Crystallization - Techniques in Organic Chemistry

Crystallization is a common method to purify substance in a laboratory. We let a substance dissolves into a hot solvent, and let the solution cool down slowly to crystallize the substance out. In the process, the substance forms lattices orderly, but the impurities could not crystallize with the substance that is because different substance has different structure of lattices, so impurities tend to remain in solution.

Acetanilide is crystallizing from hot water

Experimental Process

In this experiment, we have two sub-experiments to do.

a. Acetanilide + Methylene blue(as impurity)
b. Acetanilide + Methylene blue(as impurity) ; use decolorizing carbon to absorb impurities

Experiment without decolorizing carbon:

1.      Weight 1.000g acetanilide in to a 50ml-flask. 

 Acetanilide before crystallization

2.      Pour 20ml water and put the stir bar into to the flask.

3.      Boil the solution on a hot plate until acetanilide are all dissolved.

 Acetanilide totally dissolved in hot water

4.      Add some drops(~2.0ml) of methylene blue.

5.      Take the flask from the hot plate and let the solution cool down to about 50℃.

6.      Put the flask into ice bath to crystallize acetanilide out.

 Solution in ice bath

7.      Whip the smoothie (water and acetanilide) and filtrate it through vacuum filtration.

 Vacuum filtration apparatus(Büchnertrichter)

8.      Air-dry the final product.

9.      Calculate the recycling efficiency.

Experiment with decolorizing carbon:

1.      Weight 1.000g acetanilide in to a 50ml-flask.

2.      Pour 20ml water and put the stir bar into to the flask.

3.      Boil the solution on a hot plate until acetanilide are all dissolved.

4.      Add some drops(~2.0ml) of methylene blue.

5.      Take the flask from the hot plate and let the solution cool down to about 50℃.

6.      Add some (~0.02g) decolorizing carbon into the flask.

 Decolorizing carbon in solution

7.      Boil the solution again.

8.      Use hot filtration to separate decolorizing carbon from the solution and collect the solution into a new flask.

 Hot filtration apparatus

9.      Take the flask from the hot plate and let the solution cool down to about 50℃.

10. Boil the solution to condense it to 20mL

11.  Put the flask into ice bath to crystallize acetanilide out.

12.  Whip the smoothie (water and acetanilide) and filtrate it through vacuum filtration.

13.  Air-dry the final product.

14.  Calculate the recycling efficiency.

Experimental Record

Experiment without decolorizing carbon	Acetanilide (g)	1.020
	Filter paper (g)	0.200
	Methylene blue (ml)	2.0
	Final weight of acetanilide(g)	0.770
	Recycling efficiency (%)	75.5
	M.P. of acetanilide (℃)	113.0~114.0

Experiment with decolorizing carbon	Acetanilide (g)	1.010
	Filter paper (g)	0.180
	Methylene blue (ml)	2.0
	Decolorizing carbon (g)	0.030
	Final weight of acetanilide(g)	0.850
	Recycling efficiency (%)	84.2
	M.P. of acetanilide (℃)	114.0~115.0

 Product A: Without using decolorizing carbon. Still a little grayish.

 Product B: Using decolorizing carbon. With higher purity.

 

Analysis

		Percentage of error
Melting point of pure acetanilide (℃)	114.3	N/A
Melting point of acetanilide (without decolorizing carbon) (℃)	113.0~114.0	0.7 %
Melting point of acetanilide (with decolorizing carbon) (℃)	114.0~115.0	0.2%

 By testing melting points, we can also proof the purity of product B is higher than product A.