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krabpustelnik
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Preparation of Chromate/Dichromate
Hi, at the beginning I want to say that my english isn't perfect, sorry for that  I
will try to explain everything in simple words 
When I was going to the school some time ago, there were a lot of chemistry demonstrations. The main thing which was always repeating was "volcano
from ammonium dichromate". After that experiment there always were huge mess because of green powder Cr2O3. My teacher didn't collect it, so I was
doing it everytime. Now I have lot's of Cr2O3. I didn't know what to do with it, I was trying to dissolve it in acids like HNO3, H2SO4 or HCl, but
everytime there were no reaction so I decided to mix KNO3 with Cr2O3 and heat it up. Finally I get red-orange alloy of dichromate and other salts. I'm
not sure how the formula looks like but probably the products other than dichromate were KNO2 and some NOx
Now I have problem because I don't know how to separate products. I tried crystallization but I get red crystals mixed with withe crystals
I think about precipitation of insoluble Chromate (for example Zinc Chromate), but I have no idea what to do after that because I want to get pure
K2Cr2O7 or K2CrO4.... Only one way I see is reaction of insoluble chromate with concentrated H2SO4 to get crystals of CrO3.... What do you think about
that ? Anyone has any better idea ?
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blogfast25
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Fusion of KNO3 with Cr2O3 yields indeed K2Cr2O7, I've done this myself and it works well. You can use a stoichiometric mixture for this fusion.
Thermal recrystallisation should work to purify the K2Cr2O7 but you need to take into account the relative solubility limits of K2Cr2O7 and any
unreacted KNO3.
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krabpustelnik
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thanks, it's good to hear that it can be separated by crystallization, I'm going to recrystalize it again  but, you say that I should take stoichiometric ammount, but I'm not sure how really formula looks like... I think it
is probably this: Cr2O3 + 5 KNO3 -----> 2 K2CrO4 + KNO2 + 4 NO2
But I'm not sure... Can anyone confirm or demolish it ?
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blogfast25
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From what I read the stoichiometry is:
2 KNO3 + Cr2O3 === > K2Cr2O7 + 2 NO
That is in any case the ratio I used successfully. The reaction was nearly 100 % complete, so thermal recrystallising is really easy then. Just use
Wiki Solubility Table data.
[Edited on 15-8-2015 by blogfast25]
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krabpustelnik
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Ahh it explain why I have so much white crystals.... because of KNO3 excess
Everything clear, thanks so much!
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blogfast25
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Yes, that's the most likely cause.
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gdflp
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Funny experience and something to watch out for. I tried this reaction a while ago in a homemade charcoal-air furnace. Let's just say that it didn't
go according to plan. I started heating the mixture, I used KOH in addition to the nitrate and chromium oxide, and left it loosely covered with a
thin piece of sheet metal as a lid. I walked away for a minute to grab a metal rod to stir it and as I was walking back, I watched the lid get blown
off about 15 feet in the air along with half of the contents of the crucible. Luckily I was about 30 feet away wearing a face shield and heat
resistant clothing, but somehow I think that I still wouldn't have been all to happy taking that shower Lesson learned, heat this reaction cautiously and slowly. At least I'll always remember it, the furnace, which was
made out of plaster, now looks like a green and yellow camo finish was applied
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krabpustelnik
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wow, that's weird, I did this reaction yesterday and when I heated up the mixture, KNO3 started to "boil" slowly, nothing spectacular.... But I didn't
use KOH, maybe it makes difference.
KOH is unnecessary in this reaction, everything works perfect without it If you
want to get K2CrO4 you should alkalize the solution of the product, I think it's the best way to get it
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blogfast25
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Quote: Originally posted by krabpustelnik  | wow, that's weird, I did this reaction yesterday and when I heated up the mixture, KNO3 started to "boil" slowly, nothing spectacular.... But I didn't
use KOH, maybe it makes difference.
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It only needs mild heat, enough to melt the KNO3. The 'boiling' you're seeing is the NO escaping. It works remarkably well as a way to prepare K2Cr2O7
and yet is relatively poorly known, for some reason...
[Edited on 16-8-2015 by blogfast25]
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j_sum1
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I like the sound of this one. I might give it a go.
I have plenty of Cr2O3 and KNO3 at the moment.
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krabpustelnik
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Today I wanted to do some dichromate from chromate which also contained some KNO2 and KNO3, but after acidification of K2CrO4 solution I didn't get
nice red color of K2Cr2O7 instead od that I get very deep red/nearly brown solution. I think the KNO2 was oxidized to KNO3 and chromate was reduced to
Cr3+ and this weird color is result of dichromate and Cr3+ mixture ... Am I right? now, I'm going to add some ethanol to reduce all of dichromate (because I don't want to waste all of it) and after that I want to add some sodium
carbonate to precipitate chromium III carbonate after filtration I will have some pure chromium III salt
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j_sum1
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K2CrO7 solution is a nice orange colour. I wouldn't call it red.
