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Recrystallization Lab

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Step 1: Dissolving the Solute in the Solvent 0:12
  • Hot Filtration 6:33
  • Step 2: Cooling the Solution 8:01
  • Step 3: Filtering the Crystals 12:08
  • Step 4: Removing & Drying the Crystals 16:10

Transcription: Recrystallization Lab

Hi and welcome back to

Today, we are going to be talking about recrystallization, or just crystallization, 0001

sometimes it is called, which is a really efficient and easy and inexpensive way to purify a solid compound.0005

If we have a solid compound, what we are going to do is -- I will call that the crude material.0014

It has got some impurities in it.0019

The way we might know that it has impurities is by its appearance 0021

or more importantly by measuring its melting point and finding if it has a sharp melting point.0025

We can do a recrystallization to purify it.0031

We are going to put our sample into our crude solid into an Erlenmeyer flask.0034

What is great about an Erlenmeyer flask is that with a flat bottom, it will sit nicely unassisted on a hot plate.0038

It is very easy to heat it.0045

And also with a narrow neck is going to minimize evaporation of our solvent.0047

Because the solvent as it heats, it is going to come up and hit the side of the flask and reflux.0052

As opposed to heating something in a beaker, it just evaporates way too quickly.0058

We do not want to use a beaker for a recrystallization.0062

We are going to put our sample into an Erlenmeyer instead.0064

It is also very nice because we can grab the top and we can swirl it very nicely.0067

An Erlenmeyer is ideal for mixing as well.0071

We are going to put our sample into an Erlenmeyer flask.0073

We are also going to use a beaker or maybe another Erlenmeyer.0077

We are going to use something to heat up the solvent that we are going to use.0082

If you are not provided with a solvent and you have to figure out which kind of solvent is ideal for a crystallization, 0085

then what we are looking for is something so you could test it on a very small amount of your sample.0091

Something that is not soluble at low temperatures.0096

But when you heat it up, it is soluble.0100

That is going to be our goal.0102

We are going to heat up some amount of our solvent.0104

When that is nice and hot and boiling, we are going to add a very small amount of that.0109

We can use maybe a paper towel to help handle a hot beaker.0114

The crystal tongs are nice for Erlenmeyer flasks.0119

But even then, it is a little dicey to have control.0123

Being able to use something like a paper towel gives you a little more control.0126

I can pick this up and what I’m going to do is I’m going to pour just the smallest amount of solvent in here.0132

Of course, remember I'm doing a totally dry lab here so I’m not wearing the protective equipment.0137

I’m not wearing my safety goggles.0142

I would be wearing a lab jacket to protect myself and my clothing.0143

You may or may not be wearing gloves, depending on what reaction you are doing.0148

We are going to add a small amount here.0153

We are going to place it right onto the hot plate to make sure my solvent is staying as hot as possible.0156

The goal here is to dissolve our solid in the minimum amount of hot solvent possible.0162

We do not want to use too much solvent.0169

I can swirl this around and look at it.0171

Any time we are boiling a liquid, we want to make sure that we have a boiling chip in here to promote even heating.0174

What is handy for doing a crystallization is using a crystallizing stick.0180

You just can keep that in there, that provides a surface to facilitate the boiling.0182

But unlike a boiling chip, it is very easy to remove at the end of the day, 0188

when we want to do our crystallization and we are cooling.0191

Otherwise, a boiling chip is something you are going to have to dig out later and scrape the crystals off.0195

Anyway, we are going to be doing something like that.0200

I’m going to be watching my sample dissolve in the hot solvent.0202

If there are any big chunks in there, I can use a glass stir rod to break up those chunks 0208

because that is going to help dissolving.0214

Swirling might also help dissolving.0216

Overtime, we are going to look and we are going to stay, is that enough solvent or not?0220

They dissolved everything or not?0224

It looks like you still have some undissolved solids and it is clearly boiling.0225

It is as hot as it could possibly get, that means you need a little more solvent.0231

We can add in a little more of solvent again, just a tiny amount in small portions.0234

Swirl, swirl, break it up as needed.0240

Eventually, our sample is going to be the solution or all the solid has dissolved.0243

