Lab Investigation
Option B.4 – Lipids
Determining the degree of unsaturation of food fats by bromine water titration
DCP, CE
Introduction
Fatty acids are a class of biologically important organic chemicals, consisting of a
long carbon chain with a carboxylic acid group at the end. They are the basis of all
fats and oils we consume in our diet.
Fatty acids can be classified as saturated or unsaturated fatty acids. Unsaturated
fatty acids have at least one double bond in their carbon chain, while saturated fatty
acid do not. This double bond typically disrupts the London dispersion forces
between fatty acids, giving unsaturated fatty acids a lower melting point. Because of
this, oils, which are liquid at room temperature, are made of unsaturated fatty acids,
while fats, which are solid at room temperature, are made of saturated fatty acids.
Unsaturated fatty acids can be made into saturated fatty acids by a process called
hydrogenation. Hydrogenation is an addition reaction between the unsaturated fatty
acid and hydrogen gas, with a metal (such as nickel) used to catalyze the reaction.
This process is commonly used in the manufacture of vegetable oil-based
margarines, where it is carefully controlled to make the fat more solid, but still
spreadable and rich in certain nutritionally valuable fatty acids.
The double bond in unsaturated fatty acids is also capable of reacting with other
compounds, such as molecular halogens, by a similar addition reaction. In fact, the
degree of unsaturation of fatty acids is typically expressed as the iodine number, or
the number of grams of iodine that reacts with 100 g of fat.
The iodine number can also be measured using bromine. In this method, fats are
titrated with a bromine solution (also called bromine water). Bromine loses its brown
color when it reacts with the unsaturated fatty acid, so the end point is visible as a
sudden appearance of the brown bromine color. This allows us to determine how
many moles of bromine reacted with the fatty acid double bonds, which can be
converted into an iodine number.
In this lab, we will be using this titration method to determine the iodine number of
sunflower oil.
Materials:
 0.0625M bromine water solution
 Sunflower oil
 Burette and burette stand
 Test tubes
 Hot water bath
 250 mL Erlenmeyer flask (3)
 Distilled water
Procedure:
0.
1. Weigh accurately about 0.5g of sunflower oil into the Erlenmeyer flask.
Add 25 mL of distilled water, shaking the flask to disperse the oil.
2. Set up the burette stand, then bring the burette to the fume hood.
Fill the burette with 0.0625M bromine water in the fume hood only.
Carefully bring the burette to your lab space.
(Bromine can be hazardous if the exposed surface area is large, but is relatively
safe in the burette with adequate ventilation.)
3. Titrate the butter with bromine water, making sure to shake the flask vigorously
while titrating to ensure that the bromine water is able to react with as much fat
as possible.
The end point occurs when the bromine water color persists for 30 seconds.
(This end point may be more accurate if you turn off as many lights as possible,
as bromine will decolorize naturally when exposed to light.)
4. Do as many trials as possible in the time given.
Share your data with your classmates.
Data Processing and Analysis:
0.
1. Combine your data with that found by the other student groups.
Using this data, calculate the iodine number of sunflower oil.
2. Using your results and the nutritional data for the oil, comment on the validity of
your results.
Evaluate the limitations and weaknesses of the lab procedure.
Sources:
Saunders, Nigel. "Comparing the Degree of Unsaturation of Margarine with That of
Butter." Creative Chemistry. 2003. Web. <http://www.creativechemistry.org.uk/alevel/module3/documents/N-ch3-10.pdf>.