math
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Freezing-point depression practical question
Hello,
I'm interested into a practical application of freezing-point depression.
I'd like to know which substance could be dissolved in water to depress water's freezing point significatively (though not excessively, something
around -25°C would be more than ideal, also any input between -5°C and -20°C is welcome) yet still make it drinkable (and preserve its hydrating
functions on the body, so water shouldn't be displaced in large amounts), so preferably no sodium chloride or ethanol.
Since the formula of freezing-point depression is[1]:
ΔTF = KF · b · i
ΔTF, the freezing point depression, is defined as TF (pure solvent) - TF (solution).
KF, the cryoscopic constant, which is dependent on the properties of the solvent, not the solute. Note: When conducting experiments, a higher KF
value makes it easier to observe larger drops in the freezing point. For water, KF = 1.853 C°·kg/mol.[4]
b is the molality (mol solute per kg of solvent)
i is the van 't Hoff factor (number of ion particles per individual molecule of solute, e.g. i = 2 for NaCl, 3 for BaCl2).
I think that a low molecular weight substance lowers the freezing point more efficiently than a higher mol. weight one simply because there are more
molecules in a given dissolved weight.
About edibility (or perhaps more appropriately, given the performance/danger I'm looking after, exceedingly low toxicity), I was thinking about either
sugar alcohols or short-chained aminoacids categories, which should fulfill their depression effectiveness due to low mol. weight and still have a low
enough toxicity.
At first I thought at glycerol, though a 53% solution (47% water) is needed to achieve a -26°C freezing point[2], coupled with its human (oral) TDLo
of 1428 mg/kg makes it improposable or too risky to use it as means of hydratation since it'd only need a cupful of that drink to reach the TDLo
amount, preventing any safe and meaningful hydratation.
Even a mellower -5°C f.p. depression would involve a 25/75 glycerol/water mixture but that TDLo amount would still be achieved easily in a
drink-sized amount.
Since some of you are very knowledgeable and may even remember freezing-point depression values at a glance, I'd like to listen to your solutions. Any
input is appreciated  Thank you
[1] http://en.wikipedia.org/wiki/Freezing-point_depression
[2] http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0032/...
[3] http://datasheets.scbt.com/sc-29095.pdf
[Edited on 13-8-2013 by math]
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ElizabethGreene
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One other consideration: Extraordinarily cold beverages are painful to drink. I've had Gin at -18F/-27 c and got a near-instant "ice-cream
headache".
Anecdote: This experience led me to measure the temperature of my freezer and discover that the thermostat had failed in a stuck-turned-on state.
According to the Appliance parts shop, this seems to be a recurring problem with models of my manufacturer and vintage.
Elizabeth
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Oscilllator
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I'm afraid this may well prove to be impossible. Is the concentration of solute in the water is greater than the concentration of solute in the blood,
then you will be dehydrated instead of rehydrated. This effect can be seen when drinking seawater. Since the amount of solute you have to add to
achieve this effect is relatively small, I doubt it could depress the freezing point of water to anything more than a few degrees below zero.
One other possible route is to add a relatively non-toxic liquid, e.g. ethanol. I dont think its necessary to go all the way to 40% like in gin to
acheive significant depression. Perhaps something like beer would achieve enough depression. After all, it is quite possible to drink vast quantities of beer... 
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Fantasma4500
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dont know if sugar would help at all, because ive had 3 x 1.5L soda that started freezing from the top to the bottom in my hands..
actually thats a neat way to make slush ice
anyhow the list of what you can fill in is quite narrow
there would be CO2, sugar, citric acid, citrate salts (with care, magnesium citrate is a laxative), calcium chloride which is used for jalapeno's
aswell as an antifreeze agent for lower than -20*C but ofcourse amounts will count
i think its quite limited really, whats the purpose anyways? you can somewhat heatproof stuff by wrapping i suppose alot of saran wrap around, in
which would form random airpockets and act as styrofoam
or try to find some of that styrofoam film
perhaps some kind of organic would help, but again its limited of what you can ingest (not really, but considering that you dont want to die right
after)
looking through E numbers might help..? although acetone peroxide has its own E-number 
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math
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Thanks for your reply.
Its purpose would be to prevent freezing of drinkable water in sub-zero weather, thus taking less hassle (but still some time to let water warm in
one's own mouth or by other means) than having to defrost a solid frozen bottle, specially if you'd want to avoid fires or don't have that many other
heating options.
A 10% ethanol solution (by volume) in water freezes at -4°C, while a 20% one freezes at -9°C.
Both could be valid solutions if nothing better comes up, though if for example lowering the ethanol percentage to 5% and adding sugar, whey protein
or something else mixed together to further lower the solution's f.p. I'd obviously prefer the latter.
Edit: a consideration worth noting on using gases to lower f.p.: it'd work until you release the pressure build-up. That's also a known trick to
"instantly" freeze CO2 beverages. You cool them subzero but still above their CO2-water f.p. and then when you'll open it the water will freeze
because the CO2 will have escaped thus increasing the f.p.
[Edited on 16-8-2013 by math]
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macckone
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Propylene glycol is probably the best substance from a FP depression standpoint but not from a safety standpoint.
Glycerine is better from a safety standpoint.
In either case a solution with a FP of -35 is achievable but will not be hydrating. Most salt solutions are simply dehydrating.
There is no practical substance that will do what you are interested in.
Various sugar and salt solutions will lower the FP slightly without causing dehydration but not to the degree you are looking for. Your best bet is
diluted sports drink. Out of the bottle they usually aren't that good at hydration but when diluted they are excellent. They also have the
interesting property in that they don't freeze solid but create a slush allowing them to be removed from a container and warmed up easier than ice
which tends to freeze as a block.
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smaerd
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Math are you sure it is due to escaped CO2? Or does the physical motion of the bubbles induce crystallization in an other-wise non-spontaneous
process?
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subsecret
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May I ask why you want to drink something really cold?
Fear is what you get when caution wasn't enough.
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math
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Quote: Originally posted by smaerd  | | Math are you sure it is due to escaped CO2? Or does the physical motion of the bubbles induce crystallization in an other-wise non-spontaneous
process? |
I've always thought it was the right explanation and it made sense for me, specially if you consider that about 3.25g of CO2 are soluble in 1 liter of
water (at 0°C) and that given the f.p. depression of carbonated water it could contain even more at say just slightly above its f.p.
When you open it, you release pressure, a volume of CO2 capable of filling the bottle alone is released and its expansion further cools water and
increases its f.p. at the same time
I won't like to drink something that cold, but as previously explained it'd always be a better solution than dealing with a solid-frozen bottle and if
the need arises, I'd like to have a solution prepared beforehand
Thank you
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