MEASURING IODINE NUMBER
Introduction
The ‘saturation’ of a fat refers to the number of double bonds in its fatty acid side chains. Saturated fats
containing few double bonds are generally considered less healthy than poly-unsaturated fats containing
many double bonds in their side chains.
Lauric acid,
a saturated fatty acid
α-Linolenic acid,
A poly-unsaturated fatty
acid
The double bonds present in the side-chain will react with halogen solutions, leading to the decolorisation
of the halogen. Since one double bond will react with one halogen molecule, this gives us a very handy way
to determine and compare degrees of unsaturation: simply measure the amount of halogen a fixed mass of
a fat or oil reacts with.
We call this measure the iodine number which is defined as the mass of
iodine in grams consumed by 100g of a substance. Whilst it is called
iodine number, in practice it is often calculated by reacting the
substance with bromine or chlorine and then converted to an
equivalent amount of iodine, as chlorine and bromine are more reactive
and so the reaction takes less time.
The table to the right (courtesy of Wikipedia) gives iodine numbers for a
range of different oils.
In this experiment we will be measuring the iodine number of a range
of vegetable oils by reacting them with bromine dissolved in ethanoic
acid. As we are limited for fumehoods, groups will need to work one at
a time, each doing a different oil, and will then have to share their
results with the others.
Fat/Oil
Iodine number
Coconut oil
7 – 10
Palm oil
16 – 19
Cocoa butter
35 – 40
Palm oil
44 – 51
Jojoba oil
~80
Olive oil
80 – 88
Peanut oil
84 – 105
Cottonseed oil
100 – 117
Corn oil
109 – 133
Soybean oil
120 – 136
Grape Seed oil
124 – 143
Sunflower oil
125 – 144
Tung oil
163 – 173
Linseed oil
170 – 204
Source: http://chemistry.olivet.edu/classes/chem100/pdf/Labs/Iodine%20Number%20of%20Triglycerides%20Lab.pdf
Apparatus and Materials
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Standard laboratory glassware
Latex gloves
Analytical balance
White tile
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5% by mass Br2 solution in glacial ethanoic acid
Dichloromethane
Various vegetable oils
Procedure
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1.
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5.
6.
7.
8.
9.
Due to the use of dichloromethane, bromine and ethanoic acid, this experiment must be done in a
fume hood.
Br2/Ethanoic acid solution is corrosive and toxic and should be handled with gloves in a fume hood
Waste should be disposed of in a beaker marked ‘halogenated waste’
Using a graduated pipette, measure out 5.0 cm3 of dichloromethane and place in a 100 cm3 conical
flask. Stopper the flask and weigh it.
Add 10 drops of Br2/ethanoic acid solution, stopper the flask, swirl to mix, re-weigh it and place it on a
white tile. This will be the blank flask against which you will compare later colours.
Weigh an empty 100 cm3 concical flask and record the mass, this will be your test flask.
Add drops of oil to the test flask until you reach a mass of about 0.2g, recording the exact mass.
Dilute the oil with 5.0 cm3 of dichlromethane and swirl to mix.
Weigh the test flask and record its mass.
On the white tile, add Br2/ethanoic acid solution to the test flask drop-wise and swirling continuously
until its colour matches that of the blank flask.
Weigh the test flask and record its mass.
Share your results with the rest of the class.
Analysis
As you will be repeating the same sets of calculations a number of times, it makes sense to do them using a
spreadsheet.
a. Calculate the mass of bromine added to the blank flask (remember the solution is 5% by mass).
b. Calculate the mass of bromine added to the test flask.
c. Subtract the answer to a from your answer to b, to give you the mass of bromine added at the point
when you had just reacted all the double bonds, this is the mass bromine you should use.
d. Using the mass of oil used, scale your answer to d up to give you the amount of bromine that reacts
with 100g of oil.
e. Use your knowledge of stoichiometry to convert the mass of bromine that reacted to the mass of
iodine that would react. This is your iodine number.
f. Calculate the iodine number you would expect for a triglyceride made of glycerol, one lauric acid and
two linolenic acids. You may wish to draw it first to help calculate its formula mass.
g. Compare the iodine numbers you recorded with values found online. How accurate is this method and
what could be done to make it more accurate?
Source: http://chemistry.olivet.edu/classes/chem100/pdf/Labs/Iodine%20Number%20of%20Triglycerides%20Lab.pdf