After that many useful organic reactions are requiring less than -10....-20°C, and i now have a little experience with non-dry-ice-cooling mixtures,
i dare to make a little report about these:
Generally, dissolving inorganic salt-hydrates will always cool down to a certain extent. My theory about this is because the crystal cage of the salt
is a system with a very low entropy. The driving force of the process(the destruction of the crystal-cage, which is endothermic) thus is the growing
of the entropy.
This principle also explains why salts melt ice on contact, the vapor-pressure of the ice is higher than the ones of the salts, water-molecules are
able to get on the surface of the salt-crystals, where the abovementioned process takes place.
1)The most simple cooling mixture is probably NaCl + crushed ice(-20°C).
It has to be mentioned that the parts of the ice are ideally not bigger than 0.5-1cm diameter, or a significant decrease of cooling-performance has to
be accepted.
It is also important to weigh the mixing-portions of ice and salt, although a 50:50 ratio might be a good approximation.
The mixing-ratios can be found in various books about preparative chemistry, although i think the solubility at 0°C can be taken as a basis for
calculation of the ratios.
2)The most cost-effective of the mixtures is the ammonium-nitrate-ice-bath. KAS 27-fertilizer is working well, although recrystallized ammonium
nitrate is much better because there are no insoluble additives. So far i have not found the mix tabulated, but i think at least about -25°C are
possible.
This one is a good choice for nitrations taking place at below +15°C, it allows an acceptable rate of addition of reactants.
A good idea is to take an aluminum container because of the good heat conduction...the work with thick-walled round-bottomed glass-flasks is
frustating.
3)One of the best(in my opinion) cooling mixtures is 66% H2SO4 with crushed ice (55:45). The temp in the bath sinks to -37°C.
I am not sure if the conc. of industrial waste-sulfuric acid was about 60% or if it was 30%("Dünnsäure"; Ger).
A big advantage is the low viscousity of the mix. It can be stirred with a bigger magnetic stirring bar for faster heat exchange.
4)Calcium chloride hexahydrate(not to mistake with dehydrated CaCl2, which is used in drying columns) with crushed ice.
Temp: -40°C
5)Quite expensive, but impressive is hydrated KOH with crushed ice.
Although i never tried that myself, -60°C are tabulated.BASF - 20-8-2003 at 07:53
BTW, i would still love to use dry ice, but so far i wasn´t able to find a cheap source of it. I was asking myself wether super-markets could be a
source, - sometimes dry ice is used in styrofoam-containers for transportation of (?) have to figure out....there is always something left.Nick F - 20-8-2003 at 08:45
I don't know about supermarkets, but if you have a college/university or something like that, you could try asking. I get mine from a medical
research place, it's used to keep stuff cool in transport. They even let me have the thick polystyrene box. This is good, because it allows me to
keep it for days rather than hours.Haggis - 20-8-2003 at 10:57
Around these parts in America, dry ice can be had for .99 cents per pound. They come in 2.5 pound increments if my memory serves right and most have
a bag that is already open so you can take what you need. They also don't care much if you open a bag and take what you need.BASF - 22-8-2003 at 05:28
I was hoping this would be the case...so, Haggis, this sounds like being quite uncomplicated and cost-effective...
Are these suppliers of dry-ice generally the same companies selling gases?
(maybe european "Linde"-suppliers??)
[Edited on 22-8-2003 by BASF]Mongo Blongo - 3-9-2003 at 07:10
BASF- Do you have any quantitive information on that KOH/ice reaction? Sounds great for freezing HNO3.Haggis - 3-9-2003 at 07:38
Ol' buddy Haggis is back with a bit 'o info for you...
http://www.dryiceinfo.com is another decent resource with some general information. On their selling page however, I was blankly skimming for
useful info when I saw they sold to "Mad Science". Quick exploration proves that it is a demonstration/educational company. Their logo is
the same as seen on that sticker that (IIRC) NERV found in his school. http://www.madscience.org I thought it was interesting because it all comes together oddly. BASF- I believe that gas companies often sell dry
ice as well, but the price may be steeper.
Edit: I really want one of the lab coats that they have!
[Edited on 3-9-2003 by Haggis]
[Edited on 3-9-2003 by Haggis]madscientist - 11-9-2003 at 12:40
On the off-topic subject of impressive cooling mixtures involving dry ice how
about a slurry of crushed dry ice in liquid propane (allowing the propane to evaporate, of course, as that would contribute significantly to the
mixture's ability to remove heat)?BASF - 13-9-2003 at 07:38
Thanks for the link...it turned out Linde is really selling dry-ice.
