organic / [Solute]_aqueous
Distribution Ratio (D)
In real analytical systems, the solute may dissociate or associate. The distribution ratio (D) considers the total concentration of all forms of the solute in each phase.
D = (Total concentration of solute)_org / (Total concentration of solute)_aq
Percent Extraction (%E): This relates the distribution ratio to the volumes of the phases: %E = (100 * D) / [D + (Vaq / Vorg)].
- Batch Extraction: The most common laboratory method using a separating funnel. A specific volume of solvent is added, shaken, and the layers are separated. Multiple extractions with smaller volumes are more efficient than a single extraction with a large volume.
- Continuous Extraction: Used when the distribution ratio is very low. The solvent is evaporated, condensed, and passed through the aqueous phase repeatedly.
- Counter-current Extraction: A sophisticated method for separating solutes with very similar distribution ratios.
4. Ion Exchange: Principles and Resins
Ion exchange involves the reversible exchange of ions between a liquid phase and a solid phase (the ion exchange resin).
Types of Ion Exchange Resins
- Cation Exchange Resins: Contain acidic functional groups (e.g., -SO3H or -COOH). They exchange their H+ ions for other cations in the solution.
- Anion Exchange Resins: Contain basic functional groups (e.g., quaternary ammonium groups). They exchange their OH- ions for other anions.
5. Ion Exchange Capacity and Applications
Ion Exchange Capacity: The number of milliequivalents of exchangeable ions per gram of dry resin. This determines how much solute the resin can remove before it needs regeneration.
Analytical Applications
- Deionization of Water: Passing water through both cation and anion exchangers to remove all dissolved mineral salts.
- Separation of Lanthanides: Utilizing small differences in affinity for the resin to separate chemically similar elements.
- Concentration of Trace Elements: Collecting small amounts of metal ions from large volumes of water for easier analysis.
6. Exam Focus: Tips and FAQs
Exam Tips
- Efficiency Rule: Always state that "multiple small extractions are more efficient than one large extraction." Be prepared to prove this mathematically if asked.
- Resin Types: Differentiate clearly between "Strong Acid" (Sulphonic acid) and "Weak Acid" (Carboxylic acid) cation exchangers.
- Selectivity: Factors like ionic charge and ionic radius affect how strongly an ion is held by the resin (higher charge and smaller hydrated radius lead to stronger binding).
Frequently Asked Questions
Q: What is the Nernst Distribution Law?
A: It states that at constant temperature, a solute distributes itself between two immiscible liquids such that the ratio of its concentrations in the two phases is constant.
Q: How do you regenerate a cation exchange resin?
A: By passing a concentrated solution of a strong acid (like HCl) through the column to replace the captured metal ions with H+ ions.
Q: Define "Salting Out" in solvent extraction.
A: The addition of an electrolyte (like NaCl) to the aqueous phase to decrease the solubility of the organic solute in water, thereby increasing its distribution into the organic phase.