Today we're going to practice our extraction skills and technique with another extraction

challenge.

Here we've got four fresh lemons and the challenge is can we extract pure limonene

and citric acid from these?

Citric acid is a tri-basic organic acid with six carbon atoms as you can see here.

It occurs naturally in the juice of citrus fruit, especially lemons and limes.

And limonene is a cyclic hydrocarbon known as a terpene, which is present in the rind

of the lemon and gives it its characteristic aroma.

So let's start by removing the lemon rind.

We're using an old vegetable peeler, so it takes a bit of force.

Okay here we go.

For peeled lemons and lots of nice smelling yellow rind.

We'll work with the lemon rind first and come back to the lemon rind.

We'll use the lemon juice later on.

Limonene is a hydrocarbon which is insoluble in water, but soluble in solvents.

You can simply extract the rind using an organic solvent, but we're going to try something

a bit different.

First we'll chop the rind into thin slices.

This maximizes the surface area to make our extraction process a bit more efficient.

You can also use lemons as well if you wanted to do this better.

What we're going to do is we're going to try to steam distill the limonene out of the peel

by boiling with water.

So we've got a nice big 500 ml flask set up.

Let's get the chopped lemon rind in.

Our funnel was a bit too small, but we got there in the end.

Now we've measured out approximately 250 ml of water.

Which we'll use to see if it works.

Let's steam distill.

Let's get this in the flask.

The lemon rind is all floating around nicely.

So let's set up now for distillation.

We're going to use a heating mantle so we've got good strong heat.

And we don't need a thermometer.

Let's get heating the mixture.

In theory as the water boils, the steam produced should carry the limonene with it as well,

water and limonene condensing and coming into the receiver.

And we've got distillation starting up.

Look closely and you can see there are two products coming over.

The water and also the colorless oil in little drops floating on the surface.

We've got a good vigorous boil going on now.

So we'll leave this to distill and wait until we can't see any more oily drops coming over.

If you look at the receiving adapter it's easier to see the product.

There are actually two layers in the tube.

The one at the top is the limonene collecting as it distills.

After about 100 ml of liquid has distilled we emptied the receiving flask into a container so we can see what we've got so far.

Definitely some oily material in here on the surface.

So we'll keep going with the distillation.

After about another 20 to 30 ml of distillate has come off we're looking at the receiving adapter and it doesn't look like there's any more oil coming over.

So we stop distillation at this point and allow the flask to cool down.

Once cool and we open the apparatus up.

It's interesting because most of the flask has cooled down.

The fresh zesty lemon citrus aroma has now gone.

Here's our distillate.

On a larger scale you could simply separate the layers of product.

But it's going to be difficult here so instead we're going to extract using an organic solvent at this point.

Here's 15 ml of dichloromethane.

We add this to the distillate and swirl the mixture around well.

And we've got the bottom DCM layer now which should contain the limonene.

So we place this into a separating funnel and then once completely settled drain off the bottom DCM layer.

We then wash the aqueous layer with another 6 or 7 ml of DCM just to make sure we've got it all out.

Here we go.

Here's a clean and dry flask that we pre-weighed empty.

We decant in the DCM solution.

And now we evaporate this down on a hot plate to remove the low boiling dichloromethane.

Use ventilation for this step because DCM vapors are toxic.

Eventually the DCM is gone and we're left with a small amount of colorless oil.

And here's our product.

1.2 grams of limonene.

Not very much but that's 0.3 grams or about a third of a ml per lemon that we started with.

What's interesting is the erosion.

It doesn't smell of lemons but does have a delicious orange aroma.

So we've made orange flavoring from lemons.

So let's go back to our peeled lemons now.

First we're going to juice these and we're using a plastic juicer to make this a bit easier.

It works pretty efficiently.

Strain the juice and get rid of the pits and the fleshy bits floating around.

And here we go.

About 250 ml of lemon juice.

Let's get this into a larger beaker so we can work with it.

Okay let's just have a look at the pH.

No surprise.

It's pretty acidic as there's a fair bit of citric acid present in here.

What we're going to do now is neutralize the acid and convert to the sodium salt.

To do this we'll use sodium hydroxide.

We've weighed out 12 grams here.

Which should be just a little bit more than we need.

And here's 30 ml of water to get the sodium hydroxide into a solution.

We'll also add an ice cube to moderate the temperature.

And we've got the dropper set up so we can add the sodium hydroxide solution carefully.

The addition is pretty uneventful at the start.

The pH is coming up slowly though.

So we keep on adding and checking the pH.

Just acidic now.

Let's just recap on those universal indicator colors.

Suddenly there comes a point where the mixture turns a brighter yellow and slightly orange color.

This is the sign that the overall solution is alkaline now.

There's still a lot of particulate matter in here.

So our next step is to filter this mixture and remove all of this.

We started off with a little bit of sodium hydroxide.

Started off okay.

But within a few minutes it seems completely blocked.

We tried a grade 2 sinter as well and the same thing happened.

So we tried a paper filter in a book nerd funnel.

This works better but it still blocks and the paper needs changing about every 40 to 50 ml.

The filtering process took 3 hours and was a giant pain in the ass.

It's still a bit cloudy so we filtered it again through the grade 3 glass sinter.

This was much faster.

Looking a bit better now.

We can now precipitate out the citric acid.

The citric acid is the calcium salt.

In order to do this we need a soluble calcium salt solution.

So here's 21 grams of calcium chloride dihydrate.

And we'll make up a solution using 40 ml of water.

Here we go.

A nice clear calcium chloride solution.

So let's add this to the sodium citrate lemon extract.

As you can see there's no immediate precipitate.

To get the reaction to complete we need to heat the mixture to boiling point.

So we'll use a quick and convenient way to do this.

As the mixture heats you can see a white precipitate forming and crust on top of the liquid.

Keep the mixture at nearly boiling point for a few minutes and the product should all settle out.

So let's filter this.

Wash the flask and the solid product with 100 ml of boiling water.

Then filter again.

And we wash with about 20 ml of absolute ethanol to dry the product and remove any oils or fats remaining.

Dry the product really well on the pump.

And here we go.

24.9 grams of calcium citrate.

Which we believe is the tetrahydrated salt.

This corresponds to 16.8 grams of citric acid.

And so is citric acid content in the original lemon juice of around 6.7 grams per 100 ml.

The textbooks say 5 to 6 grams per 100 ml.

So maybe these were good lemons.

We did attempt to extract pure citric acid from this and we'll show you quickly what we did.

We placed the product into a beaker.

Equipped with stirring.

And then used 30% concentrated hydrochloric acid to dissolve this completely.

It took about 20 ml which is about right.

We then very gently boiled down the resulting solution of calcium chloride and citric acid until we got down to about 15 ml of syrupy liquid.

As the literature says we got a white foam appearing and at this point chilled the mixture to see if it would crystallize.

But it just wouldn't crystallize.

Even with freezing.

And even reducing down further and trying again.

All we could get was a syrupy liquid.

It seems as though in the presence of some inorganic salts, citric acid just doesn't want to form crystals.

We also tried mixing with ice cold acetone and ice cold ethanol to see if this would force it out.

But it didn't.

So if you want citric acid crystals then you could try using dilute sulfuric acid instead with the calcium citrate and then filtering off the calcium sulfate formed.

You'll need to be very exact though with the acid because excess will be very hard to remove.

But after all the time spent filtering and boiling down we didn't do this.

Our recommendation for citric acid is just to buy it.

This was an interesting exercise to try out.

But not worth it in practice.

Citrate and limonene from fresh lemons.

Hope you enjoy this video and stay tuned for more soon.

Thanks for watching.