Acid / Base Extractions

Introduction: View and take notes on the Extractions link web page and read  pages Chapter 15 on Extraction and Washing in lab text.

Purpose: This experiment will make use of extraction techniques to separate a mixture of an acid, a base, and a neutral compound  into its individual components. 

Procedure:

* Note: Wear gloves during this experiment. If any of the chemicals used in this experiment do come in contact with your skin, wash the affected area with soap and water.  Also, some of these chemicals will stain your clothes permanently.  Therefore, it is suggested that you wear something that you don't mind possibly ruining.

In a 50-mL Erlenmeyer flask, place the sample (of benzoic acid, p-nitroaniline, and azobenzene).

Record the exact amount that you weigh.  Completely dissolve this mixture in 35 mL of methylene chloride with swirling.

Using a funnel, pour the solution into a 125 mL separatory funnel (make sure that the stopcock is closed first).  

Extract the solution with two 25-mL portions of 6 M hydrochloric acid. To do this, you will first pour 25 mL of 6 M HCl into the separatory funnel containing your methylene chloride solution. Stopper the funnel, invert, and gently shake the funnel with frequent venting. With stopcock closed, upright the separatory funnel and place it in the holder. You will see two layers in the funnel. Determine which layer is the aqueous and which is the organic.

Loosen the stopper and then drain the bottom layer into a labeled beaker. Then drain the remaining layer into a separate labeled beaker. With the stopcock closed, pour the organic layer back into the separatory funnel. Add an additional 25 mL of 6 M HCl to the solution in the funnel. Repeat the shaking/venting process as before and drain the layers into the appropriate beakers with the aqueous layer being added to the aqueous layer collected from the first extraction process. Once these steps are completed, you have performed the “extraction of the solution with two 25-mL portions”.  

Pour the organic solution back into the separatory funnel and this time extract it with two 25-mL portions of 3 M sodium hydroxide solution following the extraction procedure from above. Place the aqueous layer from this set of extractions into a third labeled beaker (separate from the aqueous layers from the extraction with acid).

At this point, the organic layer may still contain some aqueous solution. To “dry” the solution (or remove the aqueous portion), add approximately 1 g of anhydrous magnesium sulfate to the organic layer and swirl.

While the organic solution is drying over magnesium sulfate, cool each of the aqueous extract layers in an ice-water bath. Neutralize the “acid extract” (the aqueous layer from your first set of extractions) with 6 M sodium hydroxide until the solution is distinctly basic to litmus paper. Neutralize the “basic extract” (the aqueous layer from your second set of extractions) with 6 M HCl until the solution is distinctly acidic to litmus paper. Upon neutralization of each solution, a precipitate should appear.

Collect the precipitates separately by vacuum filtration. Wash each of the precipitated solids (while on the Buchner funnel) with cold distilled water. Pre-weigh two small labeled watch glasses and place the solid materials collected on each of them. Place these samples in an oven (at 90° C) to dry for approximately 1 hour.

Separate the “organic solution” from the magnesium sulfate by gravity filtration into a small labeled pre-weighed beaker. Place beaker containing organic layer in the hood overnight in order for the methylene chloride to evaporate.  

After the samples have dried, reweigh each of the watch glasses and beaker to obtain the weights of each of the separated materials recovered.

Results and Conclusions:

How were you able to separate the three components in this mixture?  Answer by using a flow chart.

What was your percent recovery for each of the three materials (show calculations in “results section” of your report)?

If your percent recovery is not exactly 100%, what could be some reasons for this?

On the basis of what you learned in this experiment, plus the information that phenol is soluble in sodium hydroxide solution but not in sodium hydrogen carbonate solution, whereas benzoic acid is soluble in both solutions, show how you could separate a mixture containing phenol, benzoic acid, p-nitroanaline, and azobenzene. (Complete your answer in the form a flow chart.)

Why is anhydrous magnesium sulfate added to the organic layer?

Why was the water used for washing the precipitates on the funnels specified to be cold?

What is the total mass of the three separate compounds? Does this amount differ from the initial mass of the mixture? Why?