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Lecture Comments (2)

1 answer

Last reply by: Professor Starkey
Tue Oct 10, 2017 1:30 AM

Post by Maryam Fayyazi on October 5, 2017

if we want to purify the crystals wouldnt it be easier to evaporate the solvent and keep the product as pure material instead of doing crystalization?

Introduction to Recrystallization

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
  • Crystallization to Purify a Solid 0:10
    • Crude Solid
    • Hot Solution
    • Crystals
    • Supernatant Liquid
  • Theory of Crystallization 2:34
    • Theory of Crystallization
  • Analysis and Obtaining a Second Crop 3:40
    • Crystals → Melting Point, TLC
    • Supernatant Liquid → Crude Solid → Pure Solid
    • Crystallize Again → Pure Solid (2nd Crop)
  • Choosing a Solvent 5:19
    • 1. Product is Very Soluble at High Temperatures
    • 2. Product has Low Solubility at Low Temperatures
    • 3. Impurities are Soluble at All Temperatures
    • Check Handbooks for Suitable Solvents
  • Why Isn't This Dissolving?! 8:46
    • If Solid Remains When Solution is Hot
    • Still Not Dissolved in Hot Solvent?
  • Where Are My Crystals?! 12:23
    • If No Crystals Form When Solution is Cooled
    • Still No Crystals?
  • Tips, Tricks and Warnings 16:26
    • Always Use a Boiling Chip or Stick!
    • Use Charcoal to Remove Colored Impurities
    • Solvent Pairs May Be Used
    • Product May 'Oil Out'

Transcription: Introduction to Recrystallization

Hi and welcome back to

Today, we will talk about recrystallization or some people like to just call it crystallization, which is a method used to purify solids.0002

Let us assume that we have a crude solid, maybe we just ran a reaction that has a solid product and0011

we have isolated that product, but it has some impurities trapped inside it.0017

The goal of a recrystallization is to first dissolve that solid in a hot solvent and that is going to get us a hot solution.0020

Our crude solid, whatever impurities are there, are going to dissolve.0030

Now we have a solution.0034

We take that hot solution and we cool it and that is going to cause a crystallization.0037

Because if we use a minimum amount of solvent here, minimum here,0042

then our hot solution is going to have a lot of our solid dissolved in it.0049

As we cool it, the solubility decreases and our solution is going to be super saturated.0055

It is going to be holding too much solute for the amount of solvent that we have.0060

It is going to cause the solute to crystallize out.0065

Now we have a solvent that is mixed with solid, that is mixed with our crystals.0071

We are going to filter, we are going to get our crystals out.0076

Now they should be a pure solid.0079

What we are filtering off is the solvent that is called the supernatant liquid or the mother liquor.0084

That solvent is hopefully going to contain the dissolved impurities.0090

The assumption here is that the amount of impurities you have in your crude solid is relatively small.0094

As we cool a solution, it is not only supersaturated.0100

The impurities are still going to be plenty as solvent to keep the impurities dissolved.0103

When we filter off the solvent, the solution from our crystals, our impurities are going to get washed away with that.0108

We are also going to have some dissolved solids.0116

We have, our desired product has some high solubility in hot solvent.0120

But when we cool it, it is unlikely to have high solubility in the cold solvent.0126

It is just going to have a lower solubility.0132

When the crystallization is done, some of our product is still going to remain dissolved in the solution.0136

What is important to recognize is that every crystallization process results in a loss of some product.0141

But the goal is that even though we have less products now, it is going to be more pure.0148

What is the theory of crystallization that works here.0155

I just said, we have a small amount of impurities, we are going to try and do our cooling very slowly.0159

And that ensures that the crystals are going to grow slowly and have a nice chance to form,0165

that is going to ensure pure crystals.0171

Our impurities should remain dissolved along with some of our desired product that remains dissolved.0174

