Sniffity
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Calcium Chloride Recrystallization - What just happened? (Long Story)
Hello,
I'd like to begin this post by saying: I have plenty of previous experience with recrystallization. I've recrystallized multiple compounds, following
many different recrystallizations techniques. However, I find myself completely stunned by what happened today. It's a long story. I've included as
much detail as possible:
I had received a big batch of CaCl2 from a certain laboratory. It was past its expiration date, so they gave it away having no use for it. My idea was
to recrystallize it to obtain a decent grade of purity.
So, I started out with a 1 L Erlenemeyer flask, filled it up to the 400 mL mark with a deionized water. I get this water from a trusted source, it's
the same I use in my previous recrystallizations.
Usually, I'll look for a solubility table for the compound, weigh out the maximum amount I can dissolve in hot solvent, and proceed to dissolve it.
However, my balance was broken, so I decided to dissolve visually until saturation.
I waited for the water to reach 100 C in temperature, and I started slowly adding CaCl2. Here's when the first odd thing happened: It felt as way,
way, way too much was dissolving. Theoretically, about 600g of CaCl2 should dissolve in 400g of water at 100C. The amount I added felt like way more
than 600g.
The water level reached a bit past the 1000 mL mark and it started looking like no more would dissolve.
I then proceeded to carry out a hot filtration of the solution, using two separate filtration flasks, since I didn't have a single one large enough to
hold the whole solution.
A usual, I pre-heated my flask and funnel to prevent recrystallization upon contact of the hot solution with the cold funnel/flask. Filtration
proceeded as normal, with a few impurities being caught in the filter paper.
I proceeded to heat both filtered solutions in their respective flasks to redissolve some of the CaCl2 that might've come out of solution as a result
of cooling during filtration.
I then transferred both filtered solutions to two separate 1L beakers to use as crystalizing dishes, wrapped them in aluminum foil and placed both of
them in the fridge overnight.
Here's when the second odd thing happened: In the morning, nothing had crystallized out of either beakers. I figured a nucleation site was needed, so
I proceeded to mix the contents of both beakers into one. Immediately upon doing this, a large amount of what looked like white powder precipitated.
My assumption is upon mixing the two solutions a nucleation site was created, which allowed for the fast recrystallization of said powder.
Now, since I lack a vacuum filtration system, I simply let the powder settle to the bottom of the beaker and decanted off as much of the water as
possible. I was left with a white, almost paste-like, powder.
I opted to transfer this to a paper towel, and started carefully transferring the powder from one paper towel to another and allowing the paper towels
to absorb the water. Obviously, this is not an optimal method, but I found myself short on materials I would otherwise use.
Anyhow, once I had absorbed the excess water, I decided to finish drying the powder by placing it inside a microwave oven. Here's where the
extremely odd thing happened: I placed the paper towel, with the powder, inside the microwave oven. I then turned the microwave on for 2
minutes. When I returned, I opened the oven to find that absolutely all the white powder was gone.
I was shocked by this. I figured I'd probably had a confusion and placed an empty paper towel inside the microwave, or something among those lines. It
wasn't that. I confirmed I had actually placed my product inside the microwave, heated it up, and it had suddenly disappeared.
Evaporated? No; the melting point of CaCl2 is over 700C.
Thermal decomposition? Unlikely in an ionic compound.
The dish was still slightly wet and the paper towel felt somewhat crunchy to my hand, but the amount of powder placed on it was way too much for it to
have sort of melted onto the towel (plus the melting point is too high). It felt like about 100-200g.
I still have the water solution from which I recovered this product, and I'm currently evaporating all the water in order to see what exactly I can
recover.
After asking my peers for their opinion on this, I'm considering two ideas:
1.- The white powder that crystallized out was actually an impurity, and the calcium chloride is still in solution.
2.- The solution was not yet saturated when I stopped dissolving CaCl2, and my CaCl2 is still in solution.
If you've got this far, I appreciate you taking the time to read this long, long story.
Any ideas would be deeply appreciated.
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Deathunter88
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First of all, most chemicals can be used well past their expiration date. Your calcium chloride is probably still very pure. I think that the crystals
disappeared because they dissolved in their own water of hydration. Yes, anhydrous calcium chloride melts at 700˚C, but the hydrates melt at lower
temperatures:
260°C for the monohydrate,
175°C for the dihydrate
45.5°C for the tetrahydrate
30°C hexahydrate
Your slush was likely a mixture of these, and when heated simply melted and got absorbed by the paper towels.
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j_sum1
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I agree with Deathhunter. CaCl2 is a beast to work with and the difference between crystal and slush is marginal at best.
Recrystallisation to purify is always going to be a PITA.
Another consideration -- not that I think it is a factor in your case but always something to be cautious with:
Al foil in the presence of chlorides (particularly hot chlorides) is prone to corrosion. It wouldn't take much to inadvertently add some AlCl3 to
your solution.
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aga
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CaCl2 will not crystallise from water in my experience.
It is so hygroscopic that it will deliquesce quite quickly in the open air.
When making it, i tend to pour the solution into a stainless steel dog bowl and boil it to dryness, break up the chunks, then dry some more in an oven
at the highest setting (~250C for my small oven).
Wiki says it is soluble in alchohols, so crystals might be possible, just not using water.
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gatosgr
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"felt like way more than 600g"
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DistractionGrating
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I'm quite inexperienced at most of the things discussed on this forum, but one thing I have done a fair amount of is recrystallizing CaCl2. The only
time I get a "white powder" is when there is an impurity, which I'm pretty sure is CaSO4. Otherwise, I tend to get largish clear crystals. I have
definitely had supersaturated CaCl2 solutions that rather suddenly start precipitating crystals once seeded: https://www.youtube.com/watch?v=MBI39y941GU Bear in mind that the crystals that will form at room temperature are CaCl2*6H2O, which has a melting
point of only 30C.
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