Transesterification of vegetable oils to produce biofuels - problems in the lab

Question

I'm an undergraduate doing a small research project on biofuels. I have 12 hours of lab time to investigate something about biofuels. I've chosen to explore how the fatty acid content of different vegetable oils affects the quality of the biofuel produced.

To investigate this I've chosen to do several different things in the lab. To determine the fatty acid content of each vegetable oil I chose I decided to compare their respective 'acid numbers'.

The acid number (or neutralization number or acidity) is defined as the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of chemical substance.

To do this I decided to do a simple acid-base titration. I made up a standard solution of KOH to a concentration of approximately 0.1M. I did some qualitative tests to determine which alcohol was best to dissolve my vegetable oil - I chose propan-1-ol to be my solvent and I used 125ml along with 20ml of a recorded mass of vegetable oil and phenolphthalein indicator in the conical flask. Because acid number requires a mass of oil I did not pipette the oil into the conical flask, I simply recorded the mass on a top-pan balance. This does result in a larger error for my acid number value but it avoids a difficult time pipetting oil and then cleaning up afterwards - especially with my limited time in the lab. I then titrated against my 0.1M standard solution of KOH and reached the endpoint incredibly quickly - my titre value was less than 1.00 cm³. My first question here is should I dilute my standard solution to a much lower concentration, as such a small titre would lead to a large apparatus error? There is already a large amount of apparatus error in this experiment because I weighed out the mass of oil rather than pipetted a known volume and then used density etc.

For my investigation I also have to synthesise a few biofuels. In my first lab session I tried out an acid catalysed transesterification reaction. I would like to stick to transesterification reactions as it's a common process in industry and in labs and can easily done in my undergraduate teaching lab. The method I used is as follows¹:

1. Combine 8 cm³ of an alcohol with a few drops of concentrated sulfuric acid (approximately 0.15 cm³) in a 100 cm³ B19 round-bottomed flask and stir well.
2. Add 20 cm³ of a vegetable oil.
3. Reflux for approximately 1 hour
4. Allow to cool and transfer mixture to a seperating funnel. Allow the layers to seperate and discard the lower layer (containing glycerin). Wash the top layer with 2-3 15 cm³ portions of 4M NaCl solution.
5. Check the final washing with pH paper to ensure the pH is neutral.
6. Transfer the biodiesel to a clean, dry conical flask and dry the liquid with anhydrous magnesium sulfate.
7. Filter by gravity to remove the magnesium sulfate.

In italics is what went wrong essentially and why I think it did. When washing with 4M NaCl I could not get the pH to stabilise, I used almost 50 cm³ and yet still it was at a pH of 1-2. This makes me think I added too much concentrated sulfuric acid. I remember adding in several drops, where in reality one small drop is all thats required for 0.15 cm³. As such when seperating out my biofuel, I lost a lot of it so my yield will be poor. I may be able to still rectify it so I can do some sort of analysis on my biofuel so my first three hours weren't a complete waste, but I am unsure how. Should I wash with more NaCl (potentially reducing my yield to almost zero) or should I try to use a stronger base like NaOH or KOH? I'm unsure as to whether these will interfere with the product that I have made. This same issue is referred to in the following paper (page 5): downloads.hindawi.com/journals/jchem/2010/689051.pdf

I am also unsure as to why neutralisation is needed in the transesterification process? On the other hand if that dosen't work I could switch to a base catalysed method for biodiesel production which is faster and can be done with an exact mass of KOH rather than adding conc. acid dropwise. On the other hand some papers seem to suggest that this method would be worse for vegetable oils with a low fatty acid content. However, I am just comparing fatty acid content to the quality of biodiesel produced with one method - I can discuss this aspect in my final report.

[1] - This procedure was adapted from Bladt, D. Murray, S. Gitch, B. Trout, H. Liberko, C.J. Chem Ed, 2010, 88, 201-203.

Answer 1

First, I believe that using mass was the appropriate way to go. Volume measurements are subject to error due to temperature changes (most glassware is calibrated at $\pu{25 ^\circ C}$, I think). Furthermore, density is also dependent on temperature (since volume changes with temperature), but mass doesn't. Also, are you measuring the density yourself, or using it from tables? For vegetable oils, I would expect the density to vary depending on the batches, so using a theoretical value might not be the most accurate choice. In the end, you need the mass, so measuring the mass would give you the least amount of error.

For your $\ce{KOH}$ solution, you can dilute it to have a larger volume used (or you can weigh more oil, of course). I wouldn't dilute it more than 10-fold for this case. Another source of error is your low accuracy for the concentration of $\ce{KOH}$, $\pu{0.1 mol L-1}$. If you're trying to minimize error from here too, I would suggest standardizing your $\ce{KOH}$ solution with potassium hydrogen phthtalate, a common primary standard for bases, and calculate the concentration with a tenth of a mililmolar acuracy (e.g. $\pu{0.1023 mol L-1}$). Furthermore, use this solution freshly prepared, as hydroxides react with $\ce{CO2}$ to form carbonates.