Keywords: Biodiesel, feedstock, free fatty acids, acid number, triglycerides, acid value, titration, vegetable oil, biofuel, soap, glycerin, saponification, transesterification, ASTM
Production of biodiesel using an alkali catalyst always produces some amount of soap. There will be more soap with recycled restaurant waste and animal fats and less with refined vegetable oils. This is because biodiesel reaction (transesterification) uses an alkaline base, NaOH or KOH, as a catalyst. Waste oil contains free fatty acids, which will react with the catalyst to make soap ( a salt of fatty acid) before the catalyst has a chance to participate in making biodiesel. This specific amount of base consumed as a side reaction is hence sacrificed to the soap-making process (saponification).
After the transesterification reaction is complete, the leftover catalyst and soap tend to concentrate in the glycerol phase. However, some soap and a small amount of catalyst may be left in the biodiesel phase. It is always useful to know the amount of soap formed, where the catalyst resides and how effective the washing process is in removing these two compounds. Both of these chemicals are responsible for filter problems, engine corrosion and fouling of the catalytic converter in your vehicle.
A simple titration procedure can be used to measure the amounts of soap and catalyst. The basic procedure consists of titrating a sample of the phase to be analyzed, such as glycerol, biodiesel, wash water, etc., with a 0.01 N solution of HCl to the phenolphthalein end point. This gives an estimate of the amount of catalyst. Then, a few drops of bromophenol blue indicator are added and the titration is continued to the color changes for that indicator. This gives an indication of the amount of soap. In the first titration, the HCl neutralizes the alkali catalyst, so when the phenolphthalein indicates that the solution has become neutral, then all of the catalyst has been accounted for. Then, if the titration is continued, the HCl, as a strong acid, begins to split the soap molecules to free fatty acids and salt. When the pH reaches about 4.6, where the bromophenol blue changes color, this indicates that the HCl has split all of the soap. It is now lowering the pH (or donating protons, H+) since the soap has all been split.
Procedure:
1. Take 10 ml of biodiesel and weight it.
2. Add the sample to 75 ml isopropyl alcohol. Stir the sample to dissolve.
3. Add 7 drops of 1% phenolphthalein indicator (1g in 100mL Isopropyl alcohol).
4. Titrate with 0.01 N hydrochloric acid (0.83 mL concentrated HCl per liter of distilled water) until you get the phenolphthalein color change (pink to clear). This indicates that you have neutralized the free catalyst in the sample. Designate this amount of solution used as “A.”
5. Add 1 ml of bromophenol blue indicator (0.4%; 0.4g/100mL of water). This indicator changes color at about pH = 4.5.
6. Titrate the solution until the color changes from blue to yellow. This indicates that the pH is low enough that all of the soap should have been split into FFA and salt. The strong acid that we are adding (hydrochloric) is more strongly attracted to the metal ion in soap than the fatty acid chain. So, the metal ion combines with the Cl- from the hydrochloric acid to produce KCl (or NaCl), and the hydrogen freed from the hydrochloric acid converts the fatty acid chain to a free fatty acid. Designate this quantity of solution used as “B.”
Now, the amount of HCl added during the first titration tells us how much free catalyst is in the sample, and the amount added during the second titration tells us the amount of soap.
Calculations:
Leftover Catalyst:
(A×0.01×56.1)/(1000×W) = grams of KOH catalyst/gram of sample
where W is the weight of the sample.
This gives the amount of free catalyst in the sample, assuming the catalyst was KOH. You can substitute the appropriate molecular weight for other catalysts (KOH = 56.1, NaOH = 40.0, NaOCH3 = 54.0).
Soap Contents:
(B×0.01×320.56)/(1000×W) = grams of soap (as potassium oleate)/gram of sample
where W is the weight of the sample.
This calculation assumed the soap was potassium oleate. When using sodium catalysts, the molecular weight of sodium oleate is 304.4 g/mole. Oleic acid is a pretty good average compound for soybean oil, but you might want to change it for rapeseed oil.
When the level of catalyst and soap are small, it is a good idea to use blank samples. Just prepare a second beaker of the acetone-water solution, but don't add any sample. Titrate the blank at the same time as the sample and subtract the quantities for the blank from the numbers from the sample.