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Author: Subject: Freezing point depression by impurities?
Fyndium
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[*] posted on 27-3-2021 at 00:28
Freezing point depression by impurities?


How big of an effect does impurities have on freezing point? Can even minor contamination of solvents decrease freezing point significantly?
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unionised
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[*] posted on 27-3-2021 at 04:35


It depends on the solvent.
https://en.wikipedia.org/wiki/Cryoscopic_constant
but the effect is usually quite small.
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Fyndium
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[*] posted on 27-3-2021 at 04:58


Strange. I got a compound that should freeze at approx -17, but it remains crystal clear at -30C and no agitation has effect on it. Every test performed on it indicates that it is what it's supposed to be, but this is out of range.
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metalresearcher
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[*] posted on 27-3-2021 at 05:02


Quote: Originally posted by Fyndium  
Strange. I got a compound that should freeze at approx -17, but it remains crystal clear at -30C and no agitation has effect on it. Every test performed on it indicates that it is what it's supposed to be, but this is out of range.


That is called Supercooling. Recently I evacuated a flask with clean (tap-)water of 14 C and it remained liquid until -4 C and then it froze.
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Fyndium
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[*] posted on 27-3-2021 at 05:59


That's why I noted the agitation, I shaked the bottle, but nothing happened. This always freezes any liquid that's supercooled.
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[*] posted on 27-3-2021 at 06:07


It's not guaranteed- especially if the liquid is viscous.
The other thing that often works is scratching the inside of the container with a glass rod.
Until you actually have the solid and liquid together, you don't know what the freezing point is.
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CharlieA
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[*] posted on 27-3-2021 at 15:51


Quote: Originally posted by unionised  
It depends on the solvent.
https://en.wikipedia.org/wiki/Cryoscopic_constant
but the effect is usually quite small.


The interesting fact here is that the depression of the freezing point can be used to estimate the equivalent mass of the solute...just another factoid from the ancient era of "wet" chemistry.:)
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[*] posted on 28-3-2021 at 04:07


Yup! I remember using the Melting point depression concept to estimate the molecular Wt. of an unknown.

But, when we talk about "freezing point", we are also talking about crystallization.

Crystallization is tricky. It can take weeks, months, years, for crystals to form.

Are you sure you have the pure stuff, free of solvents and interfering crud?

Maybe refusal to crystallize (or solidify), is telling you something about the purity of your product.
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Fyndium
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[*] posted on 29-3-2021 at 00:43


It's not 99% pure, it very likely contains at least a little of the lower fraction as I did not use fractionating column when I vacuum distilled it. I tried to use it in a reaction and it seems to indicate that it is over 90% at least.

I might do a fractional distillation to it just because I want to know how much of what is in it, and I want to get it crystallize at given temp.

On the other hand, in the topic of benzaldehyde, I remember reading two freezing points, one was -56C and other was -26C. I got the impure one partially crystallize, and the distilled one remained liquid. So I suppose one does not simply freeze all kind of stuff.
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