Poppy
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Dichromate decomposition in water
Hi, recently I was heating aluminium chloride with sodium aluminate and dichromate in a supposedly acidic media and after a while a blackish
precipitate (which showed to be greenish chromium trioxide tetroxide mix) settled as a very light and fine precipitate.
Is this supposed to be normal?
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Galinstan
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Sodium aluminate doesn't exist in acidic medium it forms a white precipitate and then would redissolve in excess acid to form an aluminium salt. and i
think chromium trioxide hydrolysises in water so it can't have been that hope this helps somewhat.
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blogfast25
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Evidence, please?
Certainly CrO3 cannot exist in water. Did you mean Cr2O3? Even that can only exist as Cr(OH)3 in very near neutral conditions.
Was your starting solution yellowish or amber (orange)?
[Edited on 11-12-2013 by blogfast25]
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woelen
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Dichromate ion is capable of oxidizing chloride ion, but only slowly and with heating. The dichromate itself then is turned to a brown suspension, or
a green solution, depending on the amount of acid present in your solution. The brown suspension almost certainly contains chromium(IV) as CrO2 and
the green solution contains chromium(III). A nice experiment is to dissolve a mix of a dichromate and a chromium(III) salt. On standing a brown
precipitate is formed over several days, maybe on heating this is going much faster. I can imagine that this occured in your experiment, with the
chloride first reacting with dichromate and then subsequent reaction with formation of CrO2.
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MrHomeScientist
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Apparently CrO2 is ferromagnetic, and was used in magnetic recording tapes. That would be a good test of your precipitate - see if it is
attracted to a magnet. If you can rule out iron impurities in your reactants, this should be pretty good confirmation of what you have.
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blogfast25
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Quote: Originally posted by MrHomeScientist  | | Apparently CrO2 is ferromagnetic, and was used in magnetic recording tapes. That would be a good test of your precipitate - see if it is
attracted to a magnet. If you can rule out iron impurities in your reactants, this should be pretty good confirmation of what you have.
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G-d, yeah: CrO2 tapes. Memory lane!
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Poppy
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It seems the necessary conditions falls in a narrow pH range. The attempt to replicate the experiment has failed.
It was orange, but didn't give any precipitates anymore, although the pH was ridiculous low this time, below 0.
The pH of the solution after the precipitation occurred was 10.4, strange no?
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blogfast25
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If it really was oxidation of chloride by dichromate in a acidic conditions then no: the reaction consumes
H<sub>3</sub>O<sup>+</sup>. An increase in pH would then be expected.
[Edited on 11-12-2013 by blogfast25]
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Poppy
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Yeah, I repeated it successfully: to a solution containing sodium aluminate and CrIV, hydrochloric acid is slowly added until reaches pH 7.0, and the
aluminium hydroxide so formed being capable of staying visible all the way through heating, after like 10 minutes this green-gray precipitate forms
and the solution no longer holds any colorations owning to chromium.
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blogfast25
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What is your purpose with the aluminate in this case?
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Poppy
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The discussion entails the dissolution of chromium, and also the overral reaction as for the precipitate does not dissolve in boiling HCl anymore,
which surprises me. Therefore it must be some sort of oxide of aluminium and chromium.
Thats a point we all can get and discuss in detail.
The supernatant is now green evidencing the definitive presence of chromium III. I will give it another try on conc. sulfuric
Is aluminium oxide supposed to be grey black?
And, according to 2 Al(OH)3(aq) -->heat--> Al2O3 + 3H2O (which is not certain as is the analogous reaction of the hydroxides of iron Fe(OH)2
-->heat--> FeO
Well, in the oxide form, how far is aluminium inert when heated in water to this point?
[Edited on 12-12-2013 by Poppy]
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