[meteorite-list] blue crystals as desiccants

[AL Mitterling]

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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