Panache
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Recrystallisation solvent for ice (frozen H2O)
Recently i got a -85 ultra-low freezer and it has had me thinking about what one could do. Not coming from a place where the temperature often gets
below 10C you don't really think about the cold ever.
So could one, in theory, recrystalise ice, say at -30-40C in a solvent, then drop the solvent temperature to -85C and get different or better ice
crystals?
I tried looking up the solubility of ice in various solvents at low temperatures but as expected i only found results that pertained to the solubility
of water in solvents at amibient or slightly sub ambient (~0C). I also got lots of hits (pun!!) relating to meth-amphetamine but typing
'recrystalisation and ice' into a search engine was bound to achieve that. I was literally swimming in results (ha another PUN!!)
I think it would be interesting to grow some ice crystals in say butane if it were possible. I should just try it.
So as to ward off confusion i mean solid H20, ice.
[Edited on 16-7-2008 by Panache]
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12AX7
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Butane isn't nearly polar enough. Try ammonia? 
I suppose the problem with most alcohols would be melting point depression; yields would be low (equivalent to recrystallizing a salt having a
relatively high solubility at low temperatures).
What kind of crystals are you expecting? Tabular, sheet, columnar, snowflake, etc.? (Snowflakes, you know, can only be grown under very specific
sublimation conditions.)
Tim
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not_important
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Ammonia and water form a eutectic, 1/3 ammonia mp 174 K. So either you'd freeze water-ice out of a water solution of ammonia, or the NH3-2H2O
eutectic out of ammonia solution.
Carbon dioxide and water form a clathrate CO2 - 7,5H2O that I think would lead to similar problems trying to crystallise water from liquid CO2.
A solution of water in ether might work. The mutual solubilities of ether and water decline with decreasing temperatures. Dropping the
temperature below freezing may result in water-ice crystallising out, although there's not a lot of water in solution.
Attachment: EXPERIMENTS IN THE NH3 – H2O SYSTEM.pdf (43kB)
This file has been downloaded 347 times
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Panache
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| Quote: | Originally posted by not_important[/i
A solution of water in ether might work. The mutual solubilities of ether and water decline with decreasing temperatures. Dropping the
temperature below freezing may result in water-ice crystallising out, although there's not a lot of water in solution. |
Well i have four ether/water solutions in the freezer now, made from sodium dried ether and Dh20, i will leave them in there for two weeks. I think
the scope of this is greater now (other than producing nice crystals), as a potential way to dry ether without using sodium. I have made some homemade
versions of the schlenck to seperate any ice from the ether. Perhaps i should also add a solution where water is a discrete small layer, as this will
readily freeze thus perhaps seeding the water in the ether to freeze out.
watch here for results soon.
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ScienceSquirrel
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Quite an effective way to make ice crystals is by sublimation.
Place some ice at one end of a closed sandwich box and place it in your deep freeze with the ice free end up against the cooling pipes.
The ice will sublime from one end of the box to the other.
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not_important
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| Quote: | Originally posted by ScienceSquirrel
Quite an effective way to make ice crystals is by sublimation. |
Quite true, in the winter this can be done out of doors keeping one end of the container completely in the shade. It even happens naturally, both at
the microscopic level within snow and occasionally on a large scale.
But that's water and air, crystallising from other solvents may give other crystal structures. Be interesting to see what Panache observes.
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ScienceSquirrel
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Solutions of ethanol in water or acetic acid will separate out ice on cooling. Eventually a eutectic will solidify.
http://en.wikipedia.org/wiki/Freeze_distillation
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Panache
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Update
Anyone have a neat photographic solution to the problem i'm having trying to photgraph these ice crystals. Basically they are in a 1L Schott reagant
bottle in solution at -85C. I whisk the bottle out, squirt some -70 MeOH on it to remove the frosting. I then have around 15secs before its refrosted.
The main problem is getting the camera to pic up the ice crystals inside, as presently each photo simply looks like a bottle of clear solvent with
insufficient contrast (is the correct parameter) to show the crystals. I want to try to record the crystals before i filter them in case filtering
them destroys them.
Second problem is assesing the remaining water content of the etheral solutions. Anyone got a method that doesn't involve expensive karl fischer
chemicals?
i hope i can get photo's that represent how pretty it looks, cos it purdy.
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not_important
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Try raking lighting, bright light from well off the camera's axis. Even better, try such lighting with a second light source from a different angle;
use colour filters on one or both or use one or more lasers (cheap pointer type with a simple lens). Play around with the lighting to get the best
positions for reflection from the crystal surfaces.
The difference isn't real large, ether 1.3525 vs ice 1.309, but it might be enough to pick out the crystal surfaces.
It also might be worth constructing a dry box type enclosure, just plastic sheeting on a frame, with well dried air providing positive pressure. This
would give you more time before the outside of the bottle frosts up.
Water content - perhaps adding a known weight of a salt such as MgSO4 to a measured volume of the solvent,letting it sit with occasional
stirring/shaking, filtering that off onto pre-weighed paper or fritted filter, and letting the ether on the salt evaporate in a desiccator, weight the
salt, then redry and weigh again.
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