Bedlasky
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Olation - mechanism of polymerization of metal hydroxides and metal anions
Finally I found this article, which I read long time ago and for a long time I wasn't able to find it. For once who are interested I post it here:
https://en.m.wikipedia.org/wiki/Olation
Article describes mechanism how metal hydroxides and some metal anions polymerize.
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teodor
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Interesting, because I am experimenting with Cr(OH)3 now and its polymerisation prevents washing out of unwanted ions, I think they either form bonds
or clathrate with those ions. I red that addition of tannine before addition of hydroxide to Cr(III) salts changes this behaviour, possible preventing
olation (long chains formation).
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Bedlasky
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If you want pure hydroxide, use ammonia instead of hydroxide. Heating will cause escape of ammonia.
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teodor
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Yes, I know, but the problem is a chloride ion. Chromium metal doesn't want go into solution without chloride but the same ion prevents conversion of
Cr[(OH)4X2]+ to Cr[(OH)6]3+.
But sure we can discuss that in another thread, the polymerisation mechanism you described here is actually very interesting. Is it possible to make
some interesting experiments based on this behaviour of hydroxides?
[Edited on 1-12-2020 by teodor]
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Bedlasky
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Maybe reduction of ammonium chromate in ammonia will work. I once read that chromate can be reduced by hydrazine in alkaline solution. I don't know if
it work. Or electrolytic reduction?
I don't know any experiment using this property. But gel of hydrated silicon dioxide can be used as medium for crystal growing of phosphorecent
crystals of calcium and barium diglycolates.
Olation also occurs when metal anions (like chromate, vanadate, molybdate etc.) polymerize. First step is protonation pf anion (for example formation
of HCrO4-). This ion condense with another to form (Cr2O7)2-.
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teodor
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I didn't recognise that CrO4- -> (Cr2O7)2- -> ... is the same mechanism.
I red in wikipedia something quite unrelated I am thinking about it now: "In addition, the (S2O6) anion is not a good reducing group. Therefore, it
has been used to form single crystals of large cation complexes in high oxidation states without reduction of the metallic complex".
I don't quite understand what does it mean exactly (which "large cation complexes" they mean) but could this property be used to shape the form of the
olated chains somehow?
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Bedlasky
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Maybe [Co(dien)2]3+ or something like that. I think that this is large cation with metal in high oxidation state.
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teodor
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Yes, probably the easiest way to get Cr(III) complexes is to reduce some Cr(VI) compound. Or to start from chromium nitrate as a compound with the
weakest ligand. But I am particularly interested (as a part of my interest to engage people to chemistry) to see how many chromium compounds amateur
can get starting from the chromium metal itself. I personally have some chromium lumps and the seller advertised it as 99.9% pure. Any route from a
CrO3 derivative to get the same purity will require analytical grade Cr(VI), not available on ebay (even in a technical grade) and in any case it is
like wasting of money from amateur's point of view. If one gets CrO3 he probably will use it as a useful reagent for organic synthesis and the
reaction waste probably could be regenerated and recrystallised as e.g. chrome alum.
But it is another story. I don't want to go as high as Cr(VI) in my experiments now, preferring to keep Cr(II)-Cr(III) all the time.
Edit: as you understand, the problem is that Cr (metal) refuses to go into solution without cloride ion (to remove the passivation layer) and then, in
aqueous solution we get this ion attached inside complex, not accessible even to AgNO3 to remove it. Last weeks I am performing experiments about
transitions like these:
Cr[(H2O)4Cl2]+ <-> Cr[(H2O)5Cl]2+ <-> Cr[(H2O)6]3+
Only getting the last "blue" chromium will finally allow detach chloride and get all other salts of chromium.
I still have some ideas to try, if somebody is interested I will share my results here (in another thread).
[Edited on 2-12-2020 by teodor]
[Edited on 2-12-2020 by teodor]
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Bedlasky
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From chromium metal you can get any other compound as from Cr(III) and Cr(VI).
Why do you avoid Cr(VI)? It's good and stable precursor for Cr(III) complexes.
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teodor
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So, how do you propose to get, let say, chrome alum starting from chromium metal?
My tries doing it through Cr(OH)3 were complicated exactly by the olation of the intermediate hydroxide, I've got it in the form of gelatinous mass
which started to be rapidly oxidised by air to chromium chromate. I used KOH and probably should try with also NH4OH and tannine.
