Purification Techniques
Sublimation, Crystallization, and Differential Extraction.
After synthesis, organic compounds are rarely pure. Depending on the nature of the compound and the impurities present, various techniques are employed for purification. This guide covers techniques for solids (Sublimation, Crystallization) and liquid-liquid separation (Extraction).
1. Sublimation
Principle
Some solid substances change directly from the solid state to the gaseous state (vapor) upon heating, without passing through the liquid state. This process is called sublimation.
It is used to separate sublimable compounds from non-sublimable impurities.
- Naphthalene
- Iodine
- Camphor
- Ammonium Chloride ($NH_4Cl$)
- Anthracene
- Benzoic Acid
2. Crystallization
Principle
This is the most common method for purifying solids. It is based on the difference in solubility of the compound and the impurities in a suitable solvent.
The compound should be sparingly soluble in the solvent at room temperature but highly soluble at higher temperatures.
Steps Involved:
- Preparation of Solution: The impure solid is dissolved in the minimum amount of hot solvent to form a saturated solution.
- Hot Filtration: Insoluble impurities are removed by filtering the hot solution.
- Crystallization: The filtrate is allowed to cool slowly. Pure crystals separate out, leaving soluble impurities in the mother liquor.
- Separation & Drying: Crystals are filtered, washed with cold solvent, and dried.
Fractional Crystallization: Used when separating a mixture of two or more substances having different solubilities in the same solvent.
3. Differential Extraction
Principle
This method involves separating an organic compound from its aqueous solution by shaking it with an organic solvent in which the compound is more soluble than in water.
- The organic solvent must be immiscible with water (e.g., Ether, Benzene, Chloroform).
- The apparatus used is a Separatory Funnel.
Where $C_{org}$ and $C_{aq}$ are concentrations of the solute in the organic and aqueous phases, respectively.
Using a specific amount of organic solvent in multiple small portions (multi-step extraction) extracts a larger amount of substance than using the entire solvent in a single operation.
Continuous Extraction: If the compound has a very low solubility in the organic solvent (low partition coefficient), continuous extraction is used (running solvent continuously through the aqueous layer).
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