Melting Point Determination and Purification by Recrystallization Flashcards
What is recrystallization?
A way to purify solids
What are the four important concepts in recrystallization?
- Solubility
- Saturation Level
- Exclusion
- Nucleation
What is the most important thing when choosing a solvent?
Dissolves the solute well at high temps, but not at low temps
What is the solvent mixture used for the recrystallization?
Water and ethanol
What is second most important when choosing a solvent?
Will not react with the solute
What is nice, but not necessary, when choosing a solvent?
Nonflammable, nontoxic, inexpensive, volatile
What is important about saturation level?
We want to use a minimal amount of solvent to dissolve at boiling (saturated at boiling)
As we cool our saturated solution…
it becomes supersaturated.
Which compound crystallizes first?
The more pure, more present compound. The impurities are excluded.
Can you crystallize a 50/50 mixture?
No, you must have more of one compound than the other
Should we cool quickly or slowly?
Slowly, if we cool too fast we just get a precipitate
What is nucleation?
Something for the crystals to grow on, scratches, dust, seed crystal
Why is it not a melting point?
The is never an exact temperature, but a melting range
Benzoic acids melting range
118-120
What is the effect of impurities on a melting range?
Impurities lower and broaden the melting range
