[meteorite-list] blue crystals as desiccants

From: Zelimir Gabelica <Zelimir.Gabelica_at_meteoritecentral.com>
Date: Thu, 29 Mar 2007 16:46:39 +0200
Message-ID: <5.0.2.1.2.20070329160228.02a1a1e0_at_pop.univ-mulhouse.fr>

Hi Al, list,

The "blue crystals" are indeed a cobalt chloride. Most of the current
colored (blue) dessicants actually consist in impregnating silica gel beads
(balls.... etc), by dehydrated cobalt chloride, that is blue.
For those who worry about the chemistry involved, let me ensure you that
(in principle) that compound, as well as silica gel, shouldn't behave
harmful to meteorites, provided the dessicant is not in direct contact with
the meteorite surface (what Al observed is therefore correct).

For those who wish to know more about what is going on, on a molecular
level, the "old popular chemistry" stated that anhydrous Co(II) chloride
(CoCl2) was blue, while once hydrated with 6 water molecules, it gets a
red-pink color, thus becoming CoCl2.6H2O.

This is actually not so.
The real reaction is as follows:

In a fully dry medium, two (Co(H2O)6)Cl2 (pink) molecules would dehydrate,
thus loose all their 12 H2O molecules, and eventually yield anhydrous
Co(CoCl4).
You can note that the coordination of Co(II) ion (or Co2+ ion) had changed.
It was initially octahedral (6 water molecules surrounding a Co2+ ion -
also noted Co(II)) and it became, upon dehydration, tetrahedrally
coordinated, thus consisting in an anion CoCl4 2-, neutralized by a Co 2+
cation.
In other words, two molecules of "hexaaquacobalt(II) chloride" transform,
upon loosing their 12 water molecules, into anhydrous
"cobalt(II)tetrachlorocobaltate(II)". The change of coordination is
basically responsible for the color change.

Sorry for those who are not familiar with (or hate) chemical formulas but
the message is that as soon as the dessicant is blue, the chloride anions
remain inside the coordination sphere of the cobalt complex as "ligands"
and (probably) won't diffuse towards the meteorite, even if the dessicant
is in contact. Upon rehydration (perfectly reversible), it is the water
that migrates inside the coordination sphere of Co(II) (that now gets an
octahedral symmetry) and the chlorides are now out of the coordination
sphere, (thus perhaps more prompt to react with the meteorite if in
contact, although probably not, because the whole salt, so neutralized, is
still very stable).

As a conclusion and whatever the chemistry be, both complexes are quite
stable and I don't believe chloride ions will ever diffuse towards the
meteorite surface if the dessicant is adequately separated from it (I mean
water, that readily diffuses through the whole system, won't bring along
the chloride ions during its migration).

Also, bear in mind that the cobalt salt is only a color indicator of the
ambient humidity (moisture). "Red" means there is water around and "blue"
meaning the environment is really anhydrous.
The silica gel is the real dessicant (it absorbs both the cobalt salt and
water into its porous texture). In other words, the color of the
impregnated Co salt indicates whether the silica gel is still empty (of
water) and thus a good drying agent (blue) or it is saturated with water
(pink), then meaning that water is all around and thus also in contact with
the meteorite.

Hoping this can help.
If collectors use other type of colors (or dyes), it is better to check the
chemical properties of the dye first.

Have fun,

Zelimir


A 07:33 29/03/2007 -0500, AL Mitterling a ?crit :
>Greetings all,
>
>I am thinking of introducing a product to help keep meteorites dry. I'll
>go into more details when I have the product ready for shipping.
>
>I was wanting input from collectors about using some blue crystals (with
>chloride) with white crystals. The idea is to know when to charge or dry
>your desiccant. While it might seem a bad idea to use the blue crystals,
>it has been my experience that they won't cause problems as long as they
>don't come into direct contact with meteorites. I know of several large
>collections that use this method and I myself have used it with no ill
>effect. However collectors have the last say by buying a product or not
>buying a product so your input is important.
>
>There are other types of colors that can be used but I wanted to stay
>away from those for fear of introducing something into the meteorites
>that could cause problems. Using those other colors would require input
>from scientists to verify that no harm would be transmitted or absorbed
>into specimens. The blue white crystals don't seem to be a problem from
>my understanding.
>
>Perhaps when I am ready, some collectors would like to test the product
>out for me.
>
>Your turn.
>
>--AL Mitterling
>______________________________________________
>Meteorite-list mailing list
>Meteorite-list at meteoritecentral.com
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Prof. Zelimir Gabelica
Universit? de Haute Alsace
ENSCMu, Lab. GSEC,
3, Rue A. Werner,
F-68093 Mulhouse Cedex, France
Tel: +33 (0)3 89 33 68 94
Fax: +33 (0)3 89 33 68 15
Received on Thu 29 Mar 2007 10:46:39 AM PDT