You can think about it as my own weird way to study chromium chemistry.
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Bedlasky
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Cr(OH)3 is stable, it don't oxidize in to chromate. Where did you get it? I never have issues during working with Cr(OH)3.
Cr alum from Cr metal? Dissolve Cr in conc. HCl and let it stay on air for a few hours or add H2O2. If you add H2O2, boil solution for a while to
destroy excess of H2O2. Then add ammonia until pH is 8. Then filter, wash precipitate well and dissolve in sulfuric acid and add potassium or ammonium
sulfate. If you precipitate chromium hydroxide from dilute solution, don't worry about chlorides. If you want, you can dissolve precipitate in
sulfuric acid and reprecipitate again with ammonia, but this isn't necessary.
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teodor
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What is the purpose of H2O2 addition? (Edit: I see, probably to remove all Cr(II). But I was not lucky to get it even in closed system with a water
lock, so I doubt there is some of it).
How do you propose to test pH of the deep green solution? (Edit: managed)
About oxidation to chromate, check my post here: https://www.sciencemadness.org/whisper/viewthread.php?tid=18...
[Edited on 3-12-2020 by teodor]
That is where the devil is. It rejects to filter. It blocks the filter paper and my vacuum unable to pull any liquid through it with speed more than 3
drops per minute. I'll try to use a good old washing by decantation method. What I red about it "after good washing it converts itself to sol". Will
see.
[Edited on 3-12-2020 by teodor]
[Edited on 3-12-2020 by teodor]
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Bedlasky
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This is weird, I never have issues with aerial oxidation of chromium hydroxide. If after dissolving in sulfuric acid some dichromate is present, add
little bit of H2O2.
pH check with pH paper. Or if you have pH meter, use it.
Boil precipitate in hot water bath for half an hour, it should be more crystalline. Use good filter papers with small pores.
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teodor
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In my first try (when I used KOH) I've got green Cr(OH)3 which I didn't wash because it had jelly consistency, it probably was too alkaline and
started oxidation on air BUT it readily dissolved in H2SO4. One drop of conc. H2SO4 makes this mass thawing with heating and fizzing.
Now I have a nice-looking very fine grey powder but it is dissolving terrible slow and I already put H2SO4 in excess. Is it because of slowness of
ligand exchange, I mean this unreactivity with H2SO4?
Edit:
I noticed that blue-gray Cr(OH)3 on action of H2SO4 first changes colour to green and then, VERY slowly and incompletely, goes into solution. I
suppose the mechanism could be:
2Cr[(H2O)4(OH)2](OH) (gray) + H2SO4 -> (fast) -> Cr[(H2O)4(OH)2]2SO4 (green) + 2H2O
Cr[(H2O)4(OH)2]2SO4 + 2H2SO4 -> (very slow) -> Cr[(H2O)4(SO4)]2SO4 + 2H2O
[Edited on 4-12-2020 by teodor]
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Bedlasky
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You must use fresh, still wet precipitate, not dry powder.
Btw. better than ammonia for precipitation is sodium carbonate, which doesn't form soluble complexes with Cr(III), so you can use it in excess.
[Edited on 4-12-2020 by Bedlasky]
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teodor
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I use fresh just vacuum filtered paste which I name "powder" mentioning its granularity only.
It goes in solution with H2SO4 very hard.
Is somewhere detailed procedure with proportions, operations etc to compare my results with?
Are you pretty sure this route can give chrome alum? I am still getting _green_ solution after dissolving in H2SO4, not a _blue_one. As you remember
my most problem was exactly converting green cation to blue one.
[Edited on 4-12-2020 by teodor]
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Bedlasky
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Of course you get from this green solution purple chrome alum. Cr-sulfato complexes are green, but chrome alum contain [Cr(H2O)6]3+ cation in its
structure, so from green solution you get purple crystals. I crystallized beautiful purple crystals of mixed Cr-Al alum, solution was green.
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teodor
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My solution of Cr(OH)3 + KOH + H2SO4 gave _green_ "crystalls".
Weird. I think it is because of H2SO4 excess I used to dissolve Cr(OH)3.
And about Cr(OH)3 reactivity, possible staying one day in water also deactivated it. But it is clear to me now that different samples can have
different level of reactivity.
Edit: I noticed that K2SO4 crystallised separately, so I edited this post.
[Edited on 7-12-2020 by teodor]
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