What happens to the pH during the saponification reaction?

During saponification, pH decreases significantly as the reaction proceeds. Sodium hydroxide starts at approximately pH 14, but as the base reacts with the coconut oil, the pH drops to around pH 13 to 12 by the end of the 60-minute reaction. This pH change reflects the consumption of hydroxide ions attacking the carbonyl groups in the oil.

How does the structure of soap molecules make them effective cleaning agents?

Soap molecules have a dual structure: a long hydrophobic carbon chain that repels water and a hydrophilic carboxylate ion at one end that attracts water. This amphipathic design allows soap to interact with both oily dirt and water, enabling effective cleaning. The hydrophobic tail embeds in grease while the hydrophilic head remains in the aqueous phase.

Why does soap produce less foam in hard water compared to soft water?

Hard water contains multivalent cations like calcium, magnesium, and iron that form insoluble complexes with soap's carboxylate groups. These insoluble salts precipitate out of solution, reducing the number of soap molecules available to create foam. Soft water, lacking these minerals, allows more soap molecules to remain in solution and form stable micelles.

What observable changes occur in the mixture during the saponification reaction?

As saponification proceeds, the reaction mixture becomes increasingly turbid and viscous. Initially, the coconut oil and aqueous sodium hydroxide phases are separate, but as the reaction progresses under heat and stirring, the mixture thickens and changes color. The aqueous phase volume decreases as more oil is hydrolyzed into soap and glycerol.

How does commercial detergent differ from lab-made soap in hard water?

Commercial liquid detergent contains alkylbenzene sulfonate, which does not precipitate in the presence of multivalent cations like calcium. Unlike lab-made soap, which forms insoluble complexes and loses foaming ability in hard water, commercial detergent maintains significant foam production even in calcium chloride solutions.

What is the stoichiometric ratio of products formed in the saponification reaction?

The saponification reaction produces three molecules of soap for every one molecule of glycerol. This 3:1 ratio results from the hydrolysis of three ester groups present in each triglyceride molecule of the oil. Hydroxide ions attack each carbonyl group, cleaving the ester bonds and generating the soap and glycerol products.

Why is coconut oil heated to 55°C before mixing with sodium hydroxide?

Coconut oil has a melting point of 30°C, so heating to 55°C ensures the oil is completely melted and in liquid form. This liquid state allows better mixing and contact between the oil and the aqueous sodium hydroxide solution, facilitating the saponification reaction. Heating also increases reaction rate by providing the energy needed for hydroxide ions to attack the carbonyl groups.

Hydrolysis of an Ester

Concepts Instructor Prep Student Protocol

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Vegetable oils and animal fats are naturally occurring lipids in plants or animals. Oils and fats are often in the form of triglycerides, which are formed from a molecule of glycerol and three fatty acids. A fatty acid is a weak carboxylic acid with a hydrocarbon chain as the functional group. In the triglyceride, the three fatty acids are linked to the molecule of glycerol by ester bonds. A triglyceride, therefore, has three ester groups, making the molecule a triester.

An ester is a type of organic molecule that is like a carboxylic acid, with the hydrogen of the hydroxyl replaced by an alkyl or aryl group. This group is labeled R-prime to show that it may be different from the R-group on the other side. In a triglyceride, R-prime is the glycerol backbone, and each R-group is the chain of a fatty acid.

Fatty acids with double bonds in their carbon chains are known as unsaturated fatty acids. These double bonds tend to be in the cis conformation, which puts a bend in the chain. Triglycerides with these bent chains can't fit closely together, so the attractive forces between them are typically long-range and weak.

Fatty acids with only single bonds between the carbon atoms are saturated. These unbent chains allow triglycerides to pack closely together, enabling stronger, short-range intermolecular interactions. This gives saturated triglycerides a higher melting point than unsaturated triglycerides, which are usually liquid at room temperature.

Triglycerides can be converted back to fatty acids and glycerol by hydrolyzing the three esters. Ester hydrolysis is a reaction that breaks an ester bond with a molecule of water or a hydroxide ion to form a carboxylic acid and an alcohol. One common use of ester hydrolysis is to create soaps, which are the salts of fatty acids from triglycerides. This process is called saponification.

In this reaction, hydroxide ions attack each of the three ester carbonyls in the triglyceride, yielding three fatty acid molecules and one molecule of glycerol. The fatty acid carboxylates associate with the counterion from the base, which is usually sodium or potassium.

But how does soap work? The long carbon chain is lipophilic and hydrophobic, meaning that it is attracted to fats but not to water. The carboxylate group is hydrophilic, meaning that it is attracted to water. When you mix soap and water, the long carbon chains tend to interact with each other and avoid interaction with water molecules, while the carboxylate groups prefer to interact with water. Ultimately, the soap molecules form clusters called micelles, with the hydrophilic carboxylates facing outward.

Now, let's assume there is hydrophobic grease on your hands. Simply rinsing your hands with water would not remove the grease. When you wash with soap, the grease interacts with the soap's carbon chains. As the soap molecules form micelles, they bring the grease into the hydrophobic core enclosing it in a hydrophilic shell that can then be rinsed away with water.

In this lab, you will perform as a saponification reaction to create soap from coconut oil and sodium hydroxide. You will then test the tolerance of your soap for hard water by comparing the foaming ability in deionized water, tap water, and a calcium chloride solution, which mimics extremely hard water.

Print Procedure Steps

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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
  
  Time of mixing (min) Viscosity of solution Number of phases Color of solution pH
  0
  10
  20
  30
  40
  50
  60
  Click Here to download Table 1
- 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.
Table 2: pH of Soap

pH
Lab-made soap in DI water
Commercial soap in DI water
Click Here to download Table 2
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.

Table 3: Level of Foam

	Initial volume of foam (mL)	Final volume of foam (mL)	ΔV (mL)
Lab-made soap in DI water
Lab-made soap in tap water
Lab-made soap in 1 M CaCl₂
Commerical soap in DI water
Commercial soap in tap water
Commercial soap in 1 M CaCl₂

Click Here to download Table 3

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.

Results
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.