biodiesel

1. Introduction Bio-diesel is quite easily manufactured in the laboratory and a number of interesting observations can be made. If the main chain of the oil is based upon linoleic acid, then the following reaction occurs, catalysed by hydroxide ions. It is described as a trans-esterification reaction.



During the reaction, the same type of covalent bonds in the oil and the methanol are broken and subsequently reformed in a different molecular environment. Thus, theoretically, the enthalpy for the reaction is about zero. There is, though, an increase in disorder and a rise in entropy. The reaction proceeds to near completion because propane-1,2,3-triol is insoluble in the product, methyl linoleate, and is removed from the reaction. 2. Making bio-diesel Students will be able to observe a reaction almost immediately on mixing the oil and catalyst. The oil will quickly become less viscous. However, it is better to prepare bio-diesel in one lesson, store it in labelled test tubes to allow the layers to separate fully, and perform any follow-up tests in the next lesson.
 * Wear goggles.
 * Place 10 cm 3 of vegetable oil in a test tube.
 * Add the contents of the test tube containing the methanol / potassium hydroxide catalyst (EXTREMELY FLAMMABLE, TOXIC & CORROSIVE) and firmly insert the stopper from the test tube previously containing the catalyst.
 * Now invert the test tube SLOWLY over 30 times to ensure adequate mixing. DO NOT SHAKE THE TEST TUBE AS THE METHANOL MAY SQUIRT OUT! If any solution gets onto your fingers, immediately wash them under a stream of water from a cold tap.
 * Observe the contents of the test tube. A lower layer of glycerol gradually forms. The top layer is bio-diesel.
 * To carry out the tests described below, it is best to wait at least 24 hours for complete separation. Make sure the test tube is labelled with your name.

3. Testing the bio-diesel

3.1. Viscometry One method of determining that a new substance has been created is by comparing the flow rate of the bio-diesel with the original vegetable oil. Preparation of the pipette viscometer (see section 7 for more details) 3.2 Burning the bio-diesel
 * Prepare a pipette viscometer. (See the box below).
 * Wear eye protection.
 * Suck up the bio-diesel and time how long it takes to flow out between the two marks.
 * Make sure the pipette viscometer is empty and repeat the procedure with the vegetable oil.
 * You will need a plastic pipette and a short piece of wire.
 * Light a Bunsen burner.
 * Heat the end of the piece of wire and pierce the top of the pipette bulb to make a small hole.
 * Using a permanent black marker pen, draw a line against the 0.5 and 2.5 cm3 positions on the pipette. The position of these marks is not critical, as long as there is a sensible distance between them for timing the fall of oil.
 * Wear eye protection.
 * Use a small crucible or bottle top with a wad of mineral wool on it.
 * Add about 2 cm3 of bio-diesel to it.
 * Set fire to the bio-diesel with a lighted splint.
 * Compare the ease with which the bio-diesel can be lit and burns with the same procedure for the original vegetable oil.

