Saponification Is The Word
|Cups of soap "curing"|
As a part of our "Organic Chemistry" unit, I decided to have students make soap. I wanted to do this activity because it connects chemistry to real life, but the last time I tried it I had not felt successful with it. Not being one to give up, I decided to try again. I reviewed numerous classroom saponification recipes and discovered that there is a wide variety of approaches. I tried five different methods, some cold process, some using heat, some using ethanol, some not. I decided on the simplest cold process "recipe", with a modification of my own: adding a little borax. I would have liked to use coconut oil, but only had olive oil available at the time, so olive oil it was. I also had corn oil available in case the olive oil ran out.
I had all students handstir the oil and lye for at least ten minutes, to get them to see the emulsion occur. Then I allowed them to use the magnetic stirring plates. I did have one handheld emulsifier that I used to help anyone "finish up" the emulsion. Some students wanted to add color, so I allowed them to melt some crayon and mix it in with the handheld emulsifier. The melted crayon immediately solidified in the soap emulsion, so it came out more flecked than colored, but the students liked it.
The cups of soap are now sitting in my prep room. I will take them out of the plastic cups when they set up a bit, and form them into cakes wrapped in wax paper. Then they will sit until they cure more, and the pH is about 8.
So for kicks, here is the lab approach I used for my classes. I kept a squirt bottle of vinegar handy to neutralize spills.
- Safety goggles, apron, gloves
- 62 mL of olive oil, 100 mL graduated cylinder to measure oil
- 400 mL beaker
- 50 mL of water, 50 mL graduated cylinder to measure water
- 250 mL erlenmeyer flask
- 10 g of NaOH, Weigh boat for massing NaOH and borax
- 1 g of Borax Glass rod for stirring
- Salt (if required)
- Stirring magnet and magnetic stirring plate (if available)
- Handheld emulsifier (if available)
- Essential oil or perfume (optional)
- Plastic cup for setting soap
Measure out the oil and water. Pour oil into a 400 mL beaker. Pour water into a 250 mL Erlenmeyer flask. Mass the NaOH in weigh boat or on a piece of paper towel. DO NOT TOUCH IT! Squeeze the weigh boat so that the solid NaOH drops into the water in the Erlenmeyer flask and swirl until completely dissolved. Mass 1 g borax and dissolve in the NaOH solution. The borax will add to the foaming ability of the soap.
SLOWLY trickle the lye (NaOH) into the oil, stirring continuously to emulsify. DON’T TOUCH IT! Keep stirring to “trace” or a thick paste forms. This could take half an hour. If you still don't have a paste after half an hour, stir in 2 spatulas of salt. Other methods of stirring include using a magnetic stirring plate and magnet, or a handheld emulsifier. Hand stirring can take anywhere from ½ hour to 3 hours; the handheld emulsifier takes just a few minutes to reach trace. Once the proper consistency is reached, you may stir in essential oil or perfume, if you want. Test the pH with a pH strip.
Pour into plastic cup and leave to cure. After a day or two, we will remove it from the plastic cup and wrap in waxed paper to aid the curing process. This type of soap needs to mature to lose its alkalinity (pH10-12 when new). Use only after at least 3 weeks of “curing”, or when pH level is around 8.