Used fryer oil is a little different from new oil. Used oil probably has water and food particles in it. Used oil also contains free fatty acids caused by cooking with it. Remember that the titration will give you the number of additional grams of sodium hydroxide per liter of waste oil, so you must multiply the number of additional grams of sodium hydroxide by the number of liters of waste oil you are using.
Use the same equipment as you did making a 5-gallon batch with new oil. There is a handy chart in From the Fryer to the Fuel Tank (see How-To Resources) for the amount of sodium hydroxide to use for various quantities of oil and various titration numbers.
Procedure
Perform a titration on the waste oil, to determine how much sodium hydroxide is needed for the reaction (in addition to the 3.5 grams per liter of new oil). Have all the materials warm, room temperature at the coolest, 130°F at the warmest. To filter waste vegetable oil, warm it in a stockpot, and pour it through a filter paper. These and the filter basket are available from restaurant supply stores. If the oil is very warm, let it cool to 120°F. Filter the oil and remove water as described above.
Follow the directions above for making a 5-gallon batch using new oil, but using the amount of sodium hydroxide called for by the titration.
