Recrystallization and Melting Point Analysis (2023)

Introduction

Now that we have covered some important separation techniques, let's take a look at a purification technique. Sometimes a chemical reaction will produce a solid, which can precipitate from solution. But these crystals typically contain impurities from solution that were trapped in the lattice as it formed. After we isolate these crystals, we can purify them via recrystallization. This is where we dissolve them in a particular hot solvent that will then retain any impurities before cooling it down such that it can crystallize again. We will also learn how to assess the purity of our recrystallized product using melting point analysis. This is neat stuff!

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For the past few tutorials we've been looking at some different separation techniques that we can perform in the laboratory.

These techniques separate the components of a mixture by taking advantage of differences in their physical and chemical properties.

Today we are going to look at another technique that is more of a purification technique.

Sometimes, when performing a chemical reaction, a solid will be produced.

We saw this occur when we performed an extraction and while working with the aqueous layer, we caused benzoic acid to precipitate.

We then separated this precipitate from the rest of the solution by using vacuum filtration.

However, we did not discuss the fact that, when a solid precipitates, it may incorporate a number of impurities from solution into the lattice structure in order to separate the product.

From these impurities or to purify the solid we will perform something called recrystallization.

Recrystallization is a technique that takes advantage of the difference in solubility between our desired product and any impurities that may be present within the way.

This works is that we will be taking our impure product and dissolving it once more in a hot solvent, so that it will dissociate and release the impurities.

The solution is, then cooled down which causes our Pure product to crystallize again, but this time the impurities remain dissolved in the solvent because of their enhanced solubility in that solvent over the product we are isolating.

The solid will initially dissolve because of the higher temperature, since solubility typically increases with temperature, but as it cools, solubility is reduced until it eventually crashes out of solution.

Once more this time, with negligible impurity to carry out a recrystallization, we will need a few materials.

First, we will need two beakers.

We will use a 250 milliliter and a 150 milliliter Beaker in this experiment, then we will need a hot plate with a magnetic stirrer or a Bunsen burner.

If a hot plate is not available, we will also need a stir bar stirring, Rod, a vacuum filtration setup, a powder funnel a watch, glass and disposable pipettes.

Lastly, we're going to need our product remember.

This is a technique that is typically applied in order to purify a solid product that has been collected at the end of a chemical reaction, in this case we're going to use impure benzoic acid, as though we are continuing the extraction We performed earlier in the series and for the solvent, we will use distilled water now.

Choosing a suitable recrystallization solvent is the most important part of the experiment.

We need our impurities to be soluble in our solvent at all temperatures.

They must dissolve as we heat things up and remain dissolved when we cool it back down.

However, the product must only be soluble at high temperatures.

It dissolves when we heat it up, but then precipitates again, as we cool it back down to find an adequate solvent.

We must look at tabulated solubility data and compare the solubilities of our product in different solvents at different temperatures.

You can find this data online, since many different compounds and solvents have been studied in this manner.

In this case, a quick search will tell us that benzoic acid has a very high solubility in boiling water 56.31 grams per liter, while it is very low in room temperature, water only 3.44 grams per liter.

This makes water a good choice for our recrystallization, solvent and specifically distilled water, as we do not want to introduce additional impurities from tap water.

We are ready to begin the recrystallization.

We will start by heating up 150 milliliters of pure distilled water in a 250 milliliter Beaker on a hot plate.

As that heats up.

Let's weigh our impure product, we're going to be using three grams of benzoic acid.

Today, let's add the powder to a clean dry, 150 milliliter Beaker.

Once the water is sufficiently heated, we're going to use a disposable pipette to add hot water to our product, based on the solubility of benzoic acid.

It looks like we should add around 100 milliliters of water, to our three grams of benzoic acid, to ensure that it will completely dissolve.

We can add a little bit of hot water at a time slowly until we can see that it has fully dissolved, add a stir bar to the solution and place it on the hot plate alongside the hot water Beaker, so that you can keep Heating and dissolving, as you add, more water.

