Jor
National Hazard
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failure cobalt NH3 complex
Today I wanted to isolate the ammine complex of cobalt.
I took some CoSO4.7H2O, dissolved this in as little as possible water, without heating. Next excess 25% ammonia was added. The solution turned VERY
dark red/brown, almost black.
I left the solution standing for a few hours to oxidise Co(II) to Co(III). Afterwards I blew a few liters of air with a syringe in the solution.
I then added 2times the volume of ethanol. I decanted the light brown liquid, and was left with a very dark almostblack syrup. I washed with ethanol,
and then with diethyl ether, trying to get it dry. But no, it was still a syrup.
Slight heating resulted in decomposition, releasing ammonia.
So what did I do wrong?
[Edited on 23-6-2008 by Jor]
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not_important
International Hazard
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It sounds like you mea the Co(III) type complexes, if so you didn't do enough to get much conversion. From Inorg Synth II:
| Quote: | Hexamminecobalt(II1) salts may be prepared by any of three methods that depend on oxidation of cobalt(I1) ion in ammoniacal solution : (1) air
oxidation, with formation of the pentammine ion, which is converted to-the hexammine by heating with aqueous ammonia under pressure;
(2) oxidation with an agent such as hydrogen peroxide, iodine, potassium permanganate, lead dioxideo,or hypochlorite solutions;
(3) oxidation in the presence of a catalyst that allows equilibrium between the pentammine and hexammine ions to be established at room temperature
and atmospheric pressure.
These compounds may also be prepared indirectly from other hexamminecobalt(II1) salts.
In the best of the catalytic methods diamminesilver ion or decolorizing charcoal is used as a catalyst. The method devised by J. Bjerruml* in which
decolorizing charcoal is the catalyst is simple, gives high yields of pure product, and is not time-consuming. A high concentration of ammonium salt
is sufficient to stabilize the hexamminecobalt(III) ion, and the carbon serves only to establish the equilibrium. Air is used as oxidant except when
the cobalt(I1) compound is slightly soluble in the ammoniacal solution, as in the preparation of [CO(NH3)6]Br3 for which hydrogen peroxide is
preferable. |
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UnintentionalChaos
International Hazard
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http://www.sciencemadness.org/talk/viewthread.php?tid=10368&...
I have a synthesis here for hexaamminecobalt (III) chloride but the sulfate can probably be prepared analagously. H2O2 is used as oxidizing agent with
activated charcoal as catalyst. I imagine air is not a very efficient oxidizer unless the mix is hot and a lot of air is injected over a long period
of time (and maybe under pressure).
The hexaammine complex is pretty damn soluble too. A few people screwed up this lab by using more water than the procedure recommended and could not
isolate any product. If you did get any product, the sulfate may simply be extremely soluble.
[Edited on 6-23-08 by UnintentionalChaos]
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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