Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA
Preparation of Soap
Soap is prepared using a saponification reaction, where a base catalyzes the hydrolysis of three ester groups of an oil, such as coconut oil. During saponification, hydroxide ions from the base attack the carbonyl group on the oil to form a ratio of three molecules of soap to one molecule of glycerol. The resulting soap molecule is a long carbon chain, which is hydrophobic, with a carboxylate ion at one end, which is hydrophilic.
In this experiment, you will prepare soap using coconut oil and sodium hydroxide, the base. As the reaction proceeds, you will observe the decrease in pH, which occurs as the sodium hydroxide is used up in the reaction.
Before you start the lab, put on your personal protective equipment, including a lab coat, safety goggles, and gloves.
Label the 50-mL beaker as ‘waste’.
Tare a beaker and then weigh 100 g of coconut oil. Bring the coconut oil back to your workspace and place the beaker on the hotplate.
Set it to 55 °C to heat the coconut oil. Wait until all of the coconut oil is melted before proceeding.
Add a stir bar to the beaker and stir on the highest setting.
Use a 100-mL graduated cylinder to measure 121 mL of 3 M NaOH. Carefully pour the sodium hydroxide solution into the beaker with the coconut oil. Note: The solution should change to a milky color.
After 10 min, stop the stirring and observe the solution. Make a note of the viscosity of the mixture by relating it to something like honey, cake batter, or water. Also, record the number of phases present and the color.
Table 1: Characteristics of saponification reaction mixture
Dip a glass rod in the mixture and spread it on the pH strip. Record the pH of the solution in your notebook.
Resume stirring and continue mixing the solution for 60 min. Stop the stirring every 10 min and report on the characteristics of the solution as well as the pH.
After 60 min, turn off the hotplate and stirring function and observe the characteristics of the mixture. Measure the pH one more time.
Move the beaker off of the hotplate and stir the mixture using a glass rod after 5 and 10 minutes. Then, remove the stir bar from the solution.
When the solution is cool, transfer the white solid to a clean 400-mL beaker using a spatula. Reserve about 10 g of the mixture for further testing. Label the beaker, ‘reserved lab-made soap’.
Scoop a small spoonful of the dye and add it to the mixture, stirring well with a glass rod until it appears homogeneous.
Add 7 – 8 drops of one of the fragrance oils to the beaker and mix it well.
Transfer the mixture to a mold and press and smooth the soap into it using a spoon or scoopula. Leave the soap to dry in the mold for one day at room temperature.
The next day, take the soap out of the mold and flip it to expose the other side to air for another day.
Testing pH and Level of Foam
One drawback to soap is that the carboxylates form insoluble complexes with minerals found in tap water, such as magnesium, iron, and calcium. The soap scum you see in sinks and bathtubs is a buildup of these insoluble salts. If you dissolve equal amounts of soap and equal volumes of soft water, which has a low amount of minerals, and hard water, which has a high amount of minerals, more soap will stay in solution in soft water. So, the mixture of soap and soft water will be foamier and have more micelles than the mixture with hard water.
In the next part of the experiment, you will test the tolerance of your lab made soap to hard water by comparing deionized and tap water to a solution of CaCl2, which mimics extremely hard water. You'll then compare the properties of your soap to a commercially available liquid detergent.
Weigh 0.5 g of the reserved lab-made soap. Note: The soap does not have to be dry.
Transfer the soap into a 50-mL volumetric flask. Fill the flask to the marked line with deionized water and then swirl it to fully dissolve the soap.
Pour about 10 mL of the soap solution into a glass vial, and then dip the pH electrode in the solution. Record the pH in your notebook. Rinse the pH electrode thoroughly with deionized water.
Tare a clean 50-mL volumetric flask and glass funnel and weigh 0.5 g of the commercial liquid soap. Add deionized water to the line.
Pour about 10 mL of the solution into a clean vial and then dip the pH probe into the commercial soap solution. Record the pH in your notebook and then thoroughly rinse the pH probe with deionized water.
Next, test the level of foam. Tare a clean 50-mL graduated cylinder and weigh 0.2 g of the reserved lab-made soap. Measure 10 mL of deionized water and pour the water into the graduated cylinder. Cover it with a stopper. Label the graduated cylinder and observe the volume of foam in mL. Record your observation in your notebook.
Invert and shake the sample 4x to generate foam. Observe and report the level and volume of foam in mL.
Repeat this test by measuring another 0.2 g of the reserved lab-made soap into a clean 50-mL graduated cylinder.
Measure 10 mL of tap water and add it to the soap. Cover the graduated cylinder with a stopper and then label it. Observe and record the volume before inverting the cylinder and shaking it 4x. Measure the volume of foam generated and record your observations in your notebook.
Repeat the procedure again, but this time, use 10 mL of 1 M CaCl2 in place of the water. As before, measure the volume of foam before and after shaking. Also, observe any precipitate or other changes.
Weigh 0.2 g of liquid soap into a clean graduated cylinder. Measure 10 mL of deionized water and add it to the cylinder as before.
After covering and labeling the cylinder, record the volume of foam. Invert and shake the graduated cylinder 4x, like before, and record the volume of foam created.
As before, repeat the procedure with 10 mL of tap water. Don't forget to label the sample and record the volume of foam before and after shaking.
Measure and record the level of foam for 0.2 g of liquid soap in 10 mL of 1 M CaCl2. Shake the graduated cylinder as before and observe the level of foam created.
To clean up from the experiment, first, dilute the excess NaOH with water. Add 10 mL of tap water for every 1 mL of NaOH.
Then, neutralize the excess NaOH using 3 M HCl provided by your instructor. Check the pH using a pH strip, and when it is neutral, pour it down the drain. The soap solutions and rinse waste can be poured down the drain as well.
Finally, wash all glassware using detergent and tap water. Rinse well with deionized water and place on a drying rack to dry.
Résultats
Consider the reaction between the coconut oil and sodium hydroxide that forms glycerol and soap. Initially, the two phases are not miscible. The coconut oil is in the upper phase, and the sodium hydroxide is in the bottom, aqueous phase. Coconut oil has a melting point of 30 °C, so as the reaction proceeds under heat and stirring, the aqueous phase volume decreases as more sodium hydroxide reacts with the oil.
Sodium hydroxide has a pH of around 14, so the solution is initially around pH 14. After the reaction occurs, the pH drops from a pH of 13 to a pH of 12. Additionally, the turbidity and thickness of the reaction mixture gradually increases during the reaction as more coconut oil is hydrolyzed to soap and glycerol.
Next, compare the pH of the two soap solutions. The lab-made soap solution should have a pH in the range of 9 – 11. While the detergent solution should have a similar pH in the range of 8 – 11. Observe the foam level of the lab-made soap. The most foam is visible in deionized water due to the lack of multivalent cations. In contrast, there is less foam in the tap water because of the numerous cations present.
The lab-made soap formed the least amount of foam in the CaCl2 solution, which is expected as the soap molecules precipitate in the presence of Ca2+. The precipitate should be observed in the graduated cylinder.
The commercial detergent foamed similarly in the distilled water and tap water, and although there was less, it still foamed in the calcium chloride. This is because the detergent in the liquid soap is alkylbenzene sulfonate, which does not precipitate in the presence of multivalent cations.
