Separation Schemes Flashcards
Typical physical manipulations done in separation schemes
extracting, washing, rinsing, filtration, evaporation of solvent, and drying.
Extracting or Washing.
This involves mixing immiscible solvents, usually using a separatory funnel – one layer will be on top and the other will be on bottom.
Precipitation
If a heterogeneous solid/liquid mixture is produced (such as with acidification or basification) and is separated, then the two components after separation are known as the filtrate (liquid) and the solute (solid), whether by gravity or vacuum filtration.
Drying agents.
When a solution of an organic compound is dried using a drying agent (such as anhydrous Na2SO4 or MgSO4), the resulting mixture is similar to that from a Precipitation process – the resulting mixture is an organic compound in solution and a solid (the hydrate) that requires removal by filtration. (Note that the drying agent never “dissolves” in an organic solvent, but simply goes from anhydrous to hydrated)
Evaporation
Usually, when the final product is in a solution with an organic solvent, such as after extraction and the use of a drying agent, the solvent is removed by evaporation. This process is officially distillation. Common methods of evaporation include using a rotary evaporator (RotoVap) or a hot plate in a hood
What are some common drying agents?
- sodium sulfate Na2SO4
- calcium chloride Ca2Cl2
- magnesium sulfate MgSO4
- potassium carbonate K2CO
Multiple Extractions vs single extractions
Multiple extractions with small volumes of organic solvent will removemore solute than a single extraction with a large volume of solvent
.
i.e. 2 extractions with 3 mL each CH2Cl2 will net you more caffeine than1 extraction with 6 mL CH2Cl2
What is the distribution coefficient?
The solute will partition between the phases according to its distribution coefficient, K (a.k.a. partition coefficient)
K = C1 / C2
Where C1 and C2 are equilibrium solubilities of the solute in each solvent
When would you wash an organic mixture with water?
Water washes are designed to remove highly polar materials, such as:
1) in-organic salts,
2) strong acids or bases, and
3) low-molecular-weight, polar substances including alcohols, carboxylic acids, and amines.
When would treat an organic mixture (extraction) with a dilute acid (1-2M HCl)?
Acid extractions are intended to remove basic impurities, especially such basic impurities as organic amines.
The resulting cationic salts are usually soluble in the aqueous solution, and they are thus extracted from the organic material.
After treating an extraction mixture with dilute HCl, how do you remove all traces of acid?
A water extraction may be used immediately following the acid extraction to ensure that all traces of acid have been removed from the organic material.
When would one treat an organic mixture (extraction) with a dilute base (sodium bicarbonate or dilute NaOH)?
Basic extractions are intended to convert acidic impurities, such as organic acids, to their corresponding anionic salts.
The resulting anionic salts are usually soluble in the aqueous solution, and they are thus extracted from the organic material.
Which layer is on top?
1) use relative densities
or
2) Test it!
Add a few drops of water—which layer do they seek?Add a few drops of the organic solvent—where do they go?
Why are drying agents necessary if we’ve already separated the aquous layer from the organic layer?
Even after separating the phases (organic from aqueous), small amounts of water remain in the organic layer
