OBLONG
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Ice and salt bath to cool to -16°C
Hi, what's the ratio of ice to salt to get an ice bath down to -16°C? Sorry if this is a really dumb question. Thanks
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NedsHead
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https://en.wikipedia.org/wiki/Cooling_bath
Water and ice baths
A bath of ice and water will maintain a temperature 0 °C since the melting point of water is 0 °C. However, adding a salt such as sodium chloride
will lower the temperature through the property of melting-point depression. Although the exact temperature can be hard to control, the weight ratio
of salt to ice influences the temperature:
−10 °C can be achieved with a 1 to 2.5 ratio by weight of calcium chloride hexahydrate to ice.
−20 °C can be achieved with a 1 to 3 ratio by weight of sodium chloride to ice.
−40 °C can be achieved with a 1 to 0.8 ratio by weight of calcium chloride hexahydrate to ice.
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OBLONG
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Thank you 
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Ashot
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Or you can try a mix of water and isopropanol at ratio 60/40 to get a solution with -18c freezing point.
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Texium
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Thread Moved
18-9-2016 at 11:26 |
BromicAcid
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If you are going to try this, cool both reagents as low as possible, that includes putting the salt in the freezer to start out with. The ice should
be in chips, not chunks, it will slow down the process. At least those are pointers from using this mix to make iced cream.
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MrHomeScientist
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Something I've always wondered about : Does simply mixing calcium chloride and ice result in a mixture at -40 C? What would cause the drop in
temperature? I get that melting point depression makes the mix freeze at a lower temperature, but would you need supplemental cooling to actually
reach -40?
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gdflp
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Quote: Originally posted by MrHomeScientist  | | Something I've always wondered about : Does simply mixing calcium chloride and ice result in a mixture at -40 C? What would cause the drop in
temperature? I get that melting point depression makes the mix freeze at a lower temperature, but would you need supplemental cooling to actually
reach -40? |
A positive enthalpy of dissolution. The salts useful in ice salt baths are both very soluble in
cold water, and have a high enthalpy of dissoution. As the surface of the ice melts, the salt begins dissolving in this water, simultaneously
lowering the freezing point of the water and lowering the temperature of the ice bath. Once all of the salt is dissolved, the bath will begin warming
up to room temperature as a normal ice bath would. Note that with calcium chloride, only the hexahydrate salt has a positive enthalpy of dissolution,
using the anhydrous salt will instead result in hot water rather than an ice bath.
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MrHomeScientist
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Yes I think that's where some of my confusion came from - I always think of calcium chloride as being hot when it dissolves, but I only ever use the
anhydrous salt (Damp Rid). Good to know!
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