Extraction is used to separate a desired chemical compound from a mixture
of compounds. In this procedure you will learn how to use the equipment
involved in extraction.
Students may wish to wear gloves when working with chemicals.
Students make all their own solutions in this lab class. You may prepare
solutions individually, in partners or as a group. For this experiment,
each experimenter needs 0.5 grams of benzoic acid combined with 0.5 grams
of 1,4-dimethoxybenzene, 20 mL of tert-butylmethyl ether, 10 mL of 2M
sodium hydroxide solution, 20 mL of a saturated sodium chloride solution
and 20 mL of distilled water. When your solutions are prepared, you are
ready to begin the separation procedure.
|Transfer the benzoic acid and
1,4-dimethoxybenzene mixture to an Erlenmeyer flask that contains 20 mL
of tert-butylmethyl ether. Swirl and stir until dissolved (no solid particles).
To minimize the time required for dissolving a solid, crush the solid beforehand
so that maximum surface area is exposed. You can use a mortar and pestle
or the bottom of a small beaker as your tool for crushing and grinding.
Once solvent is added, stir vigorously. If the solute (the solid material
you are trying to dissolve in the solvent) stubbornly refuses to dissolve,
you can heat it gently.
|To set up for extraction, begin
by attaching the ring clamp securely to the ring stand. Tighten or loosen
the nut on the stopcock of the separatory funnel as needed and then place
the separatory funnel in the ring clamp. Adjust the height so that an Erlenmeyer
flask will fit underneath and make sure the stopcock is in the closed (horizontal)
position. Be sure to pour all liquids at eye level. Transfer the solution
prepared in Step 1 to the separatory funnel using a gravity funnel.
| Add about 20 mL of distilled
water and note the development of two layers, an aqueous layer (bottom)
and an ether layer (top). The ether layer contains both the benzoic acid
and 1,4-dimethoxybenzene. Benzoic acid has a carboxylic acid functional
group, which makes it an acidic compound, while 1,4-dimethoxybenzene has
two ether groups and is neutral. Benzoic acid will react with a base like
sodium hydroxide to create sodium benzoate, which is soluble in water.
|Add 10 mL
of the 2-M sodium hydroxide solution (base) to the separatory funnel
and add the stopper. Remove the separatory funnel from the ring
clamp and grip it firmly. Hold the stopper in place. Invert the
funnel, gently swirl the mixture and then immediately vent by turning
the stopcock to the open position to release built-up pressure.
Notice that you need to hold the stopper in place with one hand
while turning the stopcock with the other.
When you vent, the tip of the separatory funnel should not point at YOU, or ME, nor your nearest neighbor.
With the funnel inverted and the stopcock firmly in place, do this: shake, shake, shake, vent. Again: shake, shake, shake, vent. One more time! Mix the contents thoroughly and then return the separatory funnel to the ring clamp. Allow the layers to settle and separate. Open the stopcock and drain the lower layer (aqueous) into flask 1. Try to cleanly separate the upper and lower layers.
|Next add 20 mL of saturated sodium
chloride solution to the separatory funnel to dry the ether layer and drive
out any suspended water molecules. As in Step 4, shake, shake, shake, vent.
One more time! Return the separatory funnel to the ring clamp and let the
layers settle and separate. Remove the stopper and drain the lower aqueous
layer containing the sodium chloride solution and water into flask 2. Set
aside to be discarded. The organic layer is poured out through the top into
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