What we do not want is precipitation.0180

If we cool it too quickly and have our solid product just precipitate out rather than crystallize out,0183

then, that precipitation can trap impurities along with it.0191

Some of your impurities, even though they want to stay dissolved, might come crashing out with the solid.0196

We want to give that a slow opportunity for it to cool as long as possible.0202

They do not necessarily end up being large crystals.0208

Some crystals grow to be very large.0211

Other crystals always end up very small, fine needles.0212

But the idea is that we want to have well-formed crystals.0216

It is sometimes possible to obtain a second crop and we also want to think about the analysis we want to do on our product.0222

After we have done our filtration, we have two components left.0230

We have our pure solid which are the crystals.0234

We have our solvent which is the mother liquor.0237

For the pure solid, we want to analyze that.0241

We could take a melting point to see if we have in fact purified the compound.0244

We can maybe take a TLC.0249

A TLC would confirm that it is a pure compound, if we had just a single spot.0251

There is analysis that we typically do after recrystallization.0255

We can also work with this mother liquor to possibly obtain a second crop.0259

What we do in this case is we are going to remove the solvent.0263

If we put this on a rotovap and strip off the solvent, we obtain a crude solid again.0266

We can crystallize again.0274

Again, dissolve this in a minimum amount of hot solvent and then slowly cool it.0276

Let the crystals form and we can get a second crop of the pure solid.0281

The problem here is that because now our crude solid has a larger amount of impurities than it had initially,0285

our first crude product had some small amount of impurities, but we have taken most of that pure product out.0293

Now whatever small amount of product is left proportionally has a much larger amount of impurities.0302

Very often, our second crop is typically less pure.0308

It still has some impurities that are going to be in that second crop, very often.0313

That second crop might be important.0321

If it is very important for you to get as high as possible and isolate as much product as possible,0322

that is when you would go and try to obtain a second crop.0326

How do we choose a solvent?0330

Sometimes we are lucky and we are following a procedure and0331

it tells you - recrystallize this from methanol or recrystallize this from ethyl.0333

It will tell you that.0337

Sometimes we have to choose our own solvents.0339

How do we do that?0341

The goal of the crystallization, we have two solubility goals.0343

We want it to be very soluble at high temperatures.0348

When we add it into that boiling solvent, when we add the boiling solvent to our crystals,0351

we want it to dissolve and we want to get a solution.0356

It needs to have high solubility at high temperatures.0358

We want it to have low solubility at low temperatures.0361

Once we cool it, we need it to be super saturated.0365

We need to have those crystals come out of solution.0368

That is the goal, is to have those very different solubility at different temperatures.0372

Now the impurities we have, we do not always get to decide what impurities are there.0377

Sometimes we kind of know what is in there.0381

We have two goals of impurities to in order for recrystallization to be successful.0383

They either need to be soluble at all temperatures and that is typically the case, which means,0387

when we dissolve it on hot solution, everything is dissolved.0394

When we crystallize our product, our impurities stay dissolved.0398

When we filter it, they get washed away.0401

Or they can be insoluble at all temperatures and that would lead to a successful crystallization as well.0403

Because then, all we need to do, let us say my solid had some dirt mixed in, I dropped it,0411

and I scooped it off the floor and then I have some dust or dirt in there, or some sand got mixed in, something like that.0417

That would not be something that dissolves in the solvent.0423

But then you can imagine, all we need to do is dissolved my product into solvent.0427

I have these insoluble contaminants that are at the bottom of my flask.0432

All I need to do is filter them off and now I have removed that impurity from my product.0436

They either need to be dissolved at all temperatures, so they get filtered away from the pure crystals at the end.0441

Or insoluble at all temperatures, in which case we filter them off before we do the cooling in the crystallization part.0447

You can check handbooks for suitable solvents, that would be a good place to start,0455

what are typical recrystallization solvents depending on your function that you are dealing with,0460

or look at similar procedures to get ideas.0465

Then, you can do some testing, take about 100 mg of the solid and put it in a test tube.0468

Add just 1 ml of solvent at room temperature and there your goal would be that it should not dissolve.0474

