How to Purify by Recrystallization

Uploaded by NurdRage on 05.12.2009

Greetings fellow nerds.
In an upcoming video we're going to make glow in the dark phosphorescent powder from the
chemicals we've been making in previous videos. However, the aluminum and strontium nitrates
must be extremely pure. Impurities will interfere with the phosphorescent process. So in this
video, we'll use the technique of recrystallization to purify them from 90% purity up to 99.9%.
Recrystallization works because crystals tend to be purer than the solutions they came from.
As long as you discard the solution before the impurities also crystallize out, you'll
have purer crystals of your product. Repeating the process further increases purity. The
downside is that you lose alot of your product as well. In addition, this works best if your
product is also reasonably pure. If you have more contaminants than product the contaminants
will crystallize before your product does.
So first let's start with aluminum nitrate. As you can see here it’s not perfectly white
as it should be, so we'll first need a pre-filtering step. Just enough water to completely dissolve
it. As you can see the aluminum nitrate actually has lots of brown particles in it. Simply
filter them out to get a clear liquid. Now we couldn't filter these out in the previous
video because when the solution is first made, the particles are too fine to be filtered
let alone be seen by eye. The solution must first be crystallized once to cause the particles
to aggregate into something big enough that can be filtered.
Here it is after filtering, nice and clear. Now dry it out and use the desiccator bag
if needed.
Now you have clear aluminum nitrate nonahydrate. Now, we can go on to the actual recrystallization.
Here I’ve mixed a lot of batches of aluminum nitrate nonahydrate and already filtered and
dried it. Accurately weigh how much you have before starting. Here i have fifty grams of
aluminum nitrate nonahydrate.
now accurately add in 20% of it's mass in distilled water. So since I have fifty grams,
I’m adding in 10 mL of distilled water.
It won't all dissolve so heat it up until it does. It'll turn slightly yellow but this
is not a problem.
Ok once the solution is completely clear, cover it up and let it slowly cool down to
room temperature.
After a few hours of cooling you can see it has formed these very large crystals of aluminum
nitrate. The impurities are now concentrated in the liquid layer.
So now, pour off the liquid and break up the crystals.
There they are.
Now weigh them again.
Ok so the total weight now is about 33g. We're going to recrystalize it one more time for
even better purity. So once again add in 20% distill water, in this case it's 6.6mL and
again heat until it dissolves. And then allow to cool.
Pour off the impurities in the liquid.
Now you can dry the crystals. I recommend using the desiccator bag.
Now we have 18 grams of ultra pure aluminum nitrate nonahydrate. Although we lost 32 grams
during the two recrystallization steps our objective was best quality not greatest quantity,
Now we're going to purify strontium nitrate.
Get a wide beaker and place in a quantity of strontium nitrate that we already made
in a previous video. Add in just enough water to dissolve the strontium nitrate. It's very
slighly cloudy so filter it off to get a clear solution. Then this time, leave in air to
We can't use the heating and cooling method like aluminum nitrate because the solubility
of strontium nitrate doesn't change that much with temperature as with aluminum nitrate.
So we'll have to evaporate the slow way.
Evaporate it down until you have about 10 to 20% liquid left. I started with ten militers
so i evaporated down to about 2mL of liquid not including the crystals.
Get rid of the liquid.
And once again repeat the process of dissolving and crystallizing.
Once again I recommend using the desiccator bag to dry the final product.
Now we have ultra-pure strontium nitrate.
We are now ready to make glow in the dark phosphorescent powder in our next video.
So please subscribe, rate, and comment.
Note: Distilled water should be used for the dissolving steps. Using tap water won't improve
the purity as much as distilled water. Deionized water can be used but it won't be as good
as distilled water.