You may have something in there that is not soluble in the solvent.0250

It is an impurity that is not soluble.0253

That is something we would want to filter off.0256

But that the point is, that is never going to dissolve.0260

You do not want to keep adding solvent, trying to dissolve something that is not your compound.0262

Sometimes you have to think about what is in there and make that judgment call.0266

Now sometimes, you are going to find that your sample is supposed to be a colorless solid, 0271

white solid, but there might be some color to your solution.0280

Some yellow or slight color.0283

There is a way to get rid of that color.0286

That color then is an impurity and there is a way to get rid of that colored impurity by using something called activated charcoal.0288

What we do for that is we add in the smallest amount of the activated charcoal and we swirl it around.0297

That is going to just provide, it is a very porous surface and all the colored compounds are going to get trapped inside that.0305

And then, what we want to do is we want to do a hot filtration.0313

One thing is you want to make sure that you are never adding that charcoal to your solution when it is hot,0317

because if it is overheated, it may cause it to foam up right away.0323

We are going very carefully add, when it is cooled.0329

If we do have to deal with activated charcoal treatment, 0333

we want to make sure our solution with the charcoal in there is very hot.0338

We want to keep it hot as we filter off the charcoal because it usually starts to cool our solution.0343

Our desired compound is going to be crystallizing now and be a solid.0351

We do not want our compound to crystallize out.0356

We need to keep the solution as hot as possible.0358

One thing that we do is we use a special kind of funnel.0361

This is a stemless funnel, this is a typical funnel.0366

This one has no stem.0369

That is because if you try to pour a hot solution through this stem, it would most definitely cool.0371

As it cooled, your crystals would come out of solution and the stem would jam up with solid.0378

This would never filter through.0383

We are going to use this to keep it hot and to avoid, if any crystals do crystallize out,0384

they are not going to stop the filtration.0390

We are going to do a hot filtration and we can use a filter paper.0393

We can fold that once and fold it in half again.0397

And then, we can just open it up and that will make a filter paper for us, a little cone.0402

We do not want to have too much surface area here because we want to minimize cooling.0407

We would wet this with just a little bit of hot solvent.0412

You can even put this, some students would keep this on a hot plate to keep it as hot as possible.0418

That is a good idea.0423

And then, what we can do is we could add, we can use our tongs here if you want or you can use a paper towel.0424

What we are going to do is we are just going to slowly filter this through the filter, in small portions.0431

Again, because if we have too much cooling up here, it is going to start cooling.0439

In small portions, we are going to, as quickly as possible filter off that charcoal.0443

And then hopefully, the solution that comes through here is now going to be colorless.0450

The activated charcoal step is only necessary if you have a colored impurity and you can check with your instructor to see if that is something that is going to be important to you.0454

If you do that, then we have to do this hot filtration with a stemless funnel.0463

Assuming we do not have our colored impurity here, we have a colorless solution here now,0470

and we do not have anything undissolved.0477

So now we are going to take this solution and we are simply going to put it onto the bench top and start letting it cool.0481

Now what is happening is our solution is now going to become super saturated.0488

Because if we will saturate it at a high temperature, as you cool it,0493

the solubility lowers and you will have too much solute dissolved in your solvent.0498

As it starts to cool, your dissolved solute will crystallize out.0504

If we let it go very slowly then it will slowly start to form crystals and you will get very nicely, well developed crystals.0512

They are not always large, it depends on the compound.0519

Sometimes perfectly pure crystals are tiny little needles.0521

Sometimes they are a big plate, sometimes they are big prisms.0525

They vary depending on the compound that you are isolating.0527

We are going to let this cool very slowly.0533

If we cool it too quickly, what can happen with that kind of shock is it could just crash out of solution.0535

We do not want a precipitation, a rapid precipitation.0542

We want a slow growth of our crystals.0544

If your sample would precipitate out then you can trap some impurities with you.0547

That is, just end up coming out of solution, even though they want to stay in the solution.0553

The idea here is as I cool this, because I have a small amount of the impurities, 0559

those will not be super saturated and those will remain dissolved at a cold temperature, 0565

just like they did at the hot temperature.0569

Ideally, what is coming out of solution now is only our pure compound.0571

Now what happens if it does not come out of solution? 0577

We are looking and we are waiting, and everyone else is crystallizing and ours is not.0579