Because we want it to have low solubility at low temperature.0481

You want to heat it to boiling and at that point, you want it to dissolve.0484

And then, that tells us that you have that significant difference in solubility at higher and lower temperatures.0489

And that sounds like it could be a good solvent to use.0495

When you cool it, you want your goal to be nice crystals forming.0499

Again, a certain compound can be very different crystals, depending on the type of solvent that is used.0503

Sometimes, you might have more than one to pick from, in terms of solubility.0510

And then, maybe you are looking at the quality of the crystals that come out.0514

A lot can go in, but what you want to do is, here is about the ratios,0517

you want to test about 100 mg in 1 ml of solvent to decide that it is a good solvent to use.0520

Sometimes you are doing your crystallization and you are heating it, and it is just not dissolving.0528

What is going on there, what can you do with that point?0533

First of all, make sure that your solution is boiling.0536

Obviously, it has a high solubility, your component has a high solubility at high temperatures.0540

But if your solvent is not boiling at the time that you are looking, then it is not as hot as it can be.0544

You can just put it back on the hot plate, make sure it is boiling.0551

At that point, you say it is really not dissolving.0554

Then, you know you need to take action.0557

As soon as it starts to cool, then we are sure it may not be dissolved.0560

It maybe crystallizing back up, that it is supposed to be doing.0564

If you have any large chunks that remain there, you could very carefully use a stir rod.0568

Make sure it is supported on the bench top.0574

And then, carefully use a stir rod to crush up any large chunks.0577

Remember, how do you get something dissolved at the surface area argument,0581

it is only the molecules at the surface that can enter into solution.0585

By crushing up the large chunks, you will have smaller chunks and it makes it dissolve much more readily.0589

Make sure you get rid of any large chunks again, before you decide that it is really not dissolving.0595

If you have done all this and you confirm, yes, it is boiling, not only large chunks, it is really not dissolving,0601

that is when you can go ahead and have more solvent, and say that you have not added enough solvent yet.0609

We are going to be adding small portions of the hot solvent, just enough until it finally dissolves.0612

If you done all that and you have added a ton of solvents and it is still not dissolving,0622

maybe you have an insoluble contaminant.0625

You want to be sure that the thing that you are looking at is likely to be your product and0627

not some other contaminant that is in there.0631

That can be something that you can just decant off into another Erlenmeyer flask and leave that residue behind.0634

Or if it is fine or you find particulates, you can gravity filter it off.0641

And then, you have your solution to work with.0647

That is where we talk about your impurities can either be soluble at all temperatures or insoluble at all temperatures.0649

We could still effectively remove them from your compound.0654

If you are going to be filtering though at a hot temperature,0659

you want to add a little excess solvent because as you are filtering a hot solution, it is going to cool a little bit.0662

We do not want our product to be crystallizing out.0667

You add a little excess solvent to reduce that.0670

You can also use stemless funnel.0672

An ordinary funnel looks something like this, it has a long stem.0675

If you put a hot solution through that, it is going to cool and we are going to get crystals.0680

It is going to get clogged.0685

What we use instead is a stemless funnel which just has the stem removed.0689

Now you can filter through and your hot liquid can come through without cooling and precipitating out.0695

But still, you very often lose a little of your crystals here as it cools, as it starts to crystallize out.0701

We want to only do this as a last resort, if we really had to.0707

As hot as you can keep it as you are doing a filtration.0714

Here we have our Erlenmeyer down here and our solution is going through here.0717

We want to keep that nice and hot, so that everything stays dissolved.0724

But we are just filtering off, put a little filter paper in here, we would be filtering off any insoluble impurity.0730

Be careful when you look at it and see if it really looks like it could be your product.0735

Otherwise, you are just adding in solvent, trying to dissolve something that is never going to dissolve.0739

