Unsaturation in fats and oils

Food labels and advertisements often refer to unsaturated fats and oils. In this experiment, students compare the amounts of unsaturated fats and oils in different foodstuffs by titrating solutions of samples with aqueous bromine (bromine water), which reacts with the carbon–carbon double bonds present in such fats and oils.

This is suitable as a class experiment for advanced classes where sufficient fume cupboard space is available, perhaps as part of a circus of experiments if such space is limited. Do not be tempted to do this in an open lab as too much bromine vapour escapes. Two such titrations, comparing two different fats or oils on a semi-quantitative basis, might be done in 45 minutes by students who already have experience of carrying out titrations.

Equipment

Apparatus

• Eye protection
• Conical flask, 100 cm³
• Dropping pipette
• Measuring cylinder, 10 cm³ or 25 cm³
• Burette, filled with bromine water (see note 6 below)
• Burette stand, or clamp and stand
• White tile
• A fume cupboard

Chemicals

• Bromine water, approximately 0.02 M (HARMFUL), provided in ready-filled burettes (see note 3 below)
• Volasil 244 (HARMFUL), 5 cm³ needed for each titration (see note 4)
• Various vegetable oils and fats, as available (see note 5)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout. The whole procedure should be carried out in a fume cupboard.
• Bromine water, Br₂ (aq), (HARMFUL) – see CLEAPSS Hazcard HC015b and CLEAPSS Recipe Book RB017. Bromine water should be freshly prepared in a fume cupboard. 1 dm³ should be sufficient for around 10 working groups carrying out two titrations each. If less is required, reduce quantities pro rata, but any excess can be stored as bromine water for future qualitative use. The experiment is only semi-quantitative – essentially comparing results for different foodstuffs.
• Volasil 244 (HARMFUL) – see CLEAPSS Hazcard HC106. Volasils are organo-silicon (silicone) solvents which are safer to use than most common organic solvents. Volasil 244 (octamethylcyclotetrasiloxane) is available from VWR International (formerly Merck/BDH) and Sigma-Aldrich.
• Coconut oil, olive oil, palm oil and sunflower oil should give a good range of comparative results. Animal fats can be tried, but should be pre-tested to ensure they dissolve sufficiently in Volasil 244; melted fats may dissolve more readily than solid fats.
• 50 cm³ burettes will not fit in many fume cupboards – 25 cm³ burettes are better, if available. The burettes should be filled in the fume cupboard with great care to avoid spillage. If the experiment is not carried out on the same day they should be loosely corked to reduce evaporative loss of bromine.

Procedure

1. Measure 5 cm³ of Volasil into the conical flask.
2. Using a dropping pipette, carefully add exactly five drops of the selected oil to the Volasil, and swirl to ensure mixing.
3. Note the initial reading of the bromine water in the burette.

4. Add the bromine water from the burette to the solution in the conical flask slowly, shaking vigorously after each addition until the bromine colour disappears.
5. As the bromine colour takes longer to fade with each addition, add less bromine water each time until there is just an excess of bromine in the flask, as shown by a permanent yellow tint.
6. Note the final reading of the burette. Subtract the initial reading to find the volume added.
7. Repeat the experiment with exactly five drops of a different oil. If a solid fat is tested, use a top-pan balance to measure out a mass equal to the mass of 5 drops of oil.

Teaching notes

The experiment gives a rough comparison of the degrees of unsaturation of the oils and fats tested, but the results cannot be translated into a quantitative comparison.

If students find the colour change to a permanent yellow difficult to distinguish, a variation can be tried using potassium manganate(VII) solution, KMnO₄(aq), 0.0005 M instead. This should be acidified using 0.1 M sulfuric acid – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB098. The purple solution turns colourless while unsaturation is still present. The procedure is the same as for bromine water, but 1 cm³ portions of the potassium manganate(VII) are added with swirling until the mixture retains a feint purple colour. The mixture requires more and more swirling as the total amount of potassium manganate(VII) used increases.

Warming fats in the Volasil using a beaker of hot water helps the fat dissolve and also speeds up the reaction.