Organic Chemistry II  | Lecture | Laboratory 

Organic Chemistry Laboratory II
Saponification: Synthesis and Analysis of Soap

Experiment Description

Caution:  This experiment involves the use of very strong NaOH solution.  Gloves must be worn for all parts of the experiment. All parts of the experiment must be done in the hood.  Safety glasses are a must! 
In this two week experiment, students will work individually to prepare soap.  Soap will be prepared through saponification (ester hydrolysis) of  triacylglycerols derived from vegetable shortening (i.e., Crisco).  Reaction of triacylglycerols with sodium hydroxide in de-ionized water will promote saponification to generate the soap, which is a mixture of carboxylate salts of fatty acids.  The soap product will be evaluated for hardness/softness, feel, solubility in hard/soft water and its ability to emulsify oils. 

Each student will prepare approximately 5g of soap. 

Weigh out 5g of vegetable shortening into a 50-ml round-bottom flask.  Add 5 ml of ethanol and 5 ml of 20% (5M) NaOH. Clamp the round bottom flask to a vertical bar.  Fit the flask with a heating mantle above a magnetic stirplate.  Grease the joint with a minimal amount of shortening.  Insert a magnetic stir bar into the round-bottomed flask.

Insert a reflux condenser into the round-bottom flask joint (See Figure 3). Connect the heating mantle to a Variac, being sure to twist the heating mantle cable connection to make proper contact.  Connect the condenser hoses to the water faucet.  (Water comes into the condenser at the bottom and out at the top) and turn on the water.  Set the Variac to full and flip the Variac switch to the "on" position.  Allow the solution to reach reflux, reduce the Variac to 60%, and continue to  reflux for 15 minutes. The reaction mixture should turn a pale yellow.



While the shortening/NaOH mixture is refluxing, measure out 50 ml of 28% NaCl (saline) into a 150ml beaker in a plastic ice bath.  After the 15 minutes, disassemble the reflux apparatus and, using a Hot Hands to hold the warm flask, carefully pour the warm reaction mixture (including the stir bar) into the beaker containing the ice cold saline.  Immediately place the beaker and ice bath on the stir plate.  Stir vigorously for 5 minutes.  A white precipitate (soap) should form. Allow the mixture to stir vigorously while setting up a vacuum filtration apparatus.

Clamp the flask to a vertical bar and insert a Buchner funnel fitted with a piece of filter paper into the flask.  Place 30ml of 28% NaCl (saline) in a 150 ml flask in the ice bath.   Use ~10ml of this cold 28% NaCl (saline) to wet the filter paper.  Turn on the vacuum and pour the soap mixture onto the Buchner funnel.  Rinse the soap with the remaining 20ml of ice cold 28% NaCl (saline).  Then wash the soap once with 20ml of cold ethanol. Watch the volume of liquid in the filtration flask and empty it periodically if it exceeds half the volume of the flask.

Transfer the washed soap to weigh boat labeled with your name and date.  Place the granular soap in the container at the instructor’s bench for week 2.

Clean up and Waste Disposal:  Dispose of all basic waste in the basic waste container.  Dispose of filter paper in the solid waste container.  Wash all glassware, spatulas etc first with soap and water, and rinse with acetone.  Be sure that no residues remain on the glassware before you return it to your drawer. 

End of Week 1

Over the course of the week the soap should dry and harden.  Loosen the soap from the mold and weigh the bar.  Calculate the yield of soap (by weight) using the mass of the vegetable shortening as the starting material.  (Crisco is hydrogenated cotton seed oil and is a complex mixture of triglycerides. An exact molecular weight is not available so yield will be based on mass not moles.) Describe properties of the soap bar (hard, soft , crumbly etc) and record your observations.

Assessment of the Soap's Ability to Emulsify Oils
One of the properties of soaps and detergents that makes these materials effective cleansing agents is their ability to emulsify or break up oils so they can be removed.  The synthesiszed soap will be evaluated and compared qualitatively to a commerical detergent and water.  Measure approximately 500mg of soap and place it in a 50 or 100ml beaker.  Add 25 ml of deionized water to the beaker and heat it to boiling until the soap completely dissolves (Add more DI water if it evaporates during he heating process).  Set up a test tube rack containing three labeled test tubes (1, 2, 3) (13 x 100mm). Place 0.5ml of mineral or other oil into each test tube.  Add 5 ml of DI water to tube #1, 5 ml of your soap solution to tube #2,  and 5 ml of detergent solution to tube #3.  Stopper each tube and shake the tube vigorously for 10 seconds.  Immediately observe the tube for emulsification and record how long the emulsion remains before the layers separate for each mixture.

Formation of Soap Scum
Many soaps form soap scum or residue when they come into contact with hard water.  Hard water contains ions like calcium, magnesium and iron that replace the sodium or potassim ions of the soap to generate insoluble compounds (i.e., soap scum).  Detergents which differ from soaps do not generate insoluble salts with hard water ions.  In this section of the experiment, the synthesized soap interaction with hard water ions will be compared with the interaction between detergents and these ions.  Set up six test tubes (13 x 100mm) and label them #1-#9.  Place 3 ml of the soap solution (generate above) in test tubes #1-3, 3ml of the detergent solution in test tubes #4-6 and 3ml of DI water in tubes #7-9 (See table below for set up).  Add 1 ml of 1% CaCl2 solution to tubes #1, #4, #7 1ml of 1% MgCl2 to tubes #2, 5 and 8,  and 1ml of 1% FeCl3 to tubes # 3, 6 and 9.  Agitate the tubes and observe each tube for the formation of foaming and residue (soap scum). Record your observations in your notebook.  Add 1 ml of 9% sodium phosphate tribasic (water softener) to each tube.  Agitate the tubes and observe for foaming and whether the residue dissolves.  Record these results in your notebook

Observations after addition of sodium phosphate
Soap + CaCl2

Soap + MgCl2

Soap + FeCl3

Detergent + CaCl2

Detergent + MgCl2

Detergent + FeCl3

DI Water + CaCl2

DI Water + MgCl2

DI Water + FeCl3

End of Week 2