What if we finished or dissolved everything, and when we finally start to cool it,0744

we slowly cool it, but no crystals come out, that happen sometimes too.0749

There are some techniques you can use to encourage crystal formation.0753

One thing you can do is add a seed crystal.0758

If someone else has some of the product they have already isolated,0761

you could take one of those crystals and drop it in there and that serves as a nucleation site.0764

Once you have a crystal, it is a lot easier for other molecules in solution to grab on to that and build onto that crystal.0770

That is what you are doing, you are encouraging with the seed crystal0778

and then you can just watch the crystal grow from that one spot.0781

It can be quite dramatic sometimes.0785

You could also try scratching at the surface of the flask with a glass rod.0787

If you have your solution in here, you just wanted to put in the glass rod and just scratch right around here.0792

That is doing two things, it is creating maybe a little imperfection on the glass surface.0800

Again, that can act as a nucleation site and the crystallization can start.0805

It also might be taking a little bit of that solution and moving it up to the wall of the flask where it might evaporate a little.0809

You might get some crystals forming just by precipitation.0816

But then, that can act as your seed crystal and it can grow from there.0820

Of course, the way that you cause a supersaturated solution is to cool it.0824

We should place it in an ice bath, eventually.0831

Put this in an ice bath and when we say ice bath, we mean ice water.0834

We never just use ice because if you have a cooler of ice and you stick your hand in it,0840

that is kind of bearable, you can handle that.0846

What if you have cooler of ice water and you are sticking your hand in it, significantly cold right.0848

You can tolerate that much less and that is because0854

that water serves to increase the contact that you have with the cold, but whatever is cold.0856

You will have much more efficient cooling.0863

Again, when we say an ice bath that means you add ice and water, and that efficiently cools it.0865

You want to make sure it is at 0°, you want to be nice and patient.0871

Then, with some of these other techniques, watch your crystal grow.0877

You want to be as patient as possible because the more time you give it, the more crystals you are going to have.0882

At some point, you are going to reach the point where it is at equilibrium and no more crystals are going to be coming out.0885

You have to make a judgment.0893

At some point, you are going to say this is as many crystals we are going to get.0894

Now it is time for me to filter.0897

What you if you do all of this and you still had no crystals?0900

You have been patient and it is taking forever.0902

Then, you probably added too much solvent.0904

If you added too much solvent, then when it was hot and then when you cool it, it is still too much solvent,0909

you still have plenty of solvent to dissolve all of your compound so it is not supersaturated.0915

There is no reason for it to ever crystallize out.0920

That is why it is very important to add just a minimum amount of solvent in that first step of the hot solvent.0923

What you can do at this point?0928

All is not lost, you just need to boil off some of the solvent.0929

We have to get rid of some of the solvent, bring it back to a supersaturated situation.0934

Maybe pour it in a beaker, so it can boil off more rapidly.0939

Stick it in a hot plate in the hood and put it in the beaker so that you can evaporate it.0948

If you still have it in your Erlenmeyer, it is not going to evaporate as quickly because you are going to get a lot of reflux.0953

Just boil off some of the solvent back down to a lower volume.0958

You might even see it getting turbid.0961

You might see that it starts to get cloudy because your compound started to precipitate out, come out of solution.0964

Then, you can add back in a little more solvent.0970

You are right at the borderline there.0973

Adding just a little bit more solvent to make a nice clear solution.0974

Now try again, stir it and cool it, and you very likely have much better success there0978

because now it will be supersaturated, once you cool it.0982

Just a few parting words of advice.0988

Do not forget, any time we are boiling liquids, we should always have a boiling chip or boiling stick in there,0990

so we do not have any splashing or bumping of superheated solutions.0996

You are going to be boiling some solvent.1002

You should have a boiling chip in there.1004

You are going to be having your recrystallization flask, that is going to have boiling liquid too.1006

Both of those should have boiling chips in there.1011

Sometimes we have colored impurities.1013

If you know that your sample is colorless and you should have a white solid, a white solution,1015

a colorless solution, when it dissolves in the solvent, sometimes you might see little color.1023

