Waste or old oil is more acidic than new oil, since free fatty acids form in oil with use. Because of the high content of free fatty acids, more sodium hydroxide catalyst is required for making biodiesel from waste oil than from new oil. As described below, when using waste oil, you will need to perform a titration to determine how much additional catalyst is needed.
While some resources recommend making multiple small batches of biodiesel, varying the quantity of sodium hydroxide until the reaction works, titration is simple enough that, overall, it will save you time and materials. Different titration methods are presented in various references. One simple method is presented below, using a chemical indicator called phenol red. This titration method should give you an accurate indication of the additional amount of sodium hydroxide needed to neutralize the free fatty acids in the waste vegetable oil. Other titration methods are similar except for the method used to determine when a solution that you prepare changes from an acid to a base.
How to do a titration Materials One bottle of isopropyl alcohol. (In the U.S., Iso-HEET® Premium Fuel System Dryer & Antifreeze, 12 fl. oz., in the red bottle, is available at auto parts stores and is about 100% isopropyl alcohol. Isopropyl alcohol is also available at pharmacies. One bottle of phenol red from the hot tub store. One liter of 0.1% sodium hydroxide in distilled water—which is another way of saying 1 gram sodium hydroxide (lye) dissolved in 1 liter of distilled water. Since you might not have particularly accurate scales, one way of achieving this is to measure out 10 grams of lye and dissolve it into 1 liter of distilled water. Now take 100 milliliters of this water and mix it with 900 milliliters distilled water. You now have pretty close to the 1 gram of lye in 1 liter of distilled water. You can also go to your local pharmacist or high school science department and ask them to do the measuring for you. Equipment One one-cup jelly jar. Two glass 1 milliliter eyedroppers with graduations marked on the side. Note that you will use one for oil, one for the lye-water mixture. Always use the same eyedropper for the same chemical; do not mix them up. The procedure
The number of milliliters of 0.1% lye solution needed is equal to the number of extra grams of pure sodium hydroxide catalyst needed to produce the proper reactions to make biodiesel. For example, if it takes 3 milliliters of 0.1% lye solution to turn the oil and isopropyl alcohol solution to a base, you will need to add 3 grams of sodium hydroxide to the 3.5 grams for new oil, or 6.5 grams total per liter of waste oil. |
Once you have determined how much sodium hydroxide you need by titration, make a small batch of biodiesel in the same way you made it with new oil, but use the total amount (new oil plus titration result) of sodium hydroxide.
One thing you may notice in your small batch is more than two layers. A white layer between the biodiesel and the glycerol is soap, and it will be present if there is any water in the waste vegetable oil or the other reactants you started with. Warming the oil will cause the water to sink to the bottom of the container, and the oil can be poured or pumped off of the top. After warming the oil, allow it to cool below 130°F, and pump the oil into another bucket. Filter the warm oil through a filter paper. A paper coffee filter, a funnel, and patience will work for small batches.
If there is a layer of unreacted vegetable oil, you didn't use enough lye. You might make a gel soap if you use too much lye. Study the Resources for clues to what went wrong and how to remedy it. You can heat the oil to help get rid of the water before reaction with the methanol. It is best to heat the oil to a fairly low temperature (200°F) and hold it there, rather than heating it to a high temperature; it takes less energy and makes fewer free fatty acids (FFAs).
If you have a good separation between the layers of biodiesel on top and glycerol on the bottom, pump the biodiesel off of the top into another container, being careful not to bring any glycerol or visible soap along with it.
Quality Testing
Soap, methanol, and lye will still be suspended in your biodiesel. You can test for these because they are chemical bases. The pH of unwashed biodiesel will be above 8, and you want your finished fuel to be pH 7, neutral. If the pH is high, you can use less lye. If you shake up some of the biodiesel with water and it won't settle out, you have soap and other impurities in the biodiesel.
