This is the third in a series of 8 chemistry labs on the fundamentals of biodiesel.
In this lab we will learn about “Determination of the Acid Number of Vegetable Oils by Titration”.
Written procedures and background information for all 8 labs is provided on the biodiesel
education website.
To make biodiesel, fats and oils, or triglycerides, react with a catalyst and an alcohol to produce
ethyl esters, or biodiesel, and glycerin.
However, if you are making biodiesel using waste vegetable oil or fat those triglycerides
may be degraded and have a lot of free fatty acids not attached to a glycerol backbone.
These free fatty acids can react with the catalyst first to create soap, which must
be removed.
Therefore, before making biodiesel it is necessary to determine the amount of free fatty acids
in the oil or fat, in order to properly adjust the chemical process.
In this laboratory exercise we are going to determine the amount of free fatty acids that
are present in a waste vegetable oil sample.
The measurement of the amount of free fatty acids is called “acid number” and is done
by “titration”.
The acid number is a measure of acidity and is expressed as the quantity of a known concentration
of potassium hydroxide, or KOH, consumed by titrating one gram of a sample in the units
of milligrams (mg) of KOH per gram of sample.
Titration is a is a common laboratory method where a solution of known concentration is
used to determine the concentration of an unknown solution.
To conduct this exercise, you will need titration equipment and a weighing scale.
Note that in this lab the instructor will prepare the solutions but know that these
solutions are highly flammable and can burn skin and harm eyes.
You must wear gloves, a lab coat or apron, and eye protection.
The strategy for this laboratory exercise is to do 3 titrations.
Two of the titrations will be vegetable oil samples and one will be for the “blank”,
or reference.
The blank is the titration solution made by the instructor and is a known concentration
which is added (titrated) to another solution to determine the concentration of a second
chemical species.
The acid number determined for the vegetable oil will be the average of the acid number
obtained from the two vegetable oils samples.
If your instructor has not provided you with a raw data table, prepare a raw data table,
like the one shown here, in your notebook.
The first step is to weigh an empty flask and record the weight on the raw data table.
Then weigh approximately 1 gram of oil sample to the nearest 0.01 gram and transfer to the
titration flask.
Record the actual sample volume in data table.
Add 125 mL of titration solvent into the flask containing the sample.
Add a couple of drops of the phenolphthalein indicator into the solution.
Swirl the solution vigorously for approximately 1 minute to ensure the uniformity of the solution.
Prepare the second vegetable oil sample for titration by repeating these steps.
Then prepare a blank/ reference with only titration solvent and phenolphthalein indicator
and no oil by again repeating the previous steps.
Fill the burette with the titrant solution.
Record the starting volume of titrant on the data table.
Carefully introduce the titrant from the burette into the flask while swirling the solution
vigorously but without spattering.
Stop introducing titrant when the solution changes color to pink.
Record the ending volume of titrant on the data table.
Repeat the process for the second sample and the blank.
Once the three titrations have been completed and the data recorded, calculate the acid
number for the two samples.
The acid number from both samples should be about the same.
If not, another titration is recommended.
Otherwise, add the numbers and divide by two for the average acid number.
Thank you for watching the “Determination of the Acid Number of Vegetable Oils by Titration”
lab on the fundamentals of biodiesel.
Please go to the biodiesel education website for a complete written transcript of this
lab exercise.