Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Sodium acetate

  1. Oct 29, 2007 #1
    i am a total newbie here....i understand basic principles like supercooling, phase change, etc, but otherwise a total dummy, i have equivalent to 1st yr university in chem, but that was a long, long time ago. so go easy on me.

    i am trying to find out the difference btwn sodium acetate trihydrate (used in handwarmers), and anhydrous sodium acetate (used as de-icer). is it the same thing? the former seems to be kind of expensive, sold by the gram, the latter seems to be cheaper, sold by the ton. specifically, i guess i want to know, does the latter share the trait to be easily supercooled?

    the reason i ask, is i like the potential for sodium acetate trihydrate to "store" heat, and would like to do this in large quantities, but it seems too expensive to do this on practical terms, but anhydrous s.a. seems a lot cheaper, can it do the same?

    i also would like to know, am i naiive here to think that you can store the heat of summer to be busted out of some huge underground storage facility in the dead of winter? (i know that you would have to separate into small quantities or otherwise you use it all in one enormous reaction.) will it remain in it's supercooled state indefinitely, as long as you don't provide a nucleation point (physically disturbing it, contaminants, etc)?

    on a similar note, could one do a similar opposite thing with ammonium nitrate?
    i am an architecture student, by the way, obsessed with the idea of saving the world thru green architecture.
  2. jcsd
  3. Oct 31, 2007 #2
    From what I can tell, the two problems you'd face with using anhydrous sodium acetate are practicality and solubility.

    Even if you could use H2O to supercool the anhydrous form (it would be more difficult, as this form is less soluble in water -- without any water molecules of its own, the H2O has a more difficult time solvating the sodium acetate), you'd need to heat it to 324 degrees Celsius (the melting point of anhydrous sodium acetate -- I can go into details regarding why this is higher if you care) to do so, as supercooling can't occur with the substance in its solid form.

    Is it theoretically possible? I don't see any reason why it wouldn't be (I'm an undergrad, though, so I'm far from being a real authority on the matter -- feel free to correct), but it'd be much less efficient. Every time you wanted to store or use heat, you'd have to provide enough energy to melt the anhydrous sodium acetate, and in mass quantities or over a long time, a difference of 266 degrees Celsius amounts to a lot of extra energy, and ergo, money spent.

    As you mentioned, the alternative, sodium acetate trihydrate, is more expensive, and I'd guess this is a result of its larger size.

    Either way, it's an expensive process, and there's not a terribly good reason for it -- it's much easier just to store potential heat as electricity, as we do now. The fuels that produce electricity are much easier to store and harness, and can be put to a wider variety of uses. It's not that your idea is impossible, only that a more practical solution has already been implemented.

    To answer your question about ammonium nitrate, the answer is yes, they can in fact be used to "store" cold, and instant cold packs which employ ammonium nitrate exist on the market. For similar reasons, however, it's still more practical to rely on conventional power and technology to produce cold on massive scales, such as for industrial refrigeration and the like.

    Hopefully this helps -- If not, I'd be happy to elaborate.

    (Also, again, if I'm wrong about any of this, please illustrate.)
  4. Nov 1, 2007 #3
    thank you wowolaf,
    ok, i just wanted to know whether it was theoretically possible....if the supercooling state had a limited time.

    i knew it was expensive, but it's all relative to other things, and it could become "affordable" with an oil crisis, or whatever else comes up. (they are already using phase change materials in ceiling panels, this was considered ludicrous a few years ago, but it's becoming more accepted now). i'm a firm believer in passive heating methods, and the more options we have, the better.

    thanks again. anyone else see any other holes in my theory, or additional ideas/comments?
  5. Nov 1, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Back in the late 70's some people I worked with looked at sodium acetate cells in walls as a heat storage method. The idea was to circulate hot water (from solar panels) through the walls and dissolve the sodium acetate until required at night. A clicker in the wall would be activated to start the crystallization and generate the heat. I wasn't around during that time and so I don't know why it wasn't pursued further. I can see lots of problems with the approach.

    Regarding the possibility of supercooling the trihydrate vs the anhydride, you should understand that as soon as you dissolve the anhydride in water you don't have anhydride anymore. The solubility of the anhydride is 54 g per 100 g of water and the trihydrate is 54 g per 100 g water (25C) based on anhydride. (Source, CRC Handbook of Chemistry and Physics, 84th edition, pg 4-83)
    Both can be supercooled when dissolved in a little water. Yes, Wowolaf, the anhydride will dissolve in water without heating to 324C! The anhydride will need a little more water to be at the same concentration, however.

    You could use ammonium nitrate as a chilling liquid for AC in the summer. There are other salts that could be used as well.
  6. Nov 1, 2007 #5
    thank you, this is very helpful, this is exactly what my idea was about. even if it has already been done, and was an abyssmal failure, that is ok, but i can't just sit back and let a potentially good idea go untried. can you give me the contact info to these people you were working with?
    and just to be clear, my idea would be similar to handwarmers, so the sodium acetate would not be dissolved in water, but rather enclosed within it's own pouch.
  7. Nov 1, 2007 #6


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That might work for the trihydrate but, without some added water, it produces temperatures of about 130F. Pretty hot. This field has been explored by many more than the people I knew. The last of the good ol' boys that I knew died in 2002.

    Try [url="http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V4S-47XFSXT-1&_user=10&_coverDate=08%2F31%2F2003&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=7d93d267e370abeb630101dacedb2554]here.[/url] Or [url="http://www.freepatentsonline.com/CCL-126-400-p4.html]here.[/url]
    Last edited: Nov 1, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook