LAB51

51 Preparation of Soap

Background

Have you ever considered that soap is one of societyís major defenses against disease? The cleaning power of soap helps rinse away many disease-causing organisms, making your home and school a healthier place than they otherwise would be. Soaps are alkali metal salts of carboxylic acids. They are generally produced by the reaction of metallic hydroxides with animal fats and vegetable oils. THE major components of these fats and oils are triglycerides, esters of glycerol, and various fatty acids. Typically, soaps are made by hydrolyzing the ester bonds of triglycerides with solutions of sodium hydroxide. This soap-making reaction is called saponification (in Greek sapon means soap). The products of the hydrolysis reaction are soap and glycerol.

In this experiment, you will saponify a vegetable oil and examine some properties of your product. You will compare the properties of the soap you make with the properties of a commercial detergent and a commercial hand soap.

Goals

Apply saponification to vegetable oil.

Observe the properties of the soap that is formed.

Compare the properties of the soap formed to the properties of commercial detergents and hand soaps.

Equipment

safety goggles	1 gas burner	1 test-tube rack
2 50-mL beakers	1 ring stand	1 dropper pipet
2 250-mL beakers	1 ring support	1 plastic wash bottle
1 10-mL graduated cylinder	1 wire gauze	1 spatula
1 glass stirring rod	3 medium test tubes	1 scoopula
4 centigram balances/class	3 cork stoppers

Materials

8 glass-marking pencils/class	laundry detergent
vegetable oil	hand soap
paper towels	0.1M calcium chloride, CaCl2
50% (v/v) ethanol-water mixture	0.1M iron (III) chloride, FeCl3
sodium hydroxide, NaOH	0.1M magnesium chloride, MgCl2
wide range indicator solution or wide-range test paper

Safety

Note the Safety Symbols used here and in the Procedure section. Review safety information on pages 7-10.

Always wear safety goggles when working in the lab.

Sodium hydroxide is corrosive and can cause severe burns. Never handle sodium hydroxide pellets with your fingers; use a small beaker and a scoopula. Solid sodium hydroxide will absorb water from the atmosphere; do not leave the container of sodium hydroxide open.

Keep ethanol and ethanol-water mixtures away from open flames.

Aqueous iron chloride will stain clothes permanently and is irritating to the skin. Avoid contact with this material.

Procedure

Copy Table 51.1 into your laboratory notebook. As you perform the experiment, record your results in this table.

Part A. Preparation of Soap

1. Pour 5 mL (5.0 g) of vegetable oil into a 250-mL beaker.

2. Measure 15 mL of 50% ethanol-water mixture into a 50-mL beaker. Slowly dissolve 2.5 g of NaOH pellets in ethanol-water mixture.

3. Add 2-3 mL of the NaOH solution to the beaker containing the oil. CAUTION: Keep your face away from the beaker. Heat the mixture over a low flame while stirring. Every few minutes, for about 20 minutes, add a portion of the ethanol-water mixture while continuing to stir. Heat and stir for about 10 more minutes. The oil should be dissolved and a homogeneous solution should be obtained.

4. Add 25 mL of cold water to the hot solution. Using a towel "handle" as shown in Figure 30.1, pour this solution into a 250-mL beaker containing 150 mL of saturated NaCl. Stir this mixture gently an allow it to cool for several minutes.

5. Using a spatula, skim off the top layer of soap and place it in a 50-mL beaker.

Part B. Properties of Soaps and Detergents

6. Place a pea-sized lump of your soap into a test tube. Use a scoopula to put a similar amount of laundry detergent in a second tube and a similar amount of hand soap in a third tube. Add 10 mL of water to each tube, stopper them, and shake the tubes thoroughly. In this step and throughout this experiment, use a test-tube rack as needed.

7. Estimate the pH of the solutions, using wide-range indicator solution or wide-range test paper. Record the results. Pour the contents of the test tubes down the drain. Rinse the test tubes and stoppers with water.

8. Mark three test tubes with the labels "CaCl2", "FeCl3", and "MgCl2," respectively.

9. Prepare a detergent solution by dissolving 0.3g of detergent in 30 mL of water. Divide this solution equally among the three test tubes. Add solutions to the test tube as follows:

CaCl2 test tube 1.0 mL (or 20 drops) of 0.1 M CaCl2

FeCl3 test tube 1. mL of 0.1 M MgCl2

MgCl2 test tube 1.0 mL of 0.1 M FeCl3

Stopper each tube and shake it to mix. Record your observations. Pour the contents of the test tubes down the drain. Rinse the test tubes and stoppers with water.

10. Repeat step 9, but replace the detergent solution with a hand-soap solution of the same strength. Record your observations.

11. Repeat step 9, but replace the detergent solution with a solution of your soap of the same strength. Record your observations.

12. Follow your teacherís instructions for proper disposal of the materials.

Data Record

Table 51.1 Properties of Soaps and Detergents

Test Substance pH of Solution CaCl2 Effect of Adding FeCl3 MGCl2

your soap

detergent

hand soap

Conclusions

1. Write the reaction for saponification of a typical fat (or oil) with sodium hydroxide. Include structural formulas.

2. How does the pH of the soap solution that you prepared compare with those of the solution of commercial laundry detergent and the solution of hand soap? Which of these products would have the harshest effect on the skin?

3. The metal ions Ca2+, Fe3+, and Mg2+ all contribute to the formation of hard water. What differences did you observe when the metal ions Ca2, Fe3+, and Mg2+ were added to a soap or a detergent? Do you think that a soap or detergent would make a better cleansing agent in hard water? Explain.