There is a few things you can do.0583

One thing you can do is you can use a glass stir rod and you can scratch at the side of the flask.0584

What that is doing is it is creating a tiny little imperfection in the glass.0592

It is also maybe taking up some of the solution and letting it evaporate.0596

You are getting some solute there, that can act as a seed crystal.0600

What we want is a nucleation site where the crystallization can start.0606

Once the crystallization starts, it continues to grow and crystals form.0608

You can either scratch the glass or you can maybe add a seed crystal.0615

That will be a place where it can start to grow as well.0621

The other important thing that we are doing is we are cooling.0626

Clearly, letting it cool through room temperature first,0629

but then placing this into an ice bath is the way to get it to the lowest possible temperature.0631

Therefore, you have the smallest possible solubility.0637

Therefore, you get the maximum amount of crystals out.0642

We can do all those steps and watch the crystals grow as it cools, how long do we let them crystallize?0645

That is totally a judgment call up to you.0652

The longer you let it sit, the more crystals you will get.0654

But at some point, we have reached the new solubility level.0657

You are in equilibrium and you will not get anymore crystals out.0662

That is kind of a judgment call, where it looks like -- there it does not seem to be any more crystals coming out.0666

Clearly, the longer you wait, the better, you can get a maximum yield.0670

What happens if it is been sitting in here, I got a seed crystals, I'm waiting and waiting, nothing is happening.0674

What do you think the problem is, you probably added too much solvent.0680

At that point, what we have to do is bring it back to the hot plate, boil some of the solvent off.0685

If you have a lot of solvents you have added, you might even need to put it into a beaker so that it evaporates more quickly.0690

But lower that volume, maybe lower it back down, 0696

sometimes you lower back down until it actually gets turbid a little bit, a little cloudy.0699

Now we are reaching the point of saturation again and your crystals starts coming out.0703

Then, you can add a little more solvent back in until it clears up.0707

Now we added just a minimum amount of solvent.0710

Sometimes you do not have a choice.0712

You have to go back, put these mixtures in the hood, as you are boiling off the solvent.0714

And then, you have the right amount of solvent.0719

Again, cool it slowly, eventually put it in the ice bath, and then you should get a nice batch of crystals.0722

Now we have our crystals in here and we still will have our solvent in here,0728

that is called the supernatant liquid or the mother liquor.0732

We want to separate the two.0736

For that, we need to do a filtration, in this case now, we are going to be using a Buchner funnel to do the filtration.0738

Buchner funnel allows for a vacuum filtration.0746

This has holes here.0749

Our filter paper that we are going to use is a small one that stays flat on the bottom and just covers those holes perfectly.0752

We put that onto an Erlenmeyer flask that has takeoff arm.0762

We are going to use vacuum tubing.0766

This tubing is different from what we use for the distillations.0768

When we want to transport water, this is going to be a thicker tubing.0771

We need thicker tubing because this is going to be evacuated.0774

It is going to have a little pressure in and you have to be able to withstand that pressure.0777

If you use regular thinner tie down tubing or latex tubing, it is just going to collapse, when you try and pull a vacuum.0781

Make sure you use the proper tubing here for Buchner funnel.0787

We are going to attach that to the side arm.0790

This is going to go to a water aspirator or vacuum pump, or whatever you have for a source of vacuum.0792

We are going to wet the filter paper with a small amount of very cold solvent.0799

You can share a little of your solvent along with your crystals in your ice bath.0805

We are going to wet that to make sure, as you are done with the vacuum now,0810

the wet filter paper is going to be stucked onto the holes.0814

And that way, none of our crystals can sneak underneath the filter paper and snick through to the bottom.0819

We will get our vacuum going and then we can use with a spatula, 0825

we can scrape all of our crystals the best we can onto the filter paper and filter them off, as the vacuum is running.0834

This is a way of very rapidly collecting your crystals and because it draws the solvent out and drying your crystals at the same time.0844

