In this video i’m going to look at the recrystallization process which would be used to purify a solid typically we would do this after an organic solid synthesis so in this little conical flask here i’ve made some aspirin i’m going to purify it by recrystallization and then once we’ve got the purified aspirin we’ll carry out some melting point tests or we
Could carry out some thin layer chromatography just to check purity anyway so i’m going to go through the recrystallization process and explain each step as i go so the first thing we need to do is establish which solvent we’re going to use so for this practical i’m going to use ethanol because aspirin is soluble when ethanol is hot but it’s not soluble or
It’s not as soluble when the ethanol is cold so you’ll see why that’s important or how that fits into the process as we go through it so we need a solvent but your solid is soluble when hot but not very soluble when cold so for aspirin ethanol is the solvent we’re going to use so now we’ve chosen the solvent we’ve got a small amount of ethanol in this conical
Flask it’s sitting on a hot plate i’m not using any naked flames ethanol is flammable so we’re getting this as hot as we can so that we can dissolve the aspirin in the minimum amount of of the hot solvent i will explain why that’s important um later on in the video okay so the ethanol is nice and hot now so what i’m going to do is transfer dropper at a time of
Ethanol into my aspirin and dissolve it but remember i don’t want to add too much minimum amount of hot solvent okay so the aspirin’s all dissolved now so i’m going to put it back on the hot plate keep this nice and hot while i explain the next stage which is the filtration so for the filtration stage we use what’s called reduced pressure filtration or suction
Filtration it’s sometimes known as and we use this apparatus here so this is called booknet operators this is a book in a flask so it’s a very thick glass um conical flask with this side arm on and we’ve got this book in a funnel comes in two parts you can take the top it off quite easily but you can see it’s it’s inside the conical flask there’s a nice seal with
This bunk so this rubber tubing here goes to this water pump so basically the action of the water running through the water pump i’ll turn the water on in a moment because it gets quite loud um it reduces the pressure inside this flask here so you’ve got a pressure gradient you’ve got high pressure obviously outside low pressure in here and that forces the air
Down obviously that’s more efficient it’s a quicker way of filtering than regular filtering under gravity so i’ll just demonstrate that now so there’s the water pump on if i put my hand on this i can lift the um flask up and see it’s quite um quite a powerful suction there turn that off now while i do the next bit okay so before i put the water back on i’m going
To put this small disc of filter paper inside the buckner funnel now sometimes you can stick the paper down with some of your solvent as soon as it gets wet it’ll stick to the to the funnel so i’ll turn the water on and i’m just going to grab my conical flask with my dissolved aspirin in and i’m going to filter that now okay so just turn that off so the purpose
Of that little stage this is the hot filtration stage remember so we’ve got our dissolved aspirin which is in here now so this is the filtrate any soluble impurities are going to be in there as well any insoluble impurities will have been trapped by the filter paper so the hot filtration is to separate out any insoluble impurities from the dissolved aspirin and
The soluble impurities so i’m just going to transfer the filtrate to a boiling tube and i’ll show you the next stage okay so the filtrate’s in here now so what i’m going to do next is i’m going to cool this down by putting it in this speaker of ice water so i’m going to leave it for a few minutes to get the crystals back to recrystallize the aspirin so i’ll just
Quickly explain why the minimum volume of hot solvent was used so remember we dissolved the aspirin initially in a minimum volume of hot solvent so what would what we’re trying to achieve there is what’s called a saturated solution so that’s a solution that’s holding its maximum amount of solute and the good thing about creating a saturated solution is as soon
As you start to cool it down the crystals form more readily so we’ll just give that a few minutes to get really really cold and then i’ll show you what’s inside and hopefully we’ll have some crystals of aspirin right it’s had a couple of minutes i’ll just bring it out and show you what we’ve got so far so you can definitely see we’ve got crystals of aspirin in
There just gonna do a little trick that you can perform sometimes you don’t get any crystals forming initially so if you just get a glass rod and you um sort of gently scratch the sides of the boiling tube in this case and what you’re doing is you’re just creating um sort of microscopic surfaces to encourage the crystals to form so if we didn’t have any crystals
In there they would definitely start to form after doing this anyway we have got some crystals so i’m just showing you this in case you do need to to do it right so i’ll just put that back in the ice for another couple of minutes and then we’ll do the next stage okay so the next step is another filtration step so if i just bring this into short to explain what
I’ve got here so i’ve got another beaker of iced water that’s my original conical flask with ethanol in but this is really cold now because it’s been sitting in the ice water for a while so before i filter i’m going to get the stern rod out so in case any of my aspirin on the sternum rod i’m just going to give this a rinse but with cold ethanol because i don’t
Want to dissolve the aspirin i’m just making sure that i leave as much aspirin in here as possible right okay so that’s washed i can take that out now and all i’m going to do now is i’m going to filter this turn the water on okay still got a bit of my aspirin in here so i can use cold solvent to rinse this through and the the sort of beauty about doing this is
I’m also washing my aspirin as i call cold solvent remember because we don’t want to dissolve our solid so obviously what we’ve done there is we’ve managed to separate out the the recrystallized aspirin from the smoothness from the soluble impurities so just quickly explain why the soluble impurities stay in solution so they’re in here now that’s because they’re
Not saturated so there’s only a tiny amount of the soluble impurities in all of that solvent so when they cool down they don’t crystallize out okay so i’ll just let that run for a few minutes and then i’ll show you what’s inside the buckner funnel okay so i’ll just turn the water off now just quickly show you what we’ve got inside hopefully you can see what’s
In there i’ll just do this hopefully you can catch we’ve got some nice shiny crystals there of aspirin so the final thing we would do is uh transfer this onto a watch glass and just let it dry overnight and then sort of the next day you would run some melting point test thin layer chromatography desk just to check purity um against the data values okay that’s
It hopefully that was helpful and all those different stages make sense all right cheers bye
Transcribed from video
Recrystallisation of aspirin By MaChemGuy
