Saponification Lab Handout (Hydrolysis of Triglycerides)

CAUTION

Sodium Hydroxide (NaOH, Lye) is highly corrosive.  In this experiment, you will prepare soap by hydrolysis with a concentrated solution of NaOH .  As always, be sure that you wear PPE including appropriate eye protection, gloves, and a lab coat or apron.  Wipe all surfaces of your bench after using the NaOH.  The experiment can be performed safely if proper precautions are observed.

Background

In this Experiment, we’ll make soap from olive oil.  So.. what is olive oil, anyway? 
Just like any oil, olive oil is a triglyceride. A triglyceride is composed of a glycerol molecule and three fatty acids.

Figure \(\PageIndex{1}\): This reaction shows the synthesis of a triglyceride from glycerol and three fatty acids, where the fatty acid tail is shown as an "R" variable group. (CC-BY-NC-SA; Kathryn Haas)

The fatty acids shown in Figure \(\PageIndex{1}\) have a variable group. "R", attached to a carboxylic acid. The R group is a long hydrocarbon tail. All fatty acids have a "fatty" part (the hydrocarbon tail) and an "acid" part (the carboxylic acid head). Some common fatty acids are shown in Figure \(\PageIndex{2}\). Note the differences in their hydrocarbon tails. 

Figure \(\PageIndex{2}\): The structures and names of some common fatty acids. (CC-BY-NC-SA; Kathryn Haas)

The trigylcerides in a natural oil, like olive oil can vary depending on the conditions in which the plants (or animals) grow.  The structure of any one molecule of olive oil varies within one sample (it is heterogeneous) and the fatty acid content varies depending on manufacturer, region, season! In general, the approximate amounts of any fatty acids in olive oil varies within the following ranges: 

Olive Oil
Palmitic acid: 7-20%
Oleic acid: 55-83%
Steric Acid: 0-5%
Linoleic Acid: 3-21%

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Making soap from olive oil

To make soap, we will perform a hydrolysis reaction on olive oil to separate the glycerol from the fatty acids (Figure \(\PageIndex{3}\)). The result will be soap that consists of glycerol and fatty acids.

Figure \(\PageIndex{3}\): A cartoon depiction of hydrolysis of a triglyceride with sodium hydroxide. (CC-BY-NC-SA; Kathryn Haas)

How much sodium hydroxide do we need?

The answer to this question isn't simple because we can't simply calculate a molecular weight for olive oil due to the fact that the chemical makeup of any container of olive oil may be different. But, we can make a good guess. Just in case, we can decide to add just a little bit less NaOH than our guess. After all, a soap that's a little oily (moisturizing) is much better than one that is corrosive and dangerous to use!

The recommended alkaline ratio is an approximate amount of sodium hydroxide that should be added to hydrolyze a given type of oil. For olive oil, the recommended alkaline ratio is 13.4 g NaOH for every 100 g of Olive oil.

 Why does soap work to remove dirt and oil?

The hydrolysis reaction shown in Figure \(\PageIndex{3}\) results in fatty acid anions; these are the conjugate bases of fatty acid molecules. The acidic hydrogen is removed from the carboxylic acid functional group to yield a carboxylate anion. The fatty acid anions make a salt with sodium cations. 

Fatty acids are amphiphilic (aka amphopathic). They have a hydrophobic (fatty) tail and a hydrophilic head. The anion is especially attracted to polar molecules, like water, while the fatty tail is not. The fatty tails interact more favorably with other nonpolar molecules, like the ones found in grease, oils, and dirt. This allows the molecule to interact favorable with both polar and nonpolar molecules at once. The fatty tail can dissolve grease, oild, and dirt, while the hydrophilic head can dissolve in water. The amphophilic properties of fatty acids allow them to act as soap (Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): This cartoon illustrates how amphiphilic fatty acid anions dissolve dirt and oil. Dirt and oil are mostly hydrophobic (nonpolar molecules), and thus do not dissolve well in polar solvent, like water. The amphiphilic properties of fatty acids allow then to pull dirt and oil into the water through formation of miscelles.   (CC-BY-NC-SA; Kathryn Haas)

Proceedure:

Figure \(\PageIndex{5}\): Hydrolysis of a triglyceride with sodium hydroxide, This is the same hydrolysis reaction shown above in Figure \(\PageIndex{3}\), but given here with skeletal structures. (Copyright; author via source)

 

1.  Weigh 100.0 g of Olive Oil into a 1 L screw-top bottle.  It is important to have at least 100.0 g, but it is ok if you weigh out slightly more than this amount.

 

2.  Get a 250 mL beaker.  In this beaker, cautiously weigh out 12.74 g of solid NaOH. Try to get this amount as close to 12.74 g as possible.  It is important to have no more than 13 g of NaOH.

 

3.  Carefully add 33.0 mL of distilled water to the sodium hydroxide.  Be careful, this solution is corrosive and will get hot as the NaOH dissolves.  Stir gently with a glass stir rod, being careful not to splash the corrosive liquid.

 

4. When the sodium hydroxide solution has completely dissolved, gently pour the NaOH solution into the 1 L bottle with the olive oil. 

 

5. Close the bottle tightly.  Be sure the cap is screwed on properly and then shake vigorously to mix the olive oil and sodium hydroxide.  After the reaction is mixed completely, pour about 3 mL of the mixture into a small beaker and set it aside.

 

6.  Each partner should label a paper cup with their name.  Put any fragrances you wish into your cup.  Then pour half of the remaining mixture into each cup. Stir well with a glass stir rod so that the fragrance is evenly mixed.  

 

7. Put your cup in the oven until the reaction is complete.  It may take a few hours before your soap is safe to use. 

 

8. Wipe your bench area with a wet sponge or cloth to ensure there are no NaOH spills on your bench.

 

Soap Analysis:  Record observations in your lab notebook.

1.  Dissolve a small drop of your unfinished soap in one test tube and a piece of finished olive oil soap (about the size of a pea) in a second test tube. Add about 5 ml of distilled water to each. Test the pH of each tube and record your results.

 

2.  Repeat the experiment above with a high purity soap, Ivory Soap. How does the Ivory Soap compare to the finished olive oil soap?

 

3.  Wash your hands with a small sample of finished olive oil soap. Does it have satisfactory cleansing qualities? Explain. (If this is an online lab during the pandemic, you can skip this question... but do wash your hands often!)

 

4.  Place 5 ml of distilled water in one test tube, 5 ml of tap water in a second test tube, and 5 ml of 1% calcium chloride in a third. Add small (pea-sized) equal quantities of finished olive oil soap to each and shake the tubes very vigorously. Describe the relative extents to which lather and foam appear in each test tube.

 

 

Data and Results:

Use a blank sheet of paper to answer the following questions.

 

1.  Below is a triglyceride that might be found in olive oil.  Draw the products of hydrolysis of this triglyceride.  Label the identity of each one of the products (glycerol or give the name of the fatty acid).

 

 

2.  Write a paragraph or two discussing what happened at the molecular level during this experiment, and focusing on the the analysis of your soap. Use the pre-lab information and the observations you made under ‘Soap Analysis’ to stimulate your discussion. You should explain your observations here using your chemical knowedge. Your paragraphs must be clear, concise, and must be MORE than JUST the observations—show us that you understand the chemistry of soap.