12.1: Making Soap Saponification Lab Procedure

Learning Objectives

• The objective of this laboratory is to make lye soap via the saponification reaction.

 

Background

Soap making has remained unchanged over the centuries. The ancient Roman tradition called for mixing rain water, potash and animal tallow (rendered form of beef or mutton fat). Making soap was a long and arduous process. First, the fat had to be rendered (melted and filtered). Then, potash solution was added. Since water and oil do not mix, this mixture had to be continuously stirred and heated sufficiently to keep the fat melted. Slowly, a chemical reaction called saponification would take place between the fat and the hydroxide which resulted in a liquid soap. When the fat and water no longer separated, the mixture was allowed to cool. At this point salt, such as sodium chloride, was added to separate the soap from the excess water. The soap came to the top, was skimmed off, and placed in wooden molds to cure. It was aged many months to allow the reaction to run to completion.

All soap is made from fats and oils, mixed with alkaline (basic) solutions. There are many kinds of fats and oils, both animal and vegetable. Fats are usually solid at room temperature, but many oils are liquid at room temperature. Liquid cooking oils originate from corn, peanuts, olives, soybeans, and many other plants. For making soap, all different types of fats and oils can be used – anything from lard to exotic tropical plant oils.

 

Saponification Reactions

Soap can emulsify fats and oils by forming micelles around oil droplets. The soap molecules surround an oil droplet so that their nonpolar tails are embedded in the oil and their charged “head” groups are on the exterior of the droplets, facing the water. If the oil droplets are small enough and if there are enough soap molecules to surround them, the oil droplets become dispersed in the water and can then easily be washed away. Therefore, using lots of soap, hot water, and agitation can help clean greasy dishes. Hot water can melt solid fats, and agitation can help break up the fats and oils into smaller droplets.

Using lots of soap makes it more likely that there will be enough soap molecules to surround and emulsify all of the fat droplets. Soaps are less effective in hard water, which is water that contains a significant concentration of magnesium, calcium and iron ions. These ions form precipitates with soap molecules, and this precipitate is often seen as a gray line on a bathtub or sink and is often called “soap scum”. Since soap forms a precipitate with these ions, it means that many of the soap molecules are no longer present in the solution. Therefore, soap will form fewer suds in hard water. “Soft water” is water that contains very few or no ions that precipitate with soap. Soap will therefore be much more effective in soft water than in hard water.

Detergents are similar to soaps in that they have a charged head group and a long nonpolar tail group, but they are not prepared from natural fats or oils. Detergents are useful because they do not form precipitates with magnesium, iron or calcium ions, which means that they work in both soft and hard water. Shown below is a typical detergent molecule, sodium lauryl sulfate (which you may recognize from ingredient lists of shampoos or other cleaning products):

Figure \(\PageIndex{1}\): Sodium Lauryl Sulfate (a non-biodegradable detergent)

After detergents started being widely used, it was discovered that they were not broken down in sewage treatment plants. Many streams and lakes became contaminated with detergents and large amounts of foam appeared in natural waters. Biodegradable detergents were then developed. Shown below is an example of a biodegradable detergent, sodium laurylbenzenesulfonate.

Figure \(\PageIndex{2}\): Sodium Laurylbenzenesulfonate (a biodegradable detergent)

Many commercial detergents also contain phosphate compounds. This can be a problem, because phosphate is a nutrient for plants. Too much phosphate in a pond, lake, or stream accelerates the growth of algae, which consumes too much of the dissolved oxygen in the water. This disturbs the ecosystem in the pond, and some organisms will die. Therefore, you will see some detergents these days that are labeled “phosphate free”. These are better for the environment than phosphatecontaining detergents.

In this experiment, you will make soap from a fat or an oil by heating it with sodium hydroxide. You will precipitate the soap by adding it to a concentrated salt solution, and then you will collect the solid soap using vacuum filtration. You will then test the soap you made for its pH and foaming ability. You will test to see how well it emulsifies oil and you will also test its behavior in hard water. You will carry out the same tests on a commercially prepared soap solution and on a commercially prepared detergent solution, and you will compare your soap to the commercial soap and detergent.

 

Experimental Procedure

Safety

• Be sure to exercise caution when dispensing the 9 M NaOH. If the chemical comes into contact with your skin, immediately rinse with water for a minimum of fifteen minutes and notify your instructor.
• Wear safety goggles, closed-toe shoes and gloves

 

Materials and Equipment

Materials: olive oil, vegetable oil or coconut oil, 9 M sodium hydroxide solution, food coloring, assorted fragrances(optional), stearic acid, 0.5 M calcium chloride solution, 0.5 M Iron (III) chloride solution, and 0.5 M magnesium chloride solution.

Equipment: tall 250 mL beaker, glass stirring rod, glass pipets and pipet bulbs, pH paper, test tubes, tube rack, 10 mL graduate cylinder, magnetic stir bar and hot/stir plate.

 

Synthesis of Soap

1. Preheat either the olive oil, vegetable oil or coconut oil to 35°C in a 150-mL beaker. Pour 10 mL of the warm oil into a tall 250 mL beaker.
2. Prior to beginning the reaction, choose your fragrance (optional). You may choose one of the following: holiday candy, island coconut, lavender, cinnamon, vanilla.
3. Add 1-2 drops of desired fragrance, using the pipet provided at front bench; do not mix fragrances.
4. Add 3 mL of 20 % sodium hydroxide solution to the beaker. This is approximately two full dropper squirts.
5. Use the glass stirring rod to mix. You must stir for 20-45 minutes; you may choose to take turns with your lab partner. The mixture will slowly become smoother and opaquer; it should thicken to a pudding-like consistency.
6. After approval by your instructor, add 2-3 drops of desired food coloring. Stir.
7. Add a dash (approximately 1/8 teaspoon or the tip of a spatula) of stearic acid. This will serve as a hardener for the liquid soap. Stir.
8. Pour into chosen mold shape. Label with your names and lab section number.
9. After pouring into the mold (small beaker), the process will continue on its own. The soap will heat up and liquefy again, then cool off slowly, harden and dry. So, the soap must be left undisturbed for at least 12 hours. You will pick up your finished soap in lab next week.

 

Properties of Synthesized Soap

In this portion of the experiment you will be comparing the properties of the soap you made with a commercial soap and a detergent. You will start out by making three separate soap solutions, as follows:

1. Add about 40 mL of distilled water to 1 g of the soap you prepared. Heat the beaker on a hot plate at medium setting and gentle stir until the soap dissolves. Cool to room temperature. In the data table, describe the appearance of the soap solution.
2. Mix 1g of commercial soap with 40 mL of warm distilled water. Stir the solution gentle and record the appearance of the soap solution.
3. Add 1 g of commercial detergent to 50 mL of warm distilled water and stir the solution gently. Record the appearance in the data table.
4. Remove 5 mL of each solution into three different test tubes and measure the pH of the soap solution along with the pH of distilled water as a reference. Record the pH of the solutions in your data table. Keep the solution for the next step.
5. To the test tubes used in step 4 add 5 drops of oil. Stopper and shake each of the tubes continuously for 10 seconds. Allow the test tube to sit for 5 minutes and describe the observation of this emulsification test in the data table.
6. Place three test tubes in a test tube rack and label them. Pour about 4 mL of the soap solution into each test tube.
7. Add 10 drops of calcium chloride solution to the first tube, 10 drops of iron (III) chloride to the second tube and 10 drops of magnesium chloride to the third tube. Stopper and shake each tube to mix the contents. Describe the color and appearance of the mixture in each test tube.