vitirol
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What is that compound?
Hi!
I made sodium dichromate,and I tried to dissolve it in ethanol to separate it from sodium sulfate.After I filtered the mixture. When the funnel dried
I saw a deep red residue dried on the surface of the funnel.
-It is slighty soluble in hot water
-The solution is yellow like potassium chromate solution
What is that thing?
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gdflp
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How did you make your sodium dichromate? Is it possible that your sodium ion source had some potassium contamination? Potassium dichromate and
potassium chromate are both insoluble in ethanol and may be the culprit.
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Metacelsus
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Furthermore, they can react with (oxidize) ethanol.
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gdflp
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Can they? I thought that without an acid, such as dilute sulfuric acid, the intermediate chromate esters don't form and the chromates/dichromates
themselves are thus incapable of oxidizing alcohols or aldehydes.
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Texium
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Yes, gdflp is correct. The dichromate would need to be acidified to oxidize the alcohol at a substantial rate. Also, if the reaction occurred, a
definitely color change would have been apparent, due to the change in color from yellow/orange Cr(VI) to green Cr(III). It is most likely still
Cr(VI).
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woelen
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The deep red residue is a mix of chromium(VI) and chromium(III) or chromium(IV). Even at non-acidic conditions, hexavalent chromium is capable of
oxidizing ethanol, albeit slowly. A small fraction of your dichromate is converted to basic chromium(III)/(IV) compounds, which are dark green or dark
brown. This, together with the orange of the dichromate, results in a dark red/brown color. The red/brown mix is partially soluble. You can leach
dichromate out of it, the remaining material is insoluble chromium at lower oxidation state.
I once did an experiment with dichromate dissolved in water and added a solution of chromium(III) sulfate to this and added no acid. The resulting
clear solution very slowly produced a dark brown precipitate (it took many days), which according to what I read is CrO2 (in the past this CrO2 was
used in tapes for audio recording). Maybe this is what happens in your situation as well.
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Amos
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In the future, rather that trying to use solvent extraction for such a strong oxidizing agent, you might want to consider concentrating and cooling
the mixed solution to fractionally crystallize out the sodium sulfate. This can also be done before the chromate is acidified to dichromate, as sodium
chromate is several times more soluble in cold water than sodium sulfate is.
Another option is to add potassium chloride to your dichromate solution, which will cause the considerably less soluble potassium dichromate to
crystallize out.
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vitirol
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Thanks for woelen and almos,i forgot to write,i tried to separate the sodium sulfate from the dichromate with adding ethanol to the solution of it,and
only sodium sulphate precipitated out (I think,because it was pure white) than i filtered it and I saw the red residue I wanted to know what is this
compound( n that case , mixture of compounds)
Thanks again! 
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Molecular Manipulations
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Can you explain the procedure you used? Why was sodium sulfate in there to begin with?
-The manipulator
We are all here on earth to help others; what on earth the others are here for I don't know. -W. H. Auden
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vitirol
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I made sodium chromate by the reaction of chromium(III)hydroxide, hydrogen peroxide and sodium hydroxide,then i boiled out the water,i measured the
weight of it, and finally i calculated out the necessary volume of sulfuric acid and i did the exeriment (i used diluted sulfuric acid,37w/w%)
[Edited on 24-3-2015 by vitirol]
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vitirol
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*...by the reaction of...
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Amos
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The same can also be done by dissolving chromium carbonate or hydroxide in sodium hypochlorite solution, producing chlorine gas as a byproduct.
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blogfast25
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Quote: Originally posted by Amos  | | The same can also be done by dissolving chromium carbonate or hydroxide in sodium hypochlorite solution, producing chlorine gas as a byproduct.
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There no such thing as chromium(III) carbonate. Cr(III) soluble salts with soluble carbonates yields Cr(OH)3 and carbon dioxide, not Cr(III)
carbonate.
Potassium chromate can also be obtained by fusing Cr(III) oxide with potassium nitrate. Leaching with water then gives a
K<sub>2</sub>CrO<sub>4</sub> solution, with very good yield.
[Edited on 25-3-2015 by blogfast25]
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Amos
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by Amos | | The same can also be done by dissolving chromium carbonate or hydroxide in sodium hypochlorite solution, producing chlorine gas as a byproduct.
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There no such thing as chromium(III) carbonate. Cr(III) soluble salts with soluble carbonates yields Cr(OH)3 and carbon dioxide, not Cr(III)
carbonate.
Potassium chromate can also be obtained by fusing Cr(III) oxide with potassium nitrate. Leaching with water then gives a
K<sub>2</sub>CrO<sub>4</sub> solution, with very good yield.
[Edited on 25-3-2015 by blogfast25] |
Ah, my mistake. Rather I should have said "the product formed from solutions of soluble chromium(III) salts and sodium carbonate". Now I know. I was
unaware of the route using potassium nitrate; what waste gases are given off as a byproduct?
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Just speculating on the equation here...
4KNO3 + Cr2O3 --> 2 K2CrO4 + 4NO2 +1 1/2 O2. Just a guess, first I've
heard of this route to potassium chomate as well.
-The manipulator
We are all here on earth to help others; what on earth the others are here for I don't know. -W. H. Auden
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woelen
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No, this reaction does not produce any NO2, nor any O2. I have tried this kind of reactions with KNO3 as oxidizer (it works with many compounds as
reductor) and never saw dense brown gases emanating from the hot mixture. I would expect N2 as waste gas.
