Making Glycerol from Biological Fats and Oils

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    Glycerol is a valuable product as a sweetener, moisturizer, lubricant and preservative for rubber, and the organic portion of some widely used explosives and medications.

    That glycerol can be made from biological fats and oils should be apparent if one notes that another name for this class of lipids is triglycerins. Looking at the structure of a fat or oil, one sees that the central molecule is none other than glycerol. The trick is merely getting the large fatty acid molecules to uncouple from the glycerol. In other words, it is the hydrolysis of the ester bonds in the fat or oil. (See the structure of a fat or oil.)

    However, until about 1830, it was not glycerol that industry was after as the main product. The valued part was the rest of the hydrolysis product - the fatty acids that were made into soap. So let's see how soap is made, and then get back later to glycerol.

    While this hydrolysis process is a rather easy chemical step to perform, it has long been an unpleasant olfactory experience with a lot of hot, hard work. Fatty slabs sliced from butchered livestock are put into great cauldrons and boiled so that the fatty materials melt and float to the surface above the watery phase where the fats congeal upon cooling into tallow (from cows) and lard (from hogs). Meanwhile other contaminants - muscle fiber, grist, etc., settle to the bottom. This steamy, stinky, hours-long process of stirring is called "rendering." You don't want to live near a rendering plant! (Are you beginning to see that this is a multidisciplinary unit - history, industry, as well as chemistry?)

    Of course extracting plant oils with primitive presses had its own rigorous labors, but the odors involved were not as unpleasant as those coming from animal rendering workshops.

    Interestingly, as long ago as 2800 BC - some say even as far back as 10,000 BC, this process was discovered in widely separated locations on the globe: rendered fat could be boiled with the leachings of wood ash and - as the ancient Romans more recently discovered - then added to certain naturally occuring acidic minerals, and a sudsy change would occur - a non-caustic, mild soap was made, and thus one of the world's great hygienic advances was made - a cleaning agent that is probably responsible for the saving or more lives than even antibiotics. (Unfortunately, the addition of the acidic agents was soon forgotten, and harsh lye soap was much too alkaline to use on skin, yet was still suitable for cleaning more inert utensils, clothing and floors.) Egyptian Pharoah Cleopatra XI's preferred soap was made from olive oil.

    This hydrolyzing process of boiling molten fat with "lye" is called saponification - and this process is an even more stinky occupation than rendering as some of the fatty acids smell terrible. Soap making was a major fall occupation around the world up to about 1840 AD. (Historians among you: why in the fall?) Purified liquids of some of them (e.g.: butyric acid) are labelled on reagent bottles with big letters "STENCH!"

    Two types of soap were made - soft and hard. Potassium soap was a soft gooey stuff that was laddled out of small barrels for use. But it could be hardened by adding NaCl.

    The following protocol will incorporate both modern and ancient methods just so that you can get the full multidisciplinary experience. Fortunately for our noses, we will use more modern reflux methods that will prevent the escape of the more noxious fumes.

1. As old-timers, we start out by considering how we are to produce our highly caustic reagent, lye. Here is the way it goes: we first must burn some wood or seaweed to make gray-white ash, which contains a lot of oxidized metals including that of sodium and potassium. These react with water as shown:

   K₂O 2 KOH
   Na₂O 2 NaOH

2. We would know that our lye was strong enough for use if a potato or a raw egg floated in it so that the above-surface exposed area was about the size of a colonial nine-pence coin (about the size of a modern day quarter). Thus, today, we could make up our solution of KOH so that an egg will do just that. But we are smarter now: we know that fat will not dissolve in hot water, and the more fat that can be dispersed in our caustic solution the better.
3. So, instead of water, we shall use caustic (alkaline, basic) alcohol. Fats dissolve somewhat in alcohol, and thus our reaction ought to go faster. Weigh ALL components to three significant figures: use 5.6 g (±0.2 g) KOH in 100 mL 95% EtOH (volume of EtOH is not critical). How many moles of KOH is that? Fill in the table accompanying this handout.
4. Add 10 gm (±2 gm) of fat or oil - again weigh to three significant figures. How many phases can be seen in your reaction chamber? (Two!)
5. Since we need to boil this, what shall we do so that we don't boil everything away? Set up your refluxer and begin boiling. (When the Variac auto-transformer is set at 65 for a 500 ml round flask, the boiling goes nicely.)
6. Reflux for about 15-20 minutes. By this time, all the fat or oil has been saponified, and you are left with what? (Glycerol, soap and ethanol, water and excess KOH.)
7. Cool to flask. You may disconnect it from the reflux set-up, and swirl it in a cool-water bath for more rapid cooling.
8. Titrate the cooled saponified solution (pH indicator = a squirt of phenolphthalein) using 2.5 M HCl (again note the volume of acid added to three significant figures). Add acid until the red just disappears. For a trial: pour a small amount of your red solution into a small beaker; titrate that until the red disappears. You thus know what you are looking for. Dump the beaker back into the main batch and continue titrating.
9. How many moles of HCl did you use? Explain the disparity between the moles of HCl and the original KOH added to the flask. You know you understand what you have done if you can explain this: Suppose you had two fats or oils, one having higher average molecular weight than the other. Which one would show the greater disparity between HCl and KOH? This disparity is related to the "saponification number" of the fat or oil.
10. Now that your mix has been given the Roman neutralization treatment, it is less dangerous to your skin, we shall proceed.
11. Put your mix in an icebath. Swirl to speed the cooling. What's that precipitating? Is it hard or soft? Scoop out a little with your finger and rub it into a damp palm of your hand. This is your byproduct. Do you think it might be marketable?
12. Next, consider how you are going to extract your primary product - GLYCEROL! - from the flask. Look back at your findings when you tested glycerol for various of its properties. (Warning! Don't even consider skimming off all the soap layer because a lot of glycerol might be dissolved in it, and you don't want to lose that!)
13. Separate the soap from the reaction mixture by either centrifugation or filtration.
14. You may now begin to boil the remaining fluid which contains ethanol, water, glycerol and KCl. Ethanol and water will boil away leaving a syrup (glycerol) with crystals of KCl.

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FURTHER READING

www.alcasoft.com/soapfact/history.html

An old-time Tom Swiftie: "Gimme the sodium hydroxide," growled Tom caustically.