sumatra
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Help with chromate synth.
So I reacted some chrome alum with an exess of NaOH and got a bit of Cr(OH)3 which I then washed with water. I want to convert it to a chromate but
the only way I know of is to firat convert the Cr(OH)3 to Cr2O3. However I'm not currently comfortable doing that because of the high temperature
that's needed. Is there any other way to convert the hydroxide to a chromate, would just adding a bunch if base be sufficient enough? (I've heard its
possible with bleach and lye)
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CobaltChloride
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It is possible to do it with bleach and lye, but I don't really like that method.
First of all, you shouldn't have reacted the chrome alum with an excess of NaOH because Cr(OH)3 dissolves in excess NaOH as Cr(OH)6 3- ions.
My favorite method is combining stoichiometric quantities of KOH and Cr(OH)3 with a 10-20% excess of H2O2 (3% stuff from the pharmacy works, but
afterwards you need to boil a lot of liquid) after the reaction: Cr(OH)3 + 3 H2O2 + 4 KOH = 2 K2CrO4 + 8 H2O
After doing this you need to boil the liquid until only a very small amount of liquid remains (this contains the KOH that might remain) and you'll get
pure K2CrO4 powder.
I prefer using potassium salts because sodium chromate is quite hygroscopic, but you most probably can also make Na2CrO4 this way (I didn't test that
reaction though).
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sumatra
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Quote: Originally posted by CobaltChloride  | It is possible to do it with bleach and lye, but I don't really like that method.
First of all, you shouldn't have reacted the chrome alum with an excess of NaOH because Cr(OH)3 dissolves in excess NaOH as Cr(OH)6 3- ions.
My favorite method is combining stoichiometric quantities of KOH and Cr(OH)3 with a 10-20% excess of H2O2 (3% stuff from the pharmacy works, but
afterwards you need to boil a lot of liquid) after the reaction: Cr(OH)3 + 3 H2O2 + 4 KOH = 2 K2CrO4 + 8 H2O
After doing this you need to boil the liquid until only a very small amount of liquid remains (this contains the KOH that might remain) and you'll get
pure K2CrO4 powder.
I prefer using potassium salts because sodium chromate is quite hygroscopic, but you most probably can also make Na2CrO4 this way (I didn't test that
reaction though).
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Thanks, I'll try it with KOH and h.peroxide and post the results
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JJay
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If you use too much peroxide it will actually reduce the chromate, but I think it would produce chromium (vi) oxide peroxide first. Under the right
conditions, you can actually extract CrO5 with a nonpolar solvent (and it forms stable compounds with pyridine and bipy) but isolating it is not
recommended: https://www.youtube.com/watch?v=MjDgXQb4_DU
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sumatra
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Unfortunately I didn't take photos, but it worked! Now to convert it to a dichromate I believe I need to add conc. HCl, no?
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fusso
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| Quote: Originally posted by sumatra | | Unfortunately I didn't take photos, but it worked! Now to convert it to a dichromate I believe I need to add conc. HCl, no? |
Any strong acids will work.
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CobaltChloride
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| Quote: Originally posted by JJay | | If you use too much peroxide it will actually reduce the chromate, but I think it would produce chromium (vi) oxide peroxide first. Under the right
conditions, you can actually extract CrO5 with a nonpolar solvent (and it forms stable compounds with pyridine and bipy) but isolating it is not
recommended: https://www.youtube.com/watch?v=MjDgXQb4_DU |
As far as I know, only dichromate reacts with hydrogen peroxide to oxidize it to O2. Chromates don't react. Also, the reaction for producing CrO5 only
takes place in an acidic solution. Here we are dealing with a basic solution.
| Quote: Originally posted by sumatra | | Unfortunately I didn't take photos, but it worked! Now to convert it to a dichromate I believe I need to add conc. HCl, no? |
HCl works best because KCl and excess HCl are easy to separate from K2Cr2O7. The reaction with HCl has to be done at room temperature to avoid Cl2
generation though. In order to convert the chromate to dichromate, just make a saturated solution of K2CrO4 and then add concentrated HCl until the pH
is acidic (less than 4 should be enough). Afterwards, cool the solution in a freezer. This should precipitate almost all of the K2Cr2O7 without
precipitating any KCl.
[Edited on 17-8-2018 by CobaltChloride]
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JJay
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Dichromate predominates at lower pH, but chromate and dichromate are always in equilibrium in aqueous solution.
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CobaltChloride
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That's true, but I never got a chromate to react with hydrogen peroxide so the important part is that the direction of the oxidation is pH dependent.
Dichromate has a slightly acidic solution, chromate has a slightly basic solution. H2O2 only reduces Cr (VI) in an acidic solution.
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JJay
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There's also this: https://pubs.acs.org/doi/abs/10.1021/ic0257975
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CobaltChloride
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I didn't know about those reactions specifically, but, to my understanding, that paper says that the reduction of chromate only takes place in the
presence of base. The method I suggested involves using a stoichiometric amount of base. This way, no reduction of chromate should occur. The excess
hydrogen peroxide is there to account for this sort of redox reactions (that I expected to happen to a certain extent) that take place before all the
KOH is consumed.
[Edited on 17-8-2018 by CobaltChloride]
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JJay
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I didn't read the paper, but according to the abstract, reduction of chromate is actually inhibited by base.
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CobaltChloride
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This paragraph at the end of the paper encompasses the situation quite well:
"Extremely acidic conditions combine with further peroxo substitution to pull away sufficient electron density and promote the reduction to
chromium(III) (reaction 1).
Under more mildly acidic conditions (1 < pH < 7), however, the disproportionation of hydrogen peroxide within the chromium coordination sphere
is fast enough to preserve the two oxo ligands required to support the hexavalent oxidation state.
In mildly basic solution, the substitution reaction slows to the point where the diperoxo species does not even accumulate.
However, under strongly basic conditions (pH > 10), a distinct peroxo substitution reaction that involves the attack of the unprotonated chromate
dianion by the hydrogen peroxide anion, along with possible base inhibition of the disproportionation reaction, allows for the exchange of more than
two oxo ligands (Scheme 3). One-electron metal reduction ensues as the third and fourth oxo ligands are exchanged. The four peroxo groups sterically
protect the chromium center and provide sufficient electron density to stabilize the pentavalent state but apparently not the hexavalent state.
Pentavalent chromium does not reclaim more electron density by further reduction or catalysis of hydrogen peroxide disproportionation."
[Edited on 17-8-2018 by CobaltChloride]
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JJay
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That seems consistent with what I've observed.
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sumatra
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Thank you all for the help I made a decent amount of relatively pure k2cro7
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fusso
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Do you mean K2CrO4 or K2Cr2O7?
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