Experiment 10
Synthesis and Properties of Soap
A natural soap is the sodium or potassium salt of long chain fatty acids produced by the
base catalyzed hydrolysis of triacylglycerols (oils or fats). In the first part of this experiment you
will prepare soap by a saponification reaction of a small sample of a vegetable oil. A generalized
saponification reaction is shown below:
O
H2 C O C R1
O
HC
O C R2
O
CH2 OH
+
3 NaOH (aq)
!
HCOH
CH2 OH
-
+
+ R1 COO Na
+
+ R2 COO Na
-
+ R3 COO Na+
H2 C O C R3
In the second part of this experiment some of the properties (pH and solubility in various
solutions) of your soap will be examined.
Procedure (Part 1): Caution must be observed as the concentrated sodium hydroxide (lye) is
corrosive and can cause burns to skin, destruction of clothing and irreversible cornea damage to
the eye. At no time are your safety glasses/goggles to be removed during this experiment.
1. Prepare a boiling water bath in a 600 mL beaker; be sure to add a couple of boiling chips.
Place 12 mL of a vegetable oil into a 250 Erlenmeyer flask, Add 10 mL of ethanol (ethyl
alcohol) and 12 mL of 6 M sodium hydroxide to the vegetable oil. Also prepare an ice
water bath using another 600 mL beaker.
2. Stir the mixture with a glass stirring rod. Using a ring stand and a clamp, secure the flask
and heat it in the boiling water bath. Continuously stir the mixture during the heating
process to preventing the mixture from foaming up the sides of the flask.
3. Heat, with stirring until the odor of the ethyl alcohol is no longer detected. This may take
15 to 30 minutes. Remove the flask from the boiling water bath.
4. Place the flask into the ice water bath. Cool the soap solution for 10 min.
5. To the contents of the flask add 20 ml of a concentrated sodium chloride solution. Using
a spatula, break up the lumps of soap as completely as possible to permit maximum
contact between the solid and the sodium chloride wash solution. Carefully decant the
solution to remove the wash solution while retaining the solid in the flask.
6. Repeat the washing and decanting step two more times. After the final washing., remove
the last traces of liquid by dumping the solid unto paper towels and carefully blotting the
soap with additional paper towels. Avoid touching the soap with your skin.
Part 2:
1. Dissolve a small pea sized piece of your soap in a small test tube containing 5 mL of
distilled water. Add 3 to 4 drops of Universal indicator. What color appears? Using the
Universal indicator reference card, determine the approximate pH of your soap solution.
Repeat this experiment using a purified commercial soap (e.g., Ivory). What color
appears? What is its approximate pH? Record your observations in your notebook.
2. In three separate clean test tubes place 5 mL each of distilled water, tap water and 1%
calcium chloride. Add a small pea sized piece of your soap to each separate test solution.
Make sure that you use equal sized amounts. Stopper each tube and shake them
vigorously. Describe the relative amounts of lather and foam that appear in each tube.
Record your observations in your notebook.
3. Dissolve a small amount of your soap in a minimum amount of distilled water; estimate
the volume of water you used. To the dissolved soap solution add an equal volume of
concentrated sodium chloride solution. Describe and record in your notebook what
happens.
Experiment 10: Preparation and Properties of Soap
Name _______________________________
1.
Write a chemical equation for the saponification of a triacylglycerol that contains
palmitic acid, oleic acid and linoleic acid as the three fatty acid moieties.
2.
What was the purpose of adding the concentrated salt solution to your soap preparation?
3.
a.
What was the pH of your soap solution?
b.
Was your soap solution acidic, basic or neutral?
c.
What was the pH of the commercial soap solution?
d.
Was the commercial soap solution acidic, basic or neutral?
e.
Based on your results to parts 3a. –3d, above, is it possible to have a neutral
solution of a pure soap in distilled water?
4.
Describe the observed behavior when your soap was added to each of the following and
shaken up:
distilled water:
tap water:
calcium chloride solution:
5.
Write a net ionic equation for the reaction of calcium ions with the anion of palmitic acid.
6.
a.
What did you observe when concentrated sodium chloride solution was added to
the dissolved soap?
b.
What does the observation in part 6a. suggest about the effectiveness of ordinary
soap in seawater?