4. Variations We used corn oil but other vegetable oils could be used. Commercial applications appear to be centred on soya and rape oil. It is possible to fill a spirit burner with the newly-made bio-diesel and measure its calorific value. The calorific value could be compared with that of real diesel. Always use fresh, low-sulfur diesel. Alternatively, you could use kerosene or paraffin instead. 5. Technician and teacher instructions Making Bio-diesel involves potassium hydroxide CORROSIVE and methanol VERY TOXIC & EXTREMELY FLAMMABLE. In an attempt to reduce the hazards for students at KS4, the activity has been tried with ethanol in place of methanol but the chemistry appears more complicated. Therefore, despite the nature of the reactants, we believe the method described here is the most effective for students. The potassium hydroxide acts as a catalyst for the reaction, rather than a reactant. (We do not recommend using sodium hydroxide instead of potassium hydroxide because it has a lower solubility in ethanol.) Methanol The workplace exposure limit (short-term) for methanol is 333 mg m-3, This means that 80 g of methanol would need to vaporise in an average laboratory (with a volume of about 240 m3) to exceed the limit. In the procedure below, only about 30 cm3 of methanol is used per lesson. In addition, methanol is consumed in the reaction so risk of exposure is quickly reduced Potassium hydroxide Some recipes for this reaction use 9 mol dm-3 potassium hydroxide solution, which is hazardous to make up and use by students. However, a safer procedure is for the technician to prepare a 5% (w/v) potassium hydroxide solution in methanol in a fume cupboard. Only 1.5 cm3 is required per group; this quantity should be dispensed in stoppered test tubes for students to use, further minimising potential exposure. 6. Preparing the catalyst 7. Making pipette viscometer
 * Wear goggles and use a fume cupboard.
 * Pour 100 cm3 of methanol (VERY TOXIC & EXTREMELY FLAMMABLE) in a beaker or conical flask on a stirrer, add 5 g of potassium hydroxide (CORROSIVE) and start the stirrer. It will take some time for the potassium hydroxide to dissolve.
 * Pour the solution into a bottle and label it, adding the hazard symbols CORROSIVE, TOXIC and EXTREMELY FLAMMABLE.
 * Dispense 1.5 cm3 of the solution into as many test tubes as required and stopper them.

Introduction The basic idea is to measure the time for the oil level to drop between two marks on a plastic disposable teat pipette (a pipette viscometer). This is easy to do at low temperatures but, between 30 and 60 °C, it requires more skill.

Preparation of the pipette viscometer
 * Light a Bunsen burner.
 * Heat a piece of wire and pierce the top of the pipette bulb to make a small hole.
 * Using a permanent black marker pen, draw a line against the 0.5 and 2.5 ml positions on the pipette (see diagram below). The position of these marks is not critical, as long as there is a sensible distance between them for timing the fall of oil. However, by standardising on the 0.5 and 2.5 ml marks, pupils within a class could reasonably expect to compare their individual results or pool them to produce a class set of data.
 * Ice is used to cool the oil. This is not essential but does help to speed up the investigation. Make sure there is plenty of ice in the freezer.

Basic procedure
 * Allow the pupils to practise the technique once or twice before commencing the measurements.
 * Half fill a test tube with the oil and insert the pipette.
 * Support this in a wide-neck 100 or 125 ml conical flask.
 * Squeeze the bulb, place a finger over the hole and release the pressure. Oil should be drawn into the pipette. Remove the finger from the hole, squeeze again, replace the finger over the hole and release the pressure. Oil should rise into the bulb.
 * Lift the pipette above the test tube and uncover the hole. When the oil level reaches the first black mark, start timing. Stop timing when it passes the lower mark.

Experimental instructions > >
 * Assemble the apparatus as in the diagram overleaf.
 * It is easiest to begin with the oil at the hottest temperature from the chosen range and subsequently to cool it. Note though that warm oil is less viscous than when cold so the time taken for the oil to drop between the marks will be relatively short - below 5 seconds. Add about 20 ml of cold water to the conical flask and then boiling water from a kettle until the level is near the top of the flask. Wait 5 minutes for the temperature of the oil and water to equilibrate. It should be about 60-65 °C.
 * Carry out the viscometer measurement, noting the time for the oil to move between the two marks and the temperature of the water. Have patience; this is quite difficult at higher temperatures.
 * As the water cools, take further measurements around every 5 to 10 °C. Cooling can be accelerated as follows.
 * When the temperature reaches 50 °C, place the conical flask into a 400 ml plastic beaker containing some cold water from the tap. As the temperature reaches about 45 °C, remove the conical flask and viscometer so that cooling is less rapid and, after 2 minutes, take a viscometer measurement and note the temperature of the water.
 * Repeat the cooling and take further measurements at lower temperatures.
 * When the temperature reaches about 30 °C, place some ice into the water. This permits readings to be taken down to 10 °C or even lower.