We are going to scrape as much as we can.0853

I know they are sticky and it is hard to get everything there but we can get them all in there.0856

And then, maybe you have someone here who can rinse them out with some clean solvent again, using ice cold solvent.0860

Because if it is warm solvent, then when you put it in here, it might redissolve your crystals.0867

You do not want to do that, you want to keep your crystals as a solid.0871

Take a little ice cold solvent, swirl it around, whoosh it around.0874

And then very often, we also want to do that because we want to rinse our crystals0877

because the crystals will still have some of that mother liquor on them, and that will have some impurities in it.0882

The solvent is still an impure solution.0888

We want to rinse that mother liquor here, again, with some cold solvent.0892

Finally, we have our nice clean crystals here.0896

We can spread them out a little bit with a spatula, to make sure they are getting as dry as possible.0899

Leave them on the Buchner funnel for a minute or two, as it draws in air,0904

it is going to be drying your crystals because we want to wait and we want to analyze them.0907

They have to be dry, we do not want any solvent on them.0912

That is what the Buchner funnel is great for.0916

When we are done with the funnel, we want to make sure we break the vacuum before we turn off our vacuum source.0919

If we have is all connected and this ends up being really strong vacuum up here, a sealed system,0926

if we just turn off the water, the water can get drawn in here and it can mix in with the organic solvent.0933

A lot of time we put traps in between.0940

If we are using water aspirator on the sink, we can put a trap between this and the sink0942

to make sure that water cannot get into our reaction and our solvents.0949

If we have so much solvent and it got up and got up to this arm, that could go down the sink, if we have an accident here. 0952

The trap make sure that that it stays safe in both directions.0959

We can take this apart and then we have broken the vacuum, we can turn off our aspirator.0963

Now we have our crystals here.0970

We can take a watch glass.0974

A convenient way to do it is take a watch glass.0976

Sometimes you can take out the entire filter paper with your crystals on it and lay it on the watch glass.0978

It is a nice way to let it dry.0988

Again, if your filter paper is already there, you can just scrape it off and get it onto a watch glass.0992

We will do that and we can let it dry for a while like this.0999

But eventually, we are going to need to scrape the crystals off of the filter paper 1002

because we never want to weigh a filter paper.1008

I know sometimes it seems a good idea, I could weigh it beforehand.1010

And then when it is dry, we can weight it after and just subtract the two.1014

You can do that for the watch glass.1018

You can tare the watch glass so that you know how many crystals you have.1020

But you cannot do that for paper because the paper has some water absorbed in it.1023

That water is variable, essentially a filter paper now has a fixed weight.1027

It can become very highly depending on whether it has some solvent, if it is wet,1033

or if there is more water initially than there was at the end, and so on.1037

We will eventually scrape it off to filter paper, let it dry for a good amount of time.1041

And then, we can weigh it away.1046

You could transfer this to a weigh paper on a balance.1048

If you have carried your watch glass, you can weight it and find out how many crystals you have.1050

You can analyze that.1057

By its weight, do your percent recovery from your crystallization.1059

I always sacrificed some sample when you do a crystallization 1062

because some of your sample always stays dissolved in the mother liquor.1064

That is a sacrifice you make, the amount you get back is going to be pure.1070

If your goal is yield and we can figure out how good job we did purifying, 1075

by taking the melting point here, hopefully it is going to be much sharper than it was initially.1079

It is really important to me to get as much of this sample recovered as possible.1084

I can now work with this mother liquor to try and recover some more samples here.1090

What I can do is I can get this into a round bottom flask, put it on a rotovap.1095

You take away the solvent.1100

Now I have another batch of crude solid.1101

This is much more concentrated in impurities than it was initially because you have taken out so many of your pure crystals.1105

This is now going to be starting, let us take that solid, put it into another Erlenmeyer.1112

We are going to be starting with a much cruder solid and unlikely to get very much back.1117

It is also quite likely that the second batch of crystals, the second crop is probably not going to be as pure as the first crop.1124

But again, it is a way of getting more simple back, if you need to.1133

Hopefully, you can see how we are going to do a recrystallization, as a way to purify solid, if you need to.1138

Good luck with your crystallization in the future.1144