I need two chemicals, both liquids, which will combine to produce an
endothermic reaction in which the solution will drop to freezing or
below and maintain that temperature for about 30 minutes (more than 30
minutes would be great).  The chemicals, and the end solution, need to
be non-toxic and able to be stored safely in a plastic container.

You may find the chemicals that combine in chemical ice/cooling packs
to be helpful, but I have found that, in many cases, one of the
chemicals is in a solid state.  While I suppose I could work with
this, I really need two liquids.  The ideal reaction will ultimately
involve a phase change (i.e., dissolution or precipitation of a
solid).


Below is a little bit of help that I have previously received, but I
need more detail, i.e., can this be done and still be non-toxic? I
will also need precise chemical equations.

"Most ?chemical ice packs? that I am familiar with involve the
endothermic dissolution reactions of a salt, such as sodium
thiosulfate or ammonium nitrate, in an aqueous solvent."

Here is a website that sells a chemical ice pack and lists the
ingredients... this may be a helpful start:
http://www.dual-ice.com/Safety/safety.html

---

Clarification of Question by robbycoats-ga on 11 Apr 2005 21:05 PDT

I previously stated that I need the solution to remain cold for 30
minutes, but I will take anything I can get... My target time is now
10-15 minutes.
I tried working with Amonium Nitrate and water... this does not stay
cold long enough.

---

Clarification of Question by robbycoats-ga on 24 Apr 2005 18:46 PDT

A solid "brick" of ice would be fine.  My primary requirements are
that the combining chemicals be liquids, and that the resultant
soultion stays cold for a good while, say 15 minutes or more.
As far as everything being non toxic, this is no longer an issue as
everything will be sufficiently contained.  I realized that finding
two chemicals that meet my needs, and are also non toxic is not likely
to happen.  I am interested in any suggestions, as long as they can be
safely contained in plastic.

The Merck Index lists these cooling mixtures:

Substance                Quantity       Q. Water      Resulting temp. (C)
ammonium nitrate           100            94                -4.0
sodium acetate              85           100                -4.7
sodium nitrate              75           100                -5.3
sodium thiosulfate cryst   110           100                -8.0
calcium chloride .6H2O     100           246 (ice)          -9.0
sodium chloride             36           100               -10.0
ammonium nitrate            45           100 (ice)         -16.8
sodium nitrate              50           100 (ice)         -17.8
ammonium thiocyanate       133           100               -18.0
sodium chloride             33           100 (ice)         -21.3
calcium chloride .6H2O     100           123 (ice)         -21.5
sodium bromide              66           100 (ice)         -28
magnesium chloride cryst    85           100 (ice)         -34
sulfuric acid (66.1%)      100           109.7 (snow)      -37.0
calcium chloride .6H2O     100            81 (ice)         -40.3
calcium chloride .6H2O     100            70 (ice)         -55
(the rest includes dry ice)


Good luck finding something liquid and non-toxic; if there is such a
substance at all...

:-) Re

Try instituting a "slow release" mechanism such as coaceravation or
microencapsulated materials.  If you have water in the system,
encapsulate the reactive materials in a maltodextrin, so that they
will slowly dissolve and release the desired encapsulated materials. 
Typically, these cold packs have a propylene glycol or ethylene glycol
base, which is mainly used because they have high specific heats.  You
can still use those, because both are soluble in water, but, you have
to remember, if you have too much water, you'll get a solid brick
which may not be what you want if you want a form fitting gel pack.

If you're still searching for a solution, you might think about using
ammonium thiocyanate. The colorless crystals are relatively non-toxic,
and gives a low temperature.

If you add a small amount of an iron-salt to the water (the water will
remain colorless, use Fe2+), you will get a color change to deep red
when you add the thiocyanate.

:-)

Hi, I was wondering if robbycoats-gacontact me.  I am currently
researching the same information and I think we might be able to help
eachother.  My email is TJR@gmail.com
  Thanks
   TJR