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blogfast25
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| Quote: Originally posted by krabpustelnik | | [...] and after that I want to add some sodium carbonate to precipitate chromium III carbonate after filtration I will have some pure chromium III
salt |
I don't think Cr(+3) carbonate exists (in watery medium), despite that references to it can found. Like Fe(+3) and Al(+3), soluble carbonates
precipitate Cr(OH)<sub>3</sub> with release of CO<sub>2</sub>.
[Edited on 22-8-2015 by blogfast25]
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MeshPL
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Did anybody point out that Cr2O3 is insoluble in acids and aqueous hydroxides? It can only be dissolved in molte hydroxides. Or oxidised.
Industrially it can be melted with NaOH/KOH and air can be passed through, but I guess for an amateur chemist required aparature may be too hard too
obtain and some temperature control may be required.
Judging by your nick we live in the same country. Lucky you, that teacher did that experiments at least once! It is unusual for teachers here to do
such demonstrations more than twice a year.
Also getting H2SO4 is hard here unless you find a good source. I resigned searching potential sources year ago.
My teacher told me it is illegal for school here to have chromates, but due to their didactive potential we have some anyway. But anything more exotic
was thrown away a long ago e.g. nickel salts, cobalt salts (those have a huge didactive potential, don't they?) and so on. When I hear about ones
borrowing mercury compounds and others stealing uranium salts from schools I am slightly anxious...
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krabpustelnik
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ah, my mistake, you're right about Cr(OH)3 but it doesn't matter for me because I want to get pure Cr3+ ions, there is no diffrence for me if it's
hydroxide or carbonate ...
MeshPL, heh speaking in english to you it's a little bit weird but it's english forum so I will do it in english My teacher was retired, but he's still working and because he's old, he never minds
about rules. Also I was the finalist of our chemistry domestic championship so he was treating me better than other students... He is very specific
guy but I love chemistry because of him. Unfortunately in our country there are less and less teachers like him. The law is also stupid, I will be not
suprised when NaCl would be banned in schools because "it's toxic, and could be dangerous" 
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blogfast25
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| Quote: Originally posted by MeshPL | Did anybody point out that Cr2O3 is insoluble in acids and aqueous hydroxides? It can only be dissolved in molten hydroxides.
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Even that is not guaranteed. A pottery grade Cr2O3 I have didn't respond to molten KOH AT ALL, much to my surprise. That same grade however converted
almost quantitatively when fused with KNO3 to K2Cr2O7.
In the case of many oxides, Cr2O3 included, how reactive it is to acids/alkalis is very grade-dependent. Freshly produced Cr(OH)3, mildly calcined to
Cr2O3, will still be quite responded to strong alkali/acids. But seriously calcined Cr2O3 (high T, long t) becomes very inert. So it is also with the
oxides of Al, Fe, Ti and many others.
[Edited on 22-8-2015 by blogfast25]
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Amos
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| Quote: Originally posted by krabpustelnik | | Today I wanted to do some dichromate from chromate which also contained some KNO2 and KNO3, but after acidification of K2CrO4 solution I didn't get
nice red color of K2Cr2O7 instead od that I get very deep red/nearly brown solution. I think the KNO2 was oxidized to KNO3 and chromate was reduced to
Cr3+ and this weird color is result of dichromate and Cr3+ mixture ... Am I right? now, I'm going to add some ethanol to reduce all of dichromate (because I don't want to waste all of it) and after that I want to add some sodium
carbonate to precipitate chromium III carbonate after filtration I will have some pure chromium III salt |
By any chance, was the acid you used for this hydrochloric acid? Hydrochloric acid is reducing and I've found it's best not to use for that reaction,
as you'll often accidentally reduce the chromium.
I'd also like to encourage you to try dissolving some of your chromium(III) oxide in household bleach to produce sodium chromate in solution, but do
it outside, as it produces chlorine gas. From there it's just acidification(more chlorine, do outside) addition of a potassium nitrate to the
solution(as a way to introduce potassium ions) and then careful evaporation to crystallize out the K2Cr2O7.