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Amos
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| Quote: Originally posted by woelen | | No, this reaction does not produce any NO2, nor any O2. I have tried this kind of reactions with KNO3 as oxidizer (it works with many compounds as
reductor) and never saw dense brown gases emanating from the hot mixture. I would expect N2 as waste gas. |
How did you determine that no oxygen was given off? Assuming the equation would be along the lines of this:
4 KNO3 + Cr2O3 = 2 K2CrO4 + 2 N2,
There are 7 more oxygens that need to go somewhere...
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blogfast25
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From my lab notes:
2 KNO3 + Cr2O3 === > K2Cr2O7 + 2 NO
I used 10 g Cr2O3 and 20 g KNO3 and heated that mixture in a nickel crucible at medium Bunsen heat for about 45 minutes. After cooling 20 ml of water
and a few drops of conc. H2SO4 were added and the crucible gently heated on a hot plate to dissolve the reaction products. The crucible content was
then transferred to a small glass beaker, a little more water added and the solution hot filtered. On cooling/chilling the K2Cr2O7 crystalized out.
The amount of unreacted Cr2O3 left on the filter was practically zero, indicating a high degree of conversion. The Cr2O3 used was a very inert grade:
attempts at fusing it with KOH previously had yielded absolutely no solubilisation whatsoever.
Molten KNO3 is a very strong oxidiser/solvent.
[Edited on 25-3-2015 by blogfast25]
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Amos
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Ah, this makes much more sense now! I'm a lot happier to see that the ratio needed is only 2 KNO3 per mole of Cr2O3. I'll have to use this method if
I ever run out of sodium dichromate.
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woelen
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This reaction also requires alkali if you want good yield at relatively low temperatures (still appr. 300 C though). The alkali has the added benefit
that it melts at relatively low temperature and then dissolves KNO3 and Cr2O3. I do not believe that NO is formed in any of such reactions. NO
immediately reacts with oxygen from air to form NO2 and you would see copious amounts of dense brown fumes. Nothing like that appears.
The following net reaction occurs:
6 KNO3 + 5 Cr2O3 + 14 KOH --> 10 K2CrO4 + 7 H2O + 3 N2
You can also do this with NaNO3 and NaOH, or KNO3 and NaOH, but in the latter case you get mixed Na/K chromate.
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blogfast25
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| Quote: Originally posted by woelen | | This reaction also requires alkali if you want good yield at relatively low temperatures (still appr. 300 C though). The alkali has the added benefit
that it melts at relatively low temperature and then dissolves KNO3 and Cr2O3. I do not believe that NO is formed in any of such reactions. NO
immediately reacts with oxygen from air to form NO2 and you would see copious amounts of dense brown fumes. Nothing like that appears.
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I carried out the reaction without ANY alkali at all, on a tip by ‘plante’.
I did not observe any NO2 either but I don’t see how the reaction equation can be balanced without loss of NO (which of course oxidises immediately
to NO2).
During fusion the mass changed from dark green to dark red/brown. The fusion product dissolved in hot acidified water very easily and was clearly
almost 100 % K2Cr2O7.
10 g of Cr2O7 would generate about 0.13 mol of NO (and thus NO2), or over 2 L at STP. Hard to miss, of course. I carried out the reaction in my shed
cum laboratory, with good ventilation, but didn't see or catch even a whiff of NO2. So I’m not sure what the explanation is here.
‘Tim’s’ (who used to post here too) chromate preparation using Cr2O3, NaOH, KClO3 (and some KCl):
http://webpages.charter.net/dawill/tmoranwms/Chem_Chromate.h...
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woelen
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Interesting that you did not use any alkali. Normally, the KNO3-assisted oxidation, or KClO3-assisted oxidation, uses alkali and Tim indeed used that
too.
I think that in your case you get the following two reactions, which run in parallel:
2 KNO3 + Cr2O3 --> K2Cr2O7 + N2 + O2
4 KNO3 + 2 Cr2O3 --> 2 K2Cr2O7 + 2N2O + O2
The reaction mechanism most likely is an indirect one. Initially some K2Cr2O7 may be formed plus tiny amounts of N2 and NO. K2Cr2O7, however, is less
stable than K2CrO4 at high temperature and loses oxygen. The resulting material then is oxidized again by the KNO3. So, after a small initial
formation of NO, further reaction products are N2, O2 (and possibly N2O). These gases do not react with each other at 300 C or so, that would require
MUCH higher temperatures and this also explains why you did not notice any brown fumes and did not smell any NO2.
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blogfast25
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| Quote: Originally posted by woelen |
The reaction mechanism most likely is an indirect one. Initially some K2Cr2O7 may be formed plus tiny amounts of N2 and NO. K2Cr2O7, however, is less
stable than K2CrO4 at high temperature and loses oxygen. The resulting material then is oxidized again by the KNO3. So, after a small initial
formation of NO, further reaction products are N2, O2 (and possibly N2O). These gases do not react with each other at 300 C or so, that would require
MUCH higher temperatures and this also explains why you did not notice any brown fumes and did not smell any NO2. |
I'm not so keen on highly speculative reaction mechanisms like that. I think it would firstly be necessary to examine the precise composition of the
released gas. I think I might try and do that.
One thing's certain: the preparation works and if it doesn't generate any NOx, that's a bonus. Not having to use any alkali is a boon too, IMHO, which
is why I wanted to try it.
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