Organic Liquid Preparation & Purification
Removal of Impurities: The organic product is often 'washed' in the separating funnel with aqueous solutions to remove specific contaminants. For example, sodium hydrogencarbonate () is used to neutralize and remove acidic impurities, while water is used to remove highly soluble salts or alcohols.
Drying Agents: After separation, the organic liquid still contains trace amounts of water which must be removed using an anhydrous inorganic salt. Common drying agents include anhydrous magnesium sulfate () or calcium chloride (), which hydrate upon contact with water.
The Drying Process: The drying agent is added to the liquid in a flask and swirled; it is added incrementally until the salt no longer clumps together and instead flows freely like a 'snow globe.' Once dry, the liquid is separated from the solid drying agent via gravity filtration.
Principle of Distillation: Distillation is the final step used to obtain a pure liquid by separating it from remaining impurities based on differences in boiling points. The liquid is heated until it vaporizes, and the vapor is then condensed and collected as a 'distillate' at a specific temperature.
Temperature Monitoring: A thermometer is positioned at the T-junction of the distillation head to measure the temperature of the vapor as it enters the condenser. A steady temperature reading that matches the known boiling point of the target compound indicates that a pure fraction is being collected.
Fractional Collection: If the mixture contains multiple volatile components, the receiver flask is changed as the temperature rises. This allows the chemist to isolate the desired product from 'pre-run' (lower boiling point) and 'post-run' (higher boiling point) impurities.
| Feature | Reflux | Distillation |
|---|---|---|
| Condenser Orientation | Vertical (returns liquid) | Slanted (collects liquid) |
| Primary Goal | Synthesis/Reaction completion | Purification/Separation |
| System State | Open to air at top | Open to air at receiver |
Safety and Setup Checks: Always ensure the apparatus is not a 'closed system' before heating; a sealed system will explode due to gas expansion. In diagrams, check that the water enters the condenser at the bottom and leaves at the top to ensure the jacket is completely filled.
Thermometer Placement: In distillation questions, the bulb of the thermometer must be exactly level with the opening to the condenser. If it is too high, it will read a temperature lower than the actual vapor; if too low, it will measure the temperature of the boiling liquid rather than the vapor.
Yield Calculations: Remember that purification steps (washing, transferring between flasks) inherently reduce the final yield. When evaluating a procedure, identify steps where 'mechanical loss' (liquid sticking to glassware) or 'chemical loss' (solubility in the aqueous layer) might occur.