Using saltpetre for cooling drinks (2013) | Hacker News Reader

Using saltpetre for cooling drinks (2013)

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50 pointsd1322 comments

I'd love to try this myself, but where I currently live - despite being an ingredient in a few traditional cured meat dishes - nitrate salts require a virtually unobtainable (to a private person) permit to obtain, because of terrorism concerns.

FWIW the dissolution of table salt in water is also endothermic, though to a much lower extent than saltpeter: the enthalpy of solution for saltpeter is +34.9kJ/mol, for table salt it's +3.87.

An other option is ammonium nitrate at +25.69, though you might also have trouble with terrorism regulations as it's both a common fertiliser and a component for explosives.

I often will use salt in ice baths specifically for that reason :)

Ammonium nitrate used to be uses in the "instant ice/cold packs" for first aid, but has since been replaced with urea - I believe due to the terrorism regulations.

Which means (as another poster suggested) I'll be trying urea as a way to chill drinks sometime soon I guess.

I think the reason for saline cooling baths is that the liquid stays liquid at lower temperature, not any additional cooling from dissolving the salt.

> where I currently live

Not surprised by your story, but still curious where you live? In the US, you can buy it in bulk quantity on Amazon. Mixing it with sugar was a ton of fun when I was a kid, smoke bombs that could totally white out a large backyard.

A Western European country.

I could easily order it from overseas online, but the consequences are pretty harsh (violates the explosives laws). Not worth it tbh.

Somewhere else I lived in Eastern Europe, it was available in shops in 100g bags for curing meat, etc, or at the garden shop in 25kg sacks as a fertiliser.

You can get it at the hardware store, it'll break down tree stumps pretty quickly (and very inexpensively).

Yep, any place that has garden supplies generally has it. That’s where I got it when I was a kid anyway, as that was years before Amazon’s existence.

Urea also is endothermic, and as it is used in diesel engines I am quite sure it's available globally.

Aye, urea is what's in the "instant ice packs" now.

It used to be ammonium nitrate, but most brands switched it to urea a few years ago - presumably for reasons of "people could use the extremely nice, uncoated, very pure ammonium nitrate for nefarious purposes" (useful for explosives and methamphetamine manufacturing!)

The instant cold packs commonly found in first-aid kits use a closely related chemical: ammonium nitrate. While technically a high explosive, it is effectively useless as such in the hands of the average person.

Technically incorrect: ammonium nitrate on its own is not an explosive.

Actually, it is you that are incorrect. Ammonium nitrate will detonate on its own, as will virtually all ammonium oxidizer salts. There was a spectacular existence proof of this in Beirut only two years ago.

There are two reasons ammonium nitrate is often mixed with other chemicals as an explosive. First, improving stoichiometric efficiency since it is excessively oxygen-rich in detonation, usually by adding hydrocarbon (like ANFO) or metal fuels to consume the excess oxygen. Second, improving its detonation sensitivity, since it is extremely insensitive and tends not to detonate to completion except under the most controlled conditions, which is wasteful. In a professional composition, there are single common additives that can address both of these issues.

If you are using a composition like ANFO that doesn't address the sensitivity issue, then you'll need strong boosters (like PETN) to achieve a good explosive yield. If you have ready access to strong boosters then ammonium nitrate is weak tea as an explosive. We use ammonium nitrate because it cheap, not because it is good, and part of that means wringing what efficiency can be cheaply obtained from it as an explosive.

But somewhat useful as a fertilizer.

AFAIK, one type of saltpetre (potassium nitrate) was also widely used to preserve cured meats. Today we tend to use nitrites to avoid the spoiling of bacon, sausages, salami, speck, jamon and other cured meats. These became more used in late 19th century, known as Chilean saltpetre.

The nitrites and nitrates is what gives cured meats their pink colour and are considered cancer inducing.

Kind of long winded but a nice article. I had no idea saltpeter was used for cooling.

I found East India Company and Bengal saltpetre link to be an intriguing one. That is the first time I have heard about it.

For those looking to skip the history lesson:

> Saltpetre was most often used to cool wines during the Regency, by cooling the water in which the wine bottles were immersed. A large wooden tub, preferably of a cylindrical shape or, better still, wider at the top than at the bottom, was the ideal shape of a vessel to be used for cooling with saltpetre. This cooling tub should be lined with sheet lead or zinc and should also have a close fitting lid which would exclude as much of the warmer ambient air as possible. The thicker the surrounding wood, the better the cooling mixture would be insulated. A cooling tub with a capacity of ten to twelve gallons should be filled with four or five gallons of water. The cooler the water, the better, so water just pumped or drawn from a well would be most effective, since the water temperature would be about 75º Fahrenheit. Five to seven pounds of saltpetre should be pulverized to the finest powder possible. This finely-powdered saltpetre should be slowly sprinkled into the water and allowed to dissolve. Within about fifteen minutes the temperature of the water would drop twenty-five to thirty degrees, within a half hour the temperature would drop another four or five degrees. At that point, the temperature of the water would remain steady for over two hours, so long as the lid was kept on the tub as much as possible. After that, the water would begin to warm at a rate of about three or four degrees per hour, unless more powdered saltpetre was added to the water.

And for the why it works:

> Saltpetre cools water by producing an endothermic reaction. This is a chemical reaction whereby, as it dissolves, the saltpetre literally pulls the heat out of the water as part of that process, thus lowering the temperature of the water. For this reason, there is a limit to how cool the water can become. Once it has become fully saturated with saltpetre, the water is not able to absorb any more.

Why is dissolving potassium nitrate in water an endothermic process ?

https://socratic.org/questions/5422ba97581e2a47cb1a8acf

"Dissolving potassium nitrate in water is an endothermic process because the hydration of the ions when the crystal dissolves does not provide as much energy as is needed to break up the lattice."

Huh, which explains why it's reusable, dry it out, grind it up and you can use it again.

Nowhere mentioned in this article, but wasn't saltpetre given to prisoners to reduce "sexual urges" within prisons? Or is this an old wives tale?

I heard a similar urban legend about it being added to army rations. People said it was added (in the 90s) to cigarettes to prevent them from going out, and that smoking could make you impotent (which I never saw actual evidence of)

This is a very long lived urban legend.

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