Let´s get ready for a trip to Vienna....Arrgh
My only hope is they also deliver it per post, ideally in cheap styrofoam-boxes...BASF - 16-9-2003 at 06:22
@mongo blongo
I have found the mixing-proportions of the KOH-ice mix here, along with another interesting one: etOH + ice
Thanks BASF. I tried NaOH thinking I might get similar results to KOH but I only got down to -25 deg.
Anyway about the KOH, I take it that only hydrated KOH will work so I am not sure how to interpret the mixing proportions from your link.
Do you think it means 310 g of anhydrous KOH is to be hydrated then mixed with the ice or 310 g of hydrated KOH solution is to be mixed with the ice?
Also does anyone know how much water is needed to "only just" hydrate KOH?
I will electrolyze some KCl solution soon to make me some KOH or maybe I will buy it as pH up from a hydroponics shop and give it a try.BASF - 17-9-2003 at 04:10
woops. I don´t know it myself...
I know that KOH is used (very effectively) as a drying agent, so it definitely has a high affinity for water...
...so, according to this info, the needed hydrate would most likely be KOH*1 1/2 water.
Hope i could help.Mongo Blongo - 19-9-2003 at 05:05
Thanks dude. I think I will add water to the anhydrous KOH until no more heat is evolved. I will post results when I get some KOH. If all goes well it
would be great for freezing HNO3 BASF - 19-9-2003 at 07:25
Maybe mixing by weight is better, because the second hydration-step might not yield so much heat, but this is just a suggestion.Mongo Blongo - 20-10-2003 at 10:40
Well the KOH/ice bath only went down to -30 deg after performing many experements with different concentrations/ice particle sizes. Shit!BASF - 21-10-2003 at 04:57
Maybe the bath was too small.
It makes the hell freeze, but it has, on the other side a low cooling-capacity, i suggest.
Maybe the room-temp was too high.
Maybe the insulation was bad.
Maybe the mixing was bad.
Maybe your KOH was not fully hydrated(use ice-water).
Maybe your KOH was hydrated, but it was too wet...
Maybe...
It´s always difficult, but -30°C are not bad tough.
[Edited on 21-10-2003 by BASF]
[Edited on 22-10-2003 by BASF]vulture - 8-2-2004 at 12:07
Try cooling the solid KOH to 0C before adding it to the ice.
Make sure it's fresh KOH, because K2CO3 is no good.
Also, gas expansion could be a feasible method for cooling down.
Deodorant spray get's stuff down to -20C in a matter of seconds. Pity that it's rather flammable and always comes with a smell.
keine Ursache, gern geschehn..... LOLMr. Wizard - 8-2-2004 at 14:59
Once I used lye NaOH pellets to clean the rust and crud out of a car radiator. The water dissolving the lye got so hot it was steaming. KOH would do
the same thing I suspect. Maybe the cooling with ice could be enhanced by dissolving the KOH in water to saturation and then cooling it in a cool down
bath, before mixing it with crushed ice?hodges - 8-2-2004 at 15:33
Quote:
Originally posted by Mr. Wizard
Once I used lye NaOH pellets to clean the rust and crud out of a car radiator. The water dissolving the lye got so hot it was steaming. KOH would do
the same thing I suspect. Maybe the cooling with ice could be enhanced by dissolving the KOH in water to saturation and then cooling it in a cool down
bath, before mixing it with crushed ice?
A bit off topic I guess but an interesting thing I found involved putting dry ice (not regular ice) in a strong NaOH solution. The CO2 neutralizes
the NaOH, giving up much heat in the process. So (if strong enough) the solution actually gets hotter, not colder. This of course makes the dry ice
react even faster, kind of a runaway reaction.
Hodgesvulture - 9-2-2004 at 07:16
The water dissolving the lye got so hot it was steaming. KOH would do the same thing I suspect. Maybe the cooling with ice could be enhanced by
dissolving the KOH in water to saturation and then cooling it in a cool down bath, before mixing it with crushed ice?
Quite on the contrary. The KOH has to be added to ice, because it's the energy needed to keep the water liquid at -60C that cools the mixture
down.
Fusion enthalpy of H2O....Mr. Wizard - 9-2-2004 at 10:03
So the cooling takes place between the solid ice and the solid KOH pellets? The equilibrium between the KOH solution and the solid ice isn't a
factor? I guess I don't understand how the water is kept liquid at -60C. My error was thinking the solution of KOH with it's depressed
freezing point was in equilibrium with the ice liquid interface.
how
about a slurry of crushed dry ice in liquid propane (allowing the propane to evaporate, of course, as that would contribute significantly to the
mixture's ability to remove heat)?