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krabpustelnik
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| Quote: | | By any chance, was the acid you used for this hydrochloric acid? |
No, I used 30% H2SO4
| Quote: | | I'd also like to encourage you to try dissolving some of your chromium(III) oxide in household bleach to produce sodium chromate in solution
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I have never heard about it but I will try it soon
I have one more question for you guys, can I use steel pot for the reaction between KNO3 and Cr2O3 ?Will it contaminate the product ? I'm asking
because I'm afraid my glassware wouldn't survive heating up with an open fire
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blogfast25
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| Quote: Originally posted by Amos |
I'd also like to encourage you to try dissolving some of your chromium(III) oxide in household bleach to produce sodium chromate in solution, but do
it outside, as it produces chlorine gas. From there it's just acidification(more chlorine, do outside) addition of a potassium nitrate to the
solution(as a way to introduce potassium ions) and then careful evaporation to crystallize out the K2Cr2O7. |
That reaction is nice for demo purposes but not much else. Bleach is weak in hypochlorite (4 - 5 %), so you need to use quite a lot of it. The
chromate solution ends up weak and contaminated with lots of NaCl.
Cl(+1) + 2 e === > Cl(-1), Cr(+3) === > Cr(+6) + 3 e. So stoichiometrically you need need about 3 mol hypochlorite per 2 mol chromate produced.
That's lots of NaCl.
[Edited on 23-8-2015 by blogfast25]
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krabpustelnik
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That's right, I have done it just now in small scale. The reaction is very slowly and no effective, melting KNO3 with Cr2O3 is the best way I think
blogfast25, are you using glassware or something else for melting KNO3 ?
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blogfast25
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I used a nickel crucible. But a ceramic one should also work. Glass will work but it does get attacked by (some) molten salts, not sure about molten
KNO3 though. Molten saltpeter oxidises steel, so that's not a good choice.
[Edited on 23-8-2015 by blogfast25]
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Alzador
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Apart from producing alkaline conditions in the reaction, is there any benefit from mixing K2CO3 to the mixture? From the phase
diagrams that I've found, mixing the carbonate in raises the melting point, which is bad because you need more heat to melt the mixture. If fusing the
nitrate with the chromium oxide yields dichromate just fine then why would you put carbonate in it?
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blogfast25
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| Quote: Originally posted by Alzador | | If fusing the nitrate with the chromium oxide yields dichromate just fine then why would you put carbonate in it? |
There's no need for K2CO3. There's no need for alkalinity either: see the stoichiometric equation higher up.
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Alzador
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by Alzador | | If fusing the nitrate with the chromium oxide yields dichromate just fine then why would you put carbonate in it? |
There's no need for K2CO3. There's no need for alkalinity either: see the stoichiometric equation higher up. |
Riiight, I'd thought as much. Only, there was a lab practice book which we used to do the experiments from in freshman year, and that book never
mentioned that you could prep the dichromate straight with this reaction. It only showed how to prep K2CrO4:
Cr2O3 + 2 K2CO3 + 3 KNO3 = 2 K2CrO4 + 3 KNO2 + 2 CO2
and then acidify it to get the dichromate.. weird.
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JJay
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I've been thinking about making some potassium dichromate. It's an extremely useful chemical, and I have only a small quantity of it. It's also fairly
expensive; the cheapest I've seen is around $30/kg.
This has been discussed several times on Sciencemadness, and people have done it successfully:
Potassium Dichromate synthesis (from Rhodium)
Ammonium Dichromate from Stainless Steel
Synthesis of Potassium Dichromate
Dichromates
It's also been done on YouTube by Tdep and Chem Player, not to mention Shiva Chemist, Elch Science, and weiming1998
The two major methods for forming chromates are to either fuse chrome oxide with a suitable oxidizer such as potassium nitrate and then extract it or
to prepare a chrome salt, which can be extremely impure with other metal salts, react it with a hydroxide or carbonate to precipitate the chrome and
other metals as insoluble hydroxides and/or carbonates, and then use an oxidizer to produce a soluble chromate salt.
It looks to me as though used silverware is a convenient source of chromium. Most of it contains around 18%. I am considering a procedure something
like:
1. Dissolve silverware in sufficient hydrochloric acid.
2. Add sufficient sodium carbonate solution to make the solution strongly alkaline.
3. Allow the solids to settle, decant the supernatant, add some water, mix, wash, decant.
4. Leech out sodium chromate with household bleach by mixing, decanting, etc.
(Alternatively, it might be better to use potassium hypochlorite solution prepared from potassium carbonate and calcium hypochlorite to leech out
potassium chromate.)
6. Acidify solution to form dichromate ions. Add potassium chloride.
7. Boil the supernatant down until crystals start to form in it. Cool, filter, and recrystallize.
People have done pretty much all of this already, and it doesn't look hard, but it does look messy, and I think it requires rather large amounts of
bleach and acid to produce a small amount of potassium dichromate. Is this worth doing, or am I better off just paying $30/kg?
[Edited on 11-3-2017 by JJay]
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