It is a little yellow or something like that.1027

Those highly colored impurities are very good at being removed, by using activated charcoal.1030

Charcoal is just a really fine black powder with a lot of surface area, a lot of pores.1037

If you add that to your solution and treat it with the activated charcoal,1044

that colored compounds are very often removed that way.1048

You want to be careful, you do not want to add the activated charcoal to a solution that is boiling or near boiling.1052

You never want to add a solid to something that is very hot,1058

because all of a sudden that allows a lot of liquid to convert to the vapor phase and1061

it can come up out of your solution and splash all over the place.1068

Make sure it is cold, before you add in your charcoal.1072

And now you have added charcoal and now you have to get rid of the charcoal also.1076

That is going to require a hot filtration again, with one of those stemless funnels.1080

You are going to have to filter off the charcoal.1085

Now we will have a colorless solution like we were meant to have.1091

Now we can begin the cooling process.1095

That is another step, not an automatic, only if you have a colored impurity that you want to try and get rid off that way.1097

Once again, anytime you are doing a hot filtration, you need to add a little more solvent on1105

so that you minimize the amount that precipitates out, as you cool it.1110

We will always lose a lot more compound, if we have to do any extra step we do in a crystallization.1114

Sometimes we use solvent pairs.1121

Instead of just using straight methanol, we might use a mixture of methanol or methyl chloride, I cannot even recall.1123

Some solvent that are common are ethanol and water, something like that.1133

That is because that solvent combination is something that is more polar and something that is less polar,1139

ends up being the ideal solvent system for your particular compound.1144

The danger here is that, sometimes what you do is you heat it in one solvent,1152

maybe you heat it with the polar solvent and then1160

you add in very small amounts of the non-polar solvent until it starts to get a little cloudy.1162

What that non-polar solvent does is it is lowering the solubility.1168

It improves your recrystallization.1173

That is one of the reasons you might use a mixed solvent.1174

The danger here is that the longer you heat it, even though you are using an Erlenmeyer flask1178

which will minimize the amount of solvent vapors that evaporate,1182

those two solvents are not evaporating at equal rate because they have different boiling points.1187

As time goes on, the longer you tinker around with your crystallization, your solvent composition is varying.1192

You may have started out with a 90-10 mixture of two solvents,1198

or 1 to 1 mixture of these two solvents, 50 minutes later, it might be 75-25.1202

You need to work a little more quickly and keep that in mind, when you are doing it.1207

Another problem when you have issues with polarity,1212

if you use a solvent that is too polar or solvent that is not polar enough.1215

All of the sudden your sample was trying to dissolve or was dissolved, and all of the sudden says,1219

I cannot be dissolved in this solvent anymore.1225

It is not the right polarity.1227

Instead of crystallizing out, it can crash out as an oil.1229

Especially, because it is hot sometimes, it is going to be melted.1234

Sometimes you get a big mess, that is called oiling out.1238

If you ever observe that in your crystallization, one goal is to maybe add some more solvent.1240

Maybe you just did not have enough solvent.1248

Adding more might help re-dissolve the oil.1251

But worse comes to worse, if you have a total mismatch with your solvent and it is just making a huge mess,1253

what you can just do is just cool your solution down and then your oil.1259

Once it cools, it will solidify because your compound is a solid.1266

It will solidify as an oil or kind of an oily solid, it might be kind of messy.1270

But then, you can take that solid, put it in a clean Erlenmeyer and start again.1274

You can try to recrystallize.1279

Try a different solvent at this point because very often, this oiling out happens when you have chosen the wrong solvent.1282

Or your solvent mixture has changed and it ends up being the wrong solvent.1292

Few things that can go wrong with your crystallization but hopefully you have some successful ones.1298

It can be really rewarding experiment, where you start out with some dingy looking, ugly, powdery crystals.1303

In the end you have some beautifully formed, bright shiny, white crystals.1310

That could be a very rewarding experience.1315

Good luck to you in your recrystallization.1317