SAPONIFICATION (SOAP MAKING)
The term saponification comes from the ancient art of soap making. Animal fats were heated with a
potassium hydroxide solution obtained by leaching wood ashes which contains potash (potassium
carbonate) with slaked lime (calcium hydroxide) solution. The term is now used to designate the general
process of reacting esters with caustic alkalies. In this experiment, you are going to react a trigylceride
ester, fat, with sodium hydroxide to produce soap and glycerol, a trihydroxy alcohol according to the
equation:
MATERIALS:
Fat (vegetable oil), methyl alcohol, 6 M sodium hydroxide, 3 - dram vial with a rubber or Teflon® lined
cap, test tube holder, sodium chloride (NaCl), ice, 1-400 ml beaker, 1-250 ml or 150-ml beaker, plastic
spoon, hot plate.
PROCEDURES:
PART I.
Preparation of the soap.
1. Heat roughly 100-ml of water in a 400-ml beaker on a hot plate until it begins to boil. While the
water is heating, add 5.0 ml of vegetable oil, 3.0 ml of methyl alcohol, and 3.0 ml of 6 M sodium
hydroxide (lye) to a 3-dram vial. Seal the vial by tightening its cap and shake to mix its contents.
Place the vial in the hot water and observe. After a minute or two, the mixture will separate into
layers with crude soap formed at the interface. When this happens, remove the vial from the hot
water bath with a test tube holder, and cool slightly by dipping the vial in tap water. Insulate your
hands by wrapping the vial in a paper towel, and shake until a homogeneous solution in obtained.
Place the vial back in the hot water and continue to heat the mixture an additional 2-3 minutes after
the reaction appears complete to ensure that all the fat has reacted.
It is important that you tighten the cap of the reaction vial very well. If the seal fails, either the
reactants will leak out into the water bath or water will leak into the vial. In either case, the
experiment is compromised. To obtain a tight seal, hold the cap and vial in opposite hands and
tighten as hard as possible. Occasionally, a vial may begin to leak during a reaction. This is
evidenced by a small stream of tiny bubbles coming from the vial cap. This means that the vial cap is
not tight enough. Simply remove the vial from the water bath, allow it to cool for a few seconds, and
carefully tighten the vial cap.
2. The solubility of organic compounds in water is markedly affected by the presence of dissolved
inorganic salts. For example, ethanol, which is perfectly miscible with pure water, is only slightly
soluble in saturated aqueous sodium chloride. The same is true of methanol, acetone, and many other
water soluble organic compounds. This phenomenon is called the "salting-out effect." In this
experiment, you are going to "salt-out" soap.
Add four heaping teaspoons of salt (sodium chloride) to 100 ml of cold tap water in a 150 or 250 ml
beaker. Place several ice cubes in the mixture and stir to dissolve the salt, cooling the mixture at the
same time. Add additional salt, if necessary, to produce a saturated salt solution.
3. Remove the vial from the hot water with the test tube holder, and cool the outside of the vial slightly
by dipping it in tap water. Avoid excessive cooling as the soap will solidify out of the mixture. It
solid soap does precipitate out before you have a change to pour the mixture into the ice bath. Reheat
the vial in the hot water bath until the soap redissolves. Pour the saponification mixture into the
beaker containing the cold, saturated salt solution and stir with a plastic spoon until the soap floats to
the surface in a soap powder-like mass. Skim the precipitated soap from the surface, drain, and
collect on a paper towel.
PART II. What are the properties of your soap?
MATERIALS:
Beral pipet, small test tube (13x100mm), 100 ml beaker, paper clip, deionized or distilled water
PROCEDURES:
4. Add a split pea-size lump of your soap to one or two milliliters of deionized or distilled water in a
small test tube. Place your thumb over the top of the test tube and shake.
Q1.
What evidence is there that you produced a soap?
5. Observe the properties of your soap on the paper towel. Note whether it is crystalline or amorphous
(no definite form). Rub some of your soap between your fingers. Is it hard and brittle like an ionic
compound, or soft, similar to paraffin (wax) or another covalent compound?
Q2.
According to your observations, which class of compound, ionic or covalent, is your soap?
Explain your rationale.
6. Smell your soap.
Q3.
What does the odor of your soap tell your about the completeness of your reaction?
7. Water molecules at the surface of the liquid behave differently than those in the interior because
surface molecules can only interact with the molecules below them and those surrounding them. This
phenomenon leads to an inward force of attraction contracting the surface of the water and making it
behave as if it had a “skin.” The toughness of this skin is measured by its surface tension. Water has
an extraordinary high surface tension between its molecules due to the extensive hydrogen bonding
between its molecules. Surfactant (surface-active agents), such as soaps and detergents, disrupt the
orderly arrangement of water molecules by interrupting these normal hydrogen bonds causing this
contraction to be released.
Surface tension can be tested using a tensiometer. A type of tensiometer will be provided with a
reservoir for liquid samples. Place the end of the tensiometer into some distilled water and observe
what happens when you pull back on the string as demonstrated by the instructor.
To test the surface active properties of your soap, place a small clump of your soap into the reservoir
containing the distilled water. The soap should dissolve because it is a organic salt that contains
sodium ions attached to an anionic organic ion. Dip the tensiometer into the soap solution and note
how the soap has changed the properties of the water.
ADDITIONAL QUESTIONS:
Q5. What evidence was there that a chemical reaction has occurred in this experiment? Give several
examples in your explanation.
Q6. How does the density of your soap compare with the saturated salt solution and the pure water used
in the "suds" test? List the three materials, soap, salt solution, and pure water, in order of increasing
density.
Q7. (a) Your soap is the sodium salt of an organic acid. Give the equation for the ionization of soap in
water.
Q8. What is the difference between soaps and detergents? ( look up on Internet)
Q9. Explain briefly how soaps and detergents clean clothes.
NOTES FOR TEACHERS
1.
Any vegetable oil will work in this experiment but the more saturated the fat, the harder the soap
becomes and the more readily it can be compared to laundry detergent.
2.
Only glass vials with rubber or Teflon lined cap should be used in this experiment. 2-dram vials
with Teflon lined caps are available from Flinn Scientific Co., catalog No. AP9105 for a case of 144,
and AP9106 for a package of 12. However, 3-dram vials give more soap and work just as well.
3.
To prepare 6 M NaOH , dissolve 240 grams of NaOH in water and dilute to a total volume of one
liter, or dissolve 60 grams NaOH in water and dilute to a total volume of 250 ml. Alternately, 77 ml
of 50% NaOH can to diluted to a total volume of 250 ml.
4.
If 2-dram vials are used, use to following volumes: 3 ml fat, 2 ml methanol, and 1.75 ml of 6 M
NaOH.
References:
Meszaros, Mark W., and Tom Russo, Vial Organic, Flinn Scientific Co., 1996
Pavia, Donald, L., Gary M. Lampman, and George S. Kris, Introduction to Organic Laboratory
Techniques: A Contemporary Approach, 3rd. Ed., Saunders College Publishing, New York, 1988.