Science Buddies: "Ask an Expert"

 
Hi !
I have made this project as my school experiment. Apart from coconut oil I have used rapeseed oil, palm oil, margarine and olive pomace. I have modified a bit the way I made the soap:
2. Pour cold water to sink and put beaker with 50 g of distilled water to it.
3. Add 25.25 g of NaOH to the beaker, stir. The mixture will get hot really fast, be careful not to get splashed by it. Put in the thermometer and wait till it cools down to 40 *C
4. Place 150g of coconut oil into the 1000ml beaker and warm it till it is 40 *C and melted. If the temperature is too high, wait till it cools down.
5. Once the mixtures are about 40 *C, pour NaOH to the beaker with oil, and blend it or stir with wooden spoon or blender for 20-90 minutes or till it looks like pudding.

By other fats I have used following amounts:

Rapeseed oil
18.69g of NaOH and 50 g of water.

Margarine
18.56 g of NaOH And 50 g of water.

Olive pomace
18.56 g of NaOH and 50g water.

Linen oil
18.49 g of NaOH and 50g water.

But the way I purified the soap was the same,
1.Take 30 g form the other half and add 15 mL of distilled water to the soap mixture and stir it with a stirring rod.
2.Heat 50 ml of saturated sodium chloride solution in a 100-ml beaker until it is almost boiling.
a. If you are starting with solid sodium chloride (rather than liquid), weigh 15 g of sodium chloride and put it in a 100-mL beaker.
b. Add 50 mL of water and stir until dissolved.
c. Heat the salt solution until it is almost boiling.

2. Add the hot sodium chloride solution to the soap mixture. Stir it for about 1-2 minutes, then wait for the lumps of soap to form.
I have decantated the lumps by filtering them through strainer. I made my purification only once. But my results were not always as they should be according to your website (mostly the pH was higher in the purified soap), this is why I wanted to ask if I should change something in the way I do my soap, or maybe the sodium chloride concentration should be higher ?
I would be very grateful for all your help !

Hi Hela,
I think you are doing this great project from this website:
https://www.sciencebuddies.org/science- ... p096.shtml
Thanks for including the details about your procedure. I’m not sure what went wrong with the soap conversion chemistry, but you have given a few clues. Here are the possibilities:

1. The conversion of the fats into soap required heating in the NaOH until the fats are hydrolyzed into glycerol and fatty acids. The background information section for this project includes the chemical structures of the molecules. The hot NaOH is needed to break the covalent bonds between the glycerol and the fatty acid.
The directions in the project describe boiling the NaOH and fat until all of the water has evaporated. You describe heating at 40 degrees C for 20 to 90 minutes, so it’s possible that the fatty acid was not hydrolyzed completely. Repeat your experiment with one fat only and at the point where the sample looks like pudding, take a few drops and put it in water. If you see visible oil floating on the water, then the step has not completed. You need to heat the fat with the NaOH until there is no free oil.

2. If the fats have been hydrolyzed, then the problem may be that your NaCl step has not completely converted the fatty acid molecules to the pure sodium form (fatty acid salt). The directions specify repeating the hot NaCl step 3 times and measuring the pH after every NaCl treatment. The pH will be high after the first treatment, but it should decrease after the second or third time. If the pH is still above 10, then repeat the NaCl step again, The NaCl solution is close to a saturated solution, so you can’t make it much more concentrated, but you could use a larger volume and heat it a little longer to force the conversation from the hydrogen to the sodium form.

3. How are your measuring the pH? I assume you are using pH paper. What is the pH reading of your NaCl solution before you heat it or add it to the soap? It should be about 7. It’s always a good idea to check and make sure your reagents are working properly.

4. Also, how much of each oil did you use? The directions specify 10 ml, so I assume that’s what you started with. If you used a different amount of oil, then it’s possible that the ratio of oil to NaOH was not optimum.
Since this is a science project, you will need to repeat your results. I recommend using just one oil to solve the chemistry problem. Once you can get one oil converted to soap, you will be able to do the other oils as well.

Please post again and let me know about your results, or if any of my suggestions do not solve the problem. We do want you to be successful.

Donna Hardy