While we do need the solid to dissolve completely, it is important to use the minimum amount of solvent that is possible during a recrystallization.

Do not add more solvent than is needed after everything is dissolved, remove the stirring rod and stir bar before you let the solution cool down to crystallize check for any impurities that did not dissolve.

As some impurities may be insoluble in your solvent at any temperature, you will have to filter them out using vacuum or gravity filtration.

Make sure you do this immediately before the solution cools down or you risk filtering away some of your product.

This is why we call this step hot filtration after you have made sure that your solution is clear: leave it at room temperature and cover it with a watch glass allowing it to cool down slowly.

The crystals should start to form relatively quickly and as more cooling occurs, more crystals will appear, be patient and do not move or shake the beaker.

While the cooling is happening to complete the recrystallization, you can move your Beaker to an ice bath or a fridge for five minutes, or so.

This is to ensure that all our product has precipitated.

The impurities are now in the liquid solution, and these lovely crystals represent our Pure product.

We're now going to separate the crystals from the liquid use, a stirring Rod to mix the solution a bit so that we can pour it into the filter.

We will be using our vacuum filtration setup, but gravity filtration is also possible.

Pour Your solution into the buchner funnel and the crystals will be collected.

It is important to pour more pure solvent over the crystals during filtration, as this will wash your product, meaning that it will remove the original solvent since it contains impurities- and we don't want any of those sitting on our pure crystals- we must wash them through the funnel after this.

Let the vacuum run for a few minutes to further dry.

Your product, your product, should now be almost dry.

You can leave it out to air dry at room temperature or put it in the oven.

You have now completed a recrystallization now the point of this technique was to get pure crystals.

The best way to test the purity of a solid is by measuring its melting point and comparing it to the original impure mixture, as well as the literature value for the product to perform melting point analysis.

We need a melting point tube and we need to insert a tiny sample of our solid.

We then need to use some mechanical force to get this sample to the bottom of the melting point tube.

Then we place it in the melting point apparatus turn it on and wait for the temperature to slowly approach.

The expected melting point range, as it nears begin to watch through the eyepiece.

As soon as you see the first signs of melting record, the temperature, then the moment you see that all of the sample is completely melted record the temperature.

Again, that is your melting point range.

The purified sample should give a melting point range.

That is much closer to the reported value when compared with the impure crude sample.

In addition, an impure solid will give a wide range during melting point analysis, meaning it will start and finish melting several degrees apart or more.

A highly pure, solid will have a very sharp melting point where all of it melts at a particular temperature very rapidly.

So the width of the range together with a comparison to the tabulated melting point will allow us to gauge the purity of the recrystallized product be sure to turn off the melting point apparatus.

When you are finished with it, we can also check the percent recovery of our sample by weighing the purified sample, whatever the mass of the crystals recovered.

We can divide this by the mass of the original sample and multiply by a hundred to get the percent recovery during recrystallization.

We always lose a percentage of our sample so expect for this value to be less than one hundred percent.

This is due to the impurities which have been removed from the lattice, as well as some of the pure product which is lost in the process.

There is one more thing to keep in mind: sometimes your crystals will refuse to grow, even if you use an ice bath to cool them down to a lower temperature.

This can be caused by the glass surface in the beaker being too smooth.

One solution is to use a glass rod to scrape your Beaker on the bottom and sides.

This will generate some loose glass particles upon which your crystals can grow.

Another good method to initiate crystallization is to add a few crystals of your pure product to the solution, so that other molecules can join an existing lattice.

This is called using a seed Crystal in some more advanced recrystallizations.

We may need to use multi-solvent systems in this case this first solvent dissolves the impure product, while the addition of a second solvent causes the pure product to gradually crystallize, but as that's a bit more advanced, this will be all for recrystallization.

Let's move on to some other techniques, thanks for watching subscribe to my channel for more tutorials support me on patreon, so I can keep making content and, as always, feel free to email me.

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