Extracting propan-2-ol from old food dyes

Distillation is an extremely useful technique. I will be employing it for my first time at home by attempting an extraction of propan-2-ol from old food dyes. I end up trying to dry some of the product with some anhydrous copper(II) sulfate(VI), but I am at the limit of my knowledge with parts of this experiment, and I run into a few problems. The process was at least semi-successful though, with plans to use some of the propan-2-ol in the synthesis of some esters.

I recently acquired some old food dyes from a person I know who had finally gotten around to getting rid of them when moving house. I took them as all three listed isopropyl alcohol as an ingredient. For the rest of this post I will call it by its preferred IUPAC name, propan-2-ol, but isopropyl alcohol, isopropanol, propanol and many others all basically mean the same thing.

I thought it would be a fun practice to extract the propan-2-ol by distillation. I also recently acquired basic distillation equipment so this would act as a good opportunity to get to grips with the extremely useful apparatus. I will say thank you to a few users on Sciencemadness for giving me some tips for the process.

Red, yellow and blue. Some of the food dyes have been slightly used, mainly the red, but otherwise all the bottles are pretty much full. Notice the simple ingredients. This should lead to a relatively simple extraction procedure.

The only ingredients in the food dyes are the actual dyes, sorbitol solution and propan-2-ol. The dyes should stay in the boiling flask as they should be designed to handle high temperatures and if anything they will decompose instead of boiling so should pose no harm to the purity of the distillate. Sorbitol is a sugar alcohol and should also pose little threat. Only water and propan-2-ol should come over – hopefully. Propan-2-ol has a boiling point of 82.6oC so it should be the first to come over.


Substance Hazard information
Propan-2-ol Highly flammable/Irritant
Risk of catching fire. Vapour may catch fire below room temperature. Can cause eye damage and vapours may cause drowsiness and dizziness.
  • Gloves and goggles should always be worn when handling chemicals to protect your sensitive eyes and skin.
  • Work in a well ventilated area and avoid any nearby naked flames due to the flammability of propan-2-ol and to avoid breathing any vapours in.
  • I would say your main safety worry here is just boiling a liquid. Bumping can be a dangerous problem where the liquid becomes superheated but the boiling flask is lacking nucleation points, causing the liquid to barely bubble, then suddenly boil viciously, then go back to barely bubbling. This can cause the liquid you are distilling to be thrown up into your condenser, contaminating your distillate, or, at the worse, can cause your boiling flask to crack. Usually some sort of boiling stones are used to add extra nucleation points, but I did not have anything sensible on me so I went without. Although this requires constant monitoring of the distillation in case things get out of hand, luckily my stir bar was enough and I had quite a smooth boil throughout. Immediately stop or slow the distillation if bumping occurs.
  • Depending on the dyes used, elevated temperatures could cause the dyes to decompose; be observant of any possible colour changes and/or odours that may indicate this. I did not have this problem.


My chemistry teacher was nice enough to recently lend me a retort stand and two clamps, hence the poor state of them from abuse in chemistry lessons.

I did not know how much food dye solution I was working with but I decided a 250ml round-bottom flask should do the trick. I clamped the flask and added a funnel to make the transfer easier.

This was a really messy business. I ended up having to switch my gloves. They were covered in the food dye due to an improper seal when I shook one of the bottles while washing it out with water.

The food dye bottles were a bit difficult and messy to open but once the solution was transferred it nicely filled just under half the flask.

A friend said it was worth doing the distillation just for this photo, which I totally agree with. The glassware just looks pretty. I definitely need to invest in a water pump soon though, I did not want to hook the Liebig up to the tap because I felt that would be a huge waste of clean water.

I added a stir bar to the boiling flask and set up for distillation. I decided not to run water through the Liebig condenser to act as a coolant. This is half because I should only be distilling a small volume of propan-2-ol so an air jacket should be sufficient, and the other half because I still have not managed to get a water pump yet. I would have used my distillation equipment sooner otherwise.

It looks like I have a bit of water refluxing as well as some propan-2-ol but I cannot be sure.

Heating was turned to full to initially speed the heating up and I would adjust the heating later. I would have used an oil bath or similar but I often see people heating their round-bottom flasks just by throwing it onto the hotplate surface and I wanted to try this. It seemed to work fine as the solution had begun a small reflux in the flask after a few minutes.

Notice how the evaporating liquid is colourless, and the solution in the boiling flask is keeping its yucky brown colour. This shows the dyes are keeping to the boiling flask. You can see the odd drop of brown liquid being flung at the walls of the boiling flask due to bubbles bursting as the solution boils, but this does not continue into the still head.

After a few more minutes I started to get some liquid making it up into the Liebig condenser. I did not seem to be getting any leaks at the joints which is good. I used Vaseline as the joint sealant as recommended by most amateurs.

