Freezing Points Of Gelatine And Other Additives In Water

[kelticwizard]

I just bought some icewater bottles for my knees after exercising. I use two per knee. I used to use 1 quart plastic food storage bags, three bags inside each other and tied off with twine to prevent leaks, but it became too cumbersome.

The icewater bags have a screwcap which only allows you to put one ice cube in at a time. This takes time for four icebags, two ice trays per bag-I like my icewater cold.

The various wraps and bags with blue glop they sell for this purpose I find unsatisfactory. Only ice water does the trick for me.

The icebags are fabric treated with an internal coating of latex.

There are trays which make smaller icecubes, as well as icemakers which make small cubes, but I decided to try something different.

Instead of adding icecubes, I decided to try adding some kind of additive to the water in the bags to prevent freezing and keep them in the freezer, removing them only when treating my knees. I realize that this will likely make the bags too cold to contact the skin, but I figure dunking them in a 10 gallon bucket of room temperature water for a few minutes will warm them up to the point they will be cold, but suitable for use on the skin.

I was considering salt as an additive, but I read somewhere that latex reacts badly with salt. I was considering ethylene or propylene glycol, but it also said that latex does not like solvents or petroleum products or much of anything else, apparently. I don't know if the glycols are either of these, but I suspect they might be.

The icebag manufacturer is little help, they tell you to put water or ice and nothing else in there.

Researching on the net, I read that sugar and gelatin dissolve in water but are non-ionizing, which I presume means non corrosive as well. I mean, acids and alkalis and salts of same are highly ionizing, and latex doesn't like those, so latex probably doesn't mind nonionized solvents, right?

So I got some sugar and pure, nonsweetend gelatin, and I wonder if anybody can tell me how much gelatin or how much sugar I should use in four quarts of water to prevent freezing when the freezer goes down to plus 15 deg Fahrenheit. I am leaning toward gelatin because latex is the sap of a plant and I suspect there might be some form of sugar in it which might react with the table sugar I am putting in the water. After all, there's plenty of sugar in the sap of a maple tree, why not at least a little sugar in the sap of a rubber plant?

Anybody with knowledge or experience in the chemical field or working with these materials?

Thanks.

[SY]

Ethanol will do nicely. Salt will do even better; latex should be fine with it.

[kelticwizard]

Thank you. I think I will try salt, then.

Any idea how many teaspoons, or ounces, or milligrams of salt should be used to prevent freezing down to plus 15 deg Fahrenheit for four quarts of water?

[poobah]

Keep in mind that frozen water is able to absorb much more heat than salt water at a somewhat lower lower temperature. The "heat of fusion" is a big factor:

It takes 335,000 Joules to melt one kg of ice (no change in temperature).

It takes 4216 Joules to heat that same water 1 degree C.

[SY]

Well, for water, k = 1.8 degrees C/mol. Salt gives two moles of ions for each mole of salt. You want to depress the freezing point about 17 degrees F which is about 9.5 degrees C.

FPD = m * k. Rearranging, m = FPD/k = 9.5/1.8 = 5.3 moles/liter. You've got about 4 liters, so you need about 22 moles of ions, which corresponds to 11 moles of salt. The molecular weight of salt is about 58 g/mole, so the mass needed is 58 * 11 = 638 g.

[kelticwizard]

Thanks, Sy. So you are saying that it takes about 23 ounces of salt, approximately. That is good to know.

[poobah]

So are you saying that 1.16 kg of saltwalter at 17F will suck up the same heat as 1 kg of ice (32F)... assume they both end as liquid at 32F?

I had no idea that salt water had such a different heat capacity.

Hey, what is that gooey blue stuff the "freezey packs" are filled with?

[kelticwizard]

Originally posted by poobah
Keep in mind that frozen water is able to absorb much more heat than salt water at a somewhat lower lower temperature. The "heat of fusion" is a big factor:

It takes 335,000 Joules to melt one kg of ice (no change in temperature).

It takes 4216 Joules to heat that same water 1 degree C.

Poobah:

So you are saying that if I put two pounds of ice in cold water, and one pound of it melts while I have it on my knee, I will be drawing many times more heat out of the knee, (blood and tissues) than if I put a similar amount of cold salt water at the same temp on the knee?

Or, in other words, "stick with the ice water"?

[poobah]

Yes Keltic... It takes alot of heat to change ice at 32F to water at 32F.

But SY is a chemy, I am a dummy... I am interested in the saltwater thing too.

[kelticwizard]

Poobah:

Wait. Are you saying that the saltwater draws the same amount of heat out of the knee as pure ice in pure water after all, because of the different characteristics of saltwater from ice water?

Keeping everything liquid is more convenient, but if it is better for the knee to add ice to pure water, I can do that. There are ways to get smaller ice cubes that pour into the icewater bag.