[meteorite-list] blue crystals as desiccants

From: Mike Jensen <meteoriteplaya_at_meteoritecentral.com>
Date: Thu, 29 Mar 2007 11:20:45 -0600
Message-ID: <6f9da8300703291020v3da4e71bx5420f15ab8a95342_at_mail.gmail.com>

Hi Zelimir
Thanks for the fascinating explanation about how the chemistry of
cobalt chloride works. You actually made me say something I thought I
would never say...."I'm really glad I took college chemistry". Of
course that was 25 years ago and I was kind of foolish then.

Mike Jensen
Jensen Meteorites
16730 E Ada PL
Aurora, CO 80017-3137
IMCA 4264
website: www.jensenmeteorites.com
On 3/29/07, Zelimir Gabelica <Zelimir.Gabelica at uha.fr> wrote:
> 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
> >http://six.pairlist.net/mailman/listinfo/meteorite-list
> 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
> ______________________________________________
> Meteorite-list mailing list
> Meteorite-list at meteoritecentral.com
> http://six.pairlist.net/mailman/listinfo/meteorite-list
Received on Thu 29 Mar 2007 01:20:45 PM PDT

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