[meteorite-list] blue crystals as desiccants

From: AL Mitterling <almitt_at_meteoritecentral.com>
Date: Fri, 30 Mar 2007 06:21:52 -0500
Message-ID: <460CF2D0.5060302_at_kconline.com>

Hi Zelimir and list,

Many thanks for your well written information on desiccants and taking
the time to share with all of us. While I didn't know the reasons
chemically behind the crystals, I did know from experience that the
crystals weren't detrimental from using them with specimens I keep. It
is great to have the chemical understanding now and I am sure I speak
for others who very much appreciate your taking the time to write this
to the list!!! All my best to you!

--AL Mitterling

Zelimir Gabelica 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
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Received on Fri 30 Mar 2007 07:21:52 AM PDT

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