INTRODUCTION Soap is produced by the saponification (hydrolysis) of a triglyceride (fat or oil). (See Figure 1.) In this process the triglyceride is reacted with a strong base such as sodium or potassium hydroxide to produce glycerol and fatty acid salts. The salt of the fatty acid is called a soap. Fatty acids are straight-chain monocarboxylic acids. The most common fatty acids range in size from 10-20 carbons and most often have an even number of carbon atoms including the carboxyl group carbon. The carbon-carbon bonds in saturated fatty acids are all single bonds, while unsaturated fatty acids have one or more carbon-carbon double bonds in their chains. One example of a saturated fatty acid is palmitic acid, CH3-(CH2)14-CO2H. Fatty acids are seldom found as free molecules in nature but are most often a part of a larger molecule called a triglyceride. Triglycerides consist of a three-membered carbon chain (glycerol backbone) with a fatty acid bonded to each of the three carbon atoms in the glycerol backbone. The bond between the fatty acid and the glycerol backbone is referred to as an ester linkage. In the saponification process the ester linkage is broken to form glycerol and soap.

Figure 1.  Saponification of a triglyceride

1. PUT ON YOUR CHEMICAL SPLASH-PROOF SAFETY GOGGLES!

Caution: Oil will be hot, and may splatter or catch fire. Wear goggles at all times.

 
 

Assembling the hot water bath

2. Assemble a hot water bath by filling an 800 mL beaker approximately 3/4 full with water and begin heating the water with a bunsen burner.

3. Add three or four boiling chips to the water in the hot water bath to prevent the water from boiling over.

    A. 15 mL of oil (or 10 g of solid shortening)

    B. 20 mL of 20% NaOH

    C. 10 mL of ethanol

    D. 3 boiling chips (These will help prevent the mixture from boiling over while it is being heating.)

5. Note the total volume (level) in the 150 mL beaker and how many layers the ingredients initially form.
 

7. Using a stirring rod, stir the reaction mixture frequently so that it does not boil over.

8. Maintain the total volume of the reaction mixture by adding small quantities of 1:1 (volume/volume) ethanol-deionized water.

9. After the initial 25 minute heating there should be no separation of layers in the beaker.
 

12.  At this point, measure out about 50 mL of saturated NaCl solution and cool it also for about 5-6 minutes in the ice bath.

13.  After the cooling time is complete for the soap reaction mixture, decant any liquid from the beaker.  (Be careful not to pour off the soap.)

14. Next add, add the 50 mL cold, saturated NaCl solution to the soap beaker and stir thoroughly with a glass rod. This process separates the soap from the glycerol and excess base and is called "salting out."

15. Collect the solid soap, using a Buchner funnel.  [Note:  Decant as much liquid off before adding the solid soap to the Buchner funnel.]

16. While the air is being drawn over the soap in the funnel, wash the soap with two 20 mL volumes of ice cold deionized water. Continue to draw air over the soap for another 3 minutes.
 

    A. Test the pH of the first tube by dipping a clean stirring rod into the solution and touching the solution to     both red and blue litmus paper. Record your observations on the Data Sheet.

    B. In the second test tube add several drops of calcium chloride solution. Record your observations.

    C. In the third test tube add several drops of iron(III) chloride solution. Record your observations.

    D. In the fourth test tube add a single drop of kerosene and shake the solution. Record your observations.

18. Pour the contents of the four test tubes into the designated waste container.
 

20. Thoroughly rinse all of the glassware with water before storing.