Preparation of Biodiesel
This activity encourages you to find out about the chemistry of esters/carboxylic acid derivatives. There are numerous articles in the press about people making biodiesel from waste chip fat in their garden sheds, and there have been reports of waste oil being stolen because of its value in the face of rising energy costs.
The activity as it is written involves the preparation of biodiesel from fresh vegetable oil, but it could be combined with an analysis of waste vegetable oil as one activity which involves the preparation of biodiesel from waste oil.
The practical is very quick to set up and you can carry out the shaking in 10 minutes.
There are a number of ways of testing the biodiesel. You could compare its calorific value with regular diesel (use low sulphur diesel) or paraffin. Another method for demonstrating the combustion of biodiesel can be found here:
It may also be useful to compare the viscosity of biodiesel with diesel and/or paraffin (as well as the vegetable oil you started from). What are the effects of using different fuels on the pumps and injector systems in the engine?
Notes on materials needed:
Note: If you can’t use methanol, you can carry out the procedure using ethanol instead (forming potassium ethoxide), although the reaction is slower.
You should wear safety glasses and aprons/labcoats. Use standard safety precautions when doing acid/base titrations. Do not so this experiment if you have a rapeseed oil allergy!
Methanol is toxic by inhalation, ingestion or skin absorption. It may be a reproductive hazard. Ingestion may be fatal. There is a risk of very serious, irreversible damage if swallowed. Exposure may cause eye, kidney, heart and liver damage. Chronic or substantial acute exposure may cause serious eye damage, including blindness. It is an irritant and a narcotic.
ii. Potassium methoxide
Potassium methoxide is stable. It reacts violently with water. It is moisture sensitive. It absorbs carbon dioxide from the air. It is incompatible with acids, strong oxidizing agents, acid chlorides, acid anhydrides, alkali metals.
Aim: To prepare a small sample of biodiesel by reacting vegetable oil with methanol in the presence of a catalyst
Biodiesel is commonly made using a process called transesterification. Vegetable oil is composed of triglycerides, which are triesters of glycerol. To make biodiesel, the triglycerides are reacted with methanol. The reaction is extremely slow, so a potassium methoxide catalyst is used to speed the reaction up.
The catalyst is made by dissolving 5g potassium hydroxide in 100cm3 methanol. The potassium hydroxide acts as a base, removing a proton from the methanol to make potassium methoxide.
Preparation of biodiesel
1. Measure out about 10ml of vegetable oil and pour into a boiling tube.
2. Add 1.5ml of the potassium methoxide catalyst dissolved in methanol and place the bung/stopper in the tube.
3. Mix the contents of the tube by carefully inverting it repeatedly (keep your finger over the stopper to avoid it spilling)
4. Pause periodically and observe any changes that occur in the properties of the contents of the tube.
5. After 10 minutes of mixing, pour the contents of the boiling tube carefully into a separating funnel and allow the contents to settle. You should eventually see two layers. This should take about 40 minutes.
The top layer is biodiesel, which is floating on top of the glycerol by-product. Unfortunately there is more glycerol than biodiesel.
Once your layers have separated turn you can analyse your product.
Analysis of your sample of biodiesel
During the analysis, think about how your sample of biodiesel has changed compared to the vegetable oil you started with? How might this make it more suitable for use as a fuel (e.g. in a car engine)?
1. Run off the lower layer of glycerol into a small beaker and your biodiesel into a separate beaker. What do you notice about the viscosity of each of these as they run through the funnel?
2. You can compare the products you have made with the starting material using a process called Thin layer Chromatography (TLC). TLC separates products depending on their size and polarity. Running a TLC of your products and starting materials should allow you check if you have any starting material remaining and if you have made a new product. (You may need to ask for help with this)
3. Draw a thin faint pencil line about 1 cm from the bottom of the TLC plate and three faint dots equally spaced on this line. Dip a pipette into the vegetable oil, shake off any excess and carefully dot onto the first spot.
4. Repeat this with clean pipettes for both the biodiesel and glycerol spots. It is important that only a small quantity of the material is applied to the plate otherwise resolution becomes very poor.
5. Pour a small amount (about 5ml) of solvent into a small beaker (the solvent is 85% petroleum ether: 15% diethyl ether and has been made up for you). It is important that the level of the solvent is not higher than the pencil line on your TLC plate.
Thin layer chromatography (TLC) is a chromatography technique used to separate non-volatile mixtures. Thin layer chromatography is performed on a sheet of glass, plastic, or aluminium foil, which is coated with a thin layer of adsorbent material, usually silica gel, aluminium oxide, or cellulose. This layer of adsorbent is known as the stationary phase.
6. Carefully place your TLC plate into the beaker and cover with a watch glass.
7. After the solvent has risen approximately 2/3 of the way up the plate, carefully remove the plate. The TLC plate can now be visualised under UV light.
YOU MUST NEVER LOOK DIRECTLY AT THE LIGHT SOURCE
I would like to point out that the hairy hands above left are not mine (Helen).
The result wasn’t brilliant. There was probably too much oil which swamped everything. The biodiesel should have risen up the paper the most and the oil the least.
8. You can compare the viscosity of the starting material and product biodiesel by drawing up 5ml of vegetable oil into a plastic syringe. Carefully remove the plunger from the top and time how long it takes for all the liquid to drain out. Repeat this with a clean syringe of the biodiesel you have made.
Time taken for veg. oil………….s Time taken for biodiesel…………….s
Why does the difference in viscosity make biodiesel a useful fuel?