The smallest flask I had with a ground-glass joint was a 500ml round-bottom flask so I ended up using a 250ml conical flask despite it not having a ground-glass joint. Anything smaller such as a 100ml conical flask would not fit to the vacuum adapter joint. I knew I was not going to get a large volume of distillate so I wanted a small receiving flask.

At about 92oC the first drops of distillate were starting to land in the receiving flask. I am still not sure quite why. Throughout the rest of the distillation, the temperature stayed in the 90oCs. I am obviously new to distillations, but surely the temperature should have maxed out at 83oC or so as the propan-2-ol is boiling, and once all the propan-2-ol had boiled then have increased to 100oC or so as the water started to boil?

The water-propan-2-ol azeotrope may have something to do with it but it has a boiling point of 80oC, which makes even less sense. Maybe I needed to use a fraction column to get a proper separation? This requires more research around azeotropes and distillation on my part.

You can see the droplets of condensation having nearly made their way to the end of the Liebig condenser in this picture, and I have quite a long Liebig condenser (300mm), conveying how the air jacket can almost no longer keep up.

I called an end to the distillation as the Liebig condenser was beginning to be flooded with droplets of condensing liquid – though not before I was able to collect two fractions in different receiving flasks. Even the vacuum adapter was showing droplets of condensation.

The first distillate I collected.

The first distillate fraction was quite small but hopefully contained a high percentage of propan-2-ol. A second receiving flask was used to collect a second distillate fraction in case I did not get all of the propan-2-ol in the first. Once I had taken the distillation equipment apart, the boiling flask no longer smelled of the industrial/acetone smell of propan-2-ol, suggesting all of it was extracted.

I quickly went into another room to light the propan-2-ol for obvious safety reasons. It produces quite a nice flame.

Both of the receiving flasks had a propan-2-ol smell, although the first fraction had it stronger as I expected. For a more scientific test I used a glass pipette to transfer a couple of drops of the first fraction to the base of a upside-down beaker, using a butane lighter to provide a flame. It immediately caught on fire producing a nice yellow and blue flame. A small amount of liquid was left which was attributed to the water content. This was repeated with the second fraction but it refused to produce an independent flame, suggesting the water content was too high.

It looks like there is 6ml of liquid from this camera angle but the meniscus does show about 5.5ml.

I transferred the first fraction to a measuring cylinder, showing me a yield of 5.5ml, however a large part of this would still be water.

I just put an arbitrary amount of copper(II) sulfate(VI) pentahydrate into a beaker and heated it at full power on my hotplate until it seemed pretty much dehydrated, converting from blue crystals to a white powder.

I decided I would try remove the water to gain relatively pure propan-2-ol from this first fraction. I did not currently have any pure magnesium sulfate(VI), so I settled for some copper(II) sulfate(VI).

The top layer only went pale blue so I decided to stop adding copper(II) sulfate(VI) at this point.

I ended up having to use a large amount of anhydrous copper(II) sulfate(VI) as every time I added some, it turned blue as it absorbed water. I stopped adding it once it stayed white in the liquid.

Some cotton here probably would have worked better than the filter paper as it seemed to soak up a lot of the propan-2-ol which probably contributed to quite some losses from evaporation from the large surface area of the filter paper.

I then tried to filter what was left to remove the solid copper(II) sulfate(VI) and hopefully recover the water free propan-2-ol, but, probably due to a number of reasons, I only recovered a few measly drops of liquid that when tested refused to ignite. In a few steps I lost about 95% of my yield and decreased the concentration of the propan-2-ol. Well done me.

If it is hard to see the liquid inside the bottle that is because there is hardly anything in there in the first place! There are only a few drops.

The second fraction fills this bottle quite comfortably. I added a quick temporary label using some masking tape.

Well, that was a shame, but it was not a total failure on my part. At least I still have a little bit of propan-2-ol as I still have the second fraction, but this is quite dilute. I am not going to risk drying the second fraction though. I transferred it to the same bottle as my few drops of filtrate from the drying of the first fraction.

I cannot really do much with about 20ml of a dilute but unknown concentration of propan-2-ol. Depending on my yield I was hoping to be able to carry out the oxidation of some of the propan-2-ol to propanone (acetone) with potassium manganate(VII) as the oxidising agent, but this is out of the question now. I might just use it to try make some esters such as propyl ethanoate for the fruity pear smell.

I am not sure but I hypothesise that I might have used too much anhydrous copper(II) sulfate(VI) which may also have not been properly dried as I only quickly heated it to drive off most of the water. The resulting hydrated copper(II) sulfate(VI) sponge must have trapped a lot of the propan-2-ol, preventing it from going through the filter. Most of it was then lost due to evaporation. If I had used magnesium sulfate(VI) I believe I would not have had such a problem as it can absorb more water to form a heptahydrate rather than only a pentahydrate. Well, it all adds to my experience; I will hopefully be less likely now to make such mistakes in the future!

2 thoughts on “Extracting propan-2-ol from old food dyes

Add yours

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

Website Powered by WordPress.com.

Up ↑

%d bloggers like this: