Recrystallization Flashcards
Explain what affect each would have on the % yield of a recrystallization
a. Too little solvent used for dissolution
The crystals will be less pure, too little solvent will create a supersaturated solution that will crystallize too quickly and trap impurities
The hot solution is cooled quickly in an ice-water bath
The crystals will be less pure. Crystals will form too quickly and trap impurities in the crystals.
Too much decolorizing agent is used
This will result in less yield, if too much colorizing agent is added, the desired product will be absorbed along with the coloring impurity
Cooled solution is left to stand, and all solvent evaporates
The crystals will be less pure.
The evaporated solution will leave behind impurities along with the desired solid
Solution does not form crystals immediately, but after sitting overnight the liquid has completely evaporated to leave solid
less pure
- the evaporated solution will leave behind impurities along with the desired solid
Why is it important to let your solution cool slowly to room temperature after placing it in an ice water bath
By slowly dropping the temperature of the solution, pure crystals of the desired compound will form properly
If the solution is cooled rapidly, impurities will be trapped within the crystals.
Why is the second crop less pure than the first
The smaller volume and high concentration of impurities in the mother liquor will cause the impurities to precipitate with the crystals of the second crop.
How do you collect a second crop of crystals
You reduce the volume of the mother liquor to ½-1/3 of the volume of the first filtered solution and allow the solution to cool to room temperature to form crystals.
Then you ice bath the solution to increase crystal yield and vacuum filter the crystals and clean the solids with a cold solvent.
If Compound A is soluble in water and ethanol at room temperature, but insoluble in dichloromethane, petroleum ether, and toluene at all temperature, how would you recrystallize the compound A
You need to use a two solvent mixed system with two miscible solvents like ethanol and dichloromethane
How much solvent should be used to recrystallize 1.0g of an unknown sample?
Just enough to dissolve the unknown sample, too much solvent will create an undersaturated solution and the crystals will not form as the solution cools.
When do you filter using the gravity method, and when is it more appropriate to using vacuum filtration
You use it to remove any decolorizing agents or insoluble impurities from the solid.
The gravity filtration is more appropriate when the LIQUID is desired NOT the SOLID
why might activated charcoal be added to a sample during recrystallization
the absorb impurities casing the recrystallization solution to have an incorrect color
if crystal growth does not start on its own after the solution in the flask has returned to room temperature, what should you do
scratch the bottom of the flask with a stirring rod
add a bit of seed crystal
