Como Hacer Biodiesel > Titrating Your Oil Sample:

Although you might be lucky and have a good source of virgin vegetable oil to make your biodiesel, for many small scale processors the source will be used vegetable oil. When using used vegetable oil as your raw material, you will have to determine how much additional catalyst you will need to produce a proper reaction.

A common procedure for testing the oil is to use a titration method. The description below is based on methods described at the biolyle.com and www.journeytoforever.org web sites:

Make up a 0.1% solution of lye in distilled water (1 g lye in 1 L distilled water, which you can use as a stock. This 1g lyle/L water stock should be replaced once a month or so when you start regularly making biodiesel. I start by adding 50 mg lye to 500 ml H20, and use this as a concentrated stock (you can keep this more concentrated solution for up to six months). I then take a 5 ml portion of this stock and dilute that by a factor of 100 to get my 1 g/L solution. I keep both in glass mason jars or other similar size glass jars.

Put 10 ml of 99% isopropyl alcohol in small white ceramic cup. Take exactly 1 ml of your waste vegetable oil sample and mix into the alcohol in the cup. If you are outside and it's cold (<15 degrees Centigrade) when you are doing this, you may need to go to an inside, warmer location for the oil to dissolve completely in the alcohol. I don't find this generally necessary.

Add 2-3 drops phenolphthalein and stir ( you can use a chopstick or disposable fork). The phenolphthalein is an acid-base indicator that's colorless in acid and red-purple in base. Note that phenolphthalein is light-sensitive, so store it in the dark, and don't use the same batch for more than a year.

Carefully begin to add the 0.1% lye solution slowly, mixing as you add. Keep track of how much you're adding. Keep adding the lye solution and mixing until patches of red-purple start to appear in the solution. Now be sure to add slowly, one drop at a time, mixing as you add, until the solution holds its red-purple color for about 10 seconds. That should be a pH of between 8 and 8.5 (check with your pH paper). Write down how much lye solution you've added. It should be anywhere from 0.3 ml to 8 ml, depending on how used the oil was.

The amount of lye solution you added translates into how many grams per liter you'll need to add to your methanol in order to properly process that batch of used oil. You need to add 3.5 g to whatever number you , since that's how much lye (in grams per liter) is needed for the reaction of virgin (fresh) oil. So, if your titration told you to add 2 g of lye, then you'll actually need to add 5.5 g lye per liter for this batch of oil (2 g + 3.5 g = 5.5 g). Then, multiply 5.5 g times the number of liters of oil you will be processing. That is how much lye you will be adding to the methanol in the processing step.

Some sites suggest you prepare 1 liter test batches in a blender, as a way of vaerifying the accuracy of your titration. For example, if your titration tells you to use 5.5 g lye per liter, you could do test batches with 5 g, 5.5 g, and 6.0 g, adding 20% methanol to however much oil your are testing. (see for a more detailed description of this step). The 6 g batch might give you a soapy layer on top, which is typical when too much lye is used. After you have done these test batches a couple of times, and if they agree with your titration results, you can probably skip the test batch step and just depend on the titration results.

It's a good idea to test the oil from a few different restaurants before you pick the one you want to use as your waste oil source for processing large batches. I tested oil from four different restaurants and found that the amount of lye needed to raise the pH to approximately 8.5 (the level needed to make the reaction proceed properly) differed dramatically; the values were 0.3 g /L, 2.7 g/L, 4.5 g/L and 6.7 g/L. The restaurants that have the cleaner oil change their fryer oil once a week or more. So as a general rule of thumb, you will probably want to work with restaurants that change their oil at least once per week.