Free fatty acids are fatty acid groups that have broken off from the vegetable oil triglyceride molecule. Unless properly accounted for, free fatty acids consume catalyst needed for transesterification lowering yield, increasing the reaction time, or potentially stopping the reaction altogether (if they consume all of the base catalyst) as well as forming soaps that can cause problems during fuel washing and separation of the fuel/wash water mixture. Determining the amount of free fatty acids present in a given vegetable oil sample is the first step to preventing problems during the base catalyzed reaction.
To perform this measurement, the following items will be needed:
- Vegetable oil to be tested;
- 1 Liter (L) of distilled or deionized water;
- Base catalyst that you’re planning on using for your reaction sodium hydroxide (caustic soda or lye) or potassium hydroxide (caustic potash);
- A means, typically a small laboratory-grade (accuracy is important) scale, of measuring out 1 gram (g) of base catalyst;
- Phenolpthalein indicator or pH strips (one noted author even recommends turmeric powder as a pH indicator);
- 99% pure isopropyl alcohol;
- Two devices (eyedropper, pipette, syringe) for measuring 1 milliLiter (mL) of fluid;
- A device for measuring 10 mL of fluid;
- A 1 Liter bottle (for storing reference solution);
- Two small bottles or jars (one for conducting the titration in, one to hold a working amount of reference solution so that you don’t risk contaminating the full liter of reference solution);
- A timepiece that can count seconds.
To measure the amount of free fatty acids in your vegetable oil sample (by titration), first prepare a reference solution. Add the 1 liter of distilled or deionized water (deionized water works better but is much harder to find) to the 1 liter reference bottle. Then, measure out 1 gram of your base catalyst and add this to the reference bottle. Mix the catalyst into the distilled or deionized water by capping the bottle and inverting the bottle several times until no solid catalyst can be seen. This mixture will now be the reference solution. This step should not have to be performed for every titration. The remaining reference solution can be saved for use in later titrations.
To conduct the actual titration itself:
- Pour a small amount (100 mL or so) of reference solution into one of the small bottles or jars. This is the solution you will work from to avoid contaminating the full bottle of reference solution.
- Perform a blank titration. Since isopropyl alcohol may be acidic itself, it is necessary to ensure that the alcohol is neutral. Failing to do this will result in overestimating the amount of acid in the oil sample and adding (and wasting) excess base catalyst to perform the reaction. Pour 10 mL of isopropyl into a jar and add a few drops of indicator (phenopthalein). Slowly add reference solution (using one of the 1 mL measuring devices) one drop at a time until the solution just turns from yellow to pink (the solution will not stay pink), be sure to swirl the solution during addition of all materials to ensure uniform mixing. Keeping track of how much reference solution is added in this step is not necessary, this step is just to neutralize any acidity in the alcohol. Keeping track of reference solution added is necessary after this step, however.
- Add 1 mL of oil to the solution (using the second 1 mL measuring device) and swirl to mix, some small beads of oil may still be visible, but there should not be separate layers of oil and water.
- Add (measure how much is added) a few drops at a time of reference solution to the solution using the first 1 mL measuring device. It is critical to keep track of how much reference solution you are adding! Swirl after each addition to ensure good mixing. When the solution begins to change color from yellow to pink, start adding only one drop of reference solution at a time, to ensure the most accurate result possible. The end of the titration occurs when the solution remains pink for at least twenty seconds while swirling. The solution will return to a yellow color, so do not keep adding reference solution until it stays pink.
- Record the amount of reference solution used to reach the endpoint of the titration.
- Repeat the entire process. If both results are close (within 5%), use the average number. If the results are not close, repeat until two readings are within 10%, then use the average of those two numbers. If it takes an excessive number of repetitions to achieve two readings within 10%, consider using a larger volume of sample (such as multiplying all quantities above – except the indicator – by 5), but remember to divide the final result by whatever scale-up factor was applied. Scaling the sample up will help to minimize the impact of small variations in measurement.
- This reading (in mL of reference solution used) will then become the number of grams of catalyst per liter of oil that needs to be added to the base catalyst amount, For example, most authors recommend using between 3.5 – 5 grams of catalyst per 1 L of oil used, assuming no free fatty acids are present. If 2.5 mL of reference solution were used to neutralize the acids in the oil sample using the above procedure, a total of 6.0 – 7.5 grams of catalyst would need to be added for each 1 L of oil to perform the base catalyzed reaction.
As a further note, if too much base is added to neutralize the free fatty acids (i.e., if there are a lot of free fatty acids in the oil), it is possible to create so much soap that a solidified soap-biodiesel mixture is made that cannot be separated. This occurs typically around a value of “5” on the titration scale for sodium hydroxide (5.0 mL of reference solution were added to the 1 mL of oil during titration) up to around a value of “8” on the titration scale for potassium hydroxide. At levels this high and above, either avoid using the oil, mix the oil with another oil with a much lower free fatty acid content, or use an acid-catalyzed pre-treatment step followed by a base-catalyzed reaction.