This is the seventh in a series of 8 chemistry labs on the fundamentals of biodiesel.
In this lab we will learn about “Using Differences in Solubility to Remove Contaminants from
Biodiesel”.
Written procedures and background information for all 8 labs is provided on the biodiesel
education website.
When triglycerides are mixed with methanol and catalyst, a reaction occurs, and two phases
separate.
The top portion is biodiesel and the glycerol portion settles to the bottom.
The glycerol will contain most of the catalyst, soap, and excess methanol, but some will also
be in the biodiesel.
If this contaminated biodiesel is used in an engine, it could damage the engine.
We can remove the contaminants by mixing the biodiesel with water, and then letting the
water settle out.
This is often referred to as “washing” the biodiesel.
This is possible because water and the contaminants that are dissolved in the biodiesel are both
more “polar” than biodiesel.
Polarity is where electrons forming a bond are unequally distributed causing a molecule
to have an electrical charge.
Since biodiesel is slightly polar, some percentage of each contaminant will remain with it.
Each time the biodiesel is washed, more and more contaminants will be removed.
The number of times you need to wash the biodiesel in order to reduce the contaminants to an
acceptable level can be determined by measuring the “partition ratio” of each contaminant.
The partition ratio equation is shown here and is the ratio of the concentrations of
a given substance in two different solvents.
By convention, the concentration of the less polar phase goes on top and the more polar
phase goes on the bottom.
For example, if after washing the biodiesel, there is 10 times more of a particular contaminant
(X) in the wash water than in the biodiesel, the partition ratio for X is 1 over 10, or
0.1.
This means that each time we wash the biodiesel, the amount of X in the biodiesel goes down
by a factor of 10.
In this laboratory, we will measure the partition ratio for potassium hydroxide, which is commonly
used as a catalyst for biodiesel production, as it partitions between water and biodiesel.
Please note you will be using isopropyl alcohol, which is flammable, and potassium hydroxide,
a caustic chemical which can harm skin and eyes.
Wear goggles and gloves when handling the contaminated biodiesel.
To conduct the exercise, you will need one 50 mL graduated titration burette with stand,
three-250 ml Erlenmeyer flasks, a 500 ml separatory funnel, 500 mL, a pipette, and a weighing
scale.
For reagent you will need 120 mL of unwashed biodiesel, distilled or de-ionized water.
Anhydrous isopropyl alcohol, 1% phenolphthalein solution in isopropyl alcohol, 0.1 N Hydrochloric
acid, and 20 grams of potassium nitrate.
The first step is to weigh a 20-gram sample of biodiesel to an accuracy of 0.01 grams
and transfer this biodiesel to the titration flask.
Record the sample size before measuring the amount of catalyst present in the biodiesel
sample by titrating with 0.1 moles per liter of hydrochloric acid, using the following
procedure: Add 125 mL of isopropyl alcohol into the flask
containing the biodiesel sample.
Add a couple of drops of phenolphthalein indicator solution into the biodiesel solution.
The solution should turn pink, which indicates that the solution is basic.
Swirl the solution for approximately 1 minute to ensure the uniformity of the solution.
Fill the burette with the known concentration hydrochloric acid and water solution.
Record the starting volume of titrant.
Carefully add the titrant from the burette into the flask while swirling the solution
to mix.
Do not allow the solution to splash.
When the solution starts to turn from pink to very light pink, add the titrant slowly,
drop-by-drop.
Stop introducing titrant when the solution changes completely to clear.
Record the ending volume of titrant.
Subtract the ending volume from the starting volume to get the volume of titrant added.
Calculate the amount of potassium hydroxide using the following equation:
Now you will wash some biodiesel.
Place about 100 mL of unwashed biodiesel into a 500 mL separatory funnel.
Add an equal volume of distilled or de-ionized water to the biodiesel.
Agitate gently by rocking the funnel back and forth for two minutes, but do not shake.
Let the water separate for 5-10 minutes.
Save the washed biodiesel and separated wash water.
Now take a 20-gram sample of this washed biodiesel and put it in a 250 mL flask.
Weigh and record the weight.
Measure the catalyst again using the titration method described before and record the data.
Now take a 20-gram sample of the wash water and put it in a 250 mL flask.
Weigh and record the weight.
Measure the catalyst in the wash water by conducting another titration as before and
record the data.
To calculate the partition ratio, the measured values of potassium hydroxide in moles of
potassium hydroxide per gram must be converted into concentrations, or moles per liter.
To do this, multiply the moles of potassium hydroxide per gram by the density of the solutions.
The density of biodiesel is 870 grams per liter and the density of water is 1000 grams
per liter.
Record the converted concentration in moles per liter from the recorded data and calculate
the partition ratio by using the following formula and record the calculated the partition
ratio into data table.
Thank you for watching the “Using Differences in Solubility to Remove Contaminants from
Biodiesel” lab on the fundamentals of biodiesel.
Please go to the biodiesel education website for a complete written transcript of this
lab exercise.