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garage chemist
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You could have heated your KCl with H2SO4 to make K2SO4, you could have used K2CO3 for this, etc... The potassium ion can even be added after the
oxidation melt.
You need to avoid chloride, otherwise your product can be contaminated by Cr2O3 or toher Cr(III) compounds.
[Edited on 22-4-2007 by garage chemist]
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12AX7
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- Fixed a typo on the page (I had said that "potassium chromate is less soluble").
I wonder what happens if I fuse KClO3 and Cr2O3. Is a strong oxidizing environment strong enough to kick out Cl2? (HCl would be preferred, but I
suppose with no hydration available, that can't happen.)
Tim
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12AX7
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So I did put Cr2O3 and KClO3 together and heated the mixture. Lo and behold, it drove me out of the room! Should've done this outdoors...
The main reaction should be something like:
2 KClO3 + Cr2O3 > K2Cr2O7 + Cl2 + O2 (I don't know that O2 is present, but it balances)
But there will be others, some releasing HCl from hydration and some leaving KCl, which appears to be stable in a CrO4 melt (I don't know about
Cr2O7).
Goes off cleanly, Cl2 notwithstanding. No boiling over, because it fuses to a permeable mush as the chlorate decomposes. That chlorine is pretty
harsh though.
Tim
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I made more K2Cr2O7 today, in two experiments: scaled up, and done outdoors  with
fire for heat.
First I tried melting a pound of KClO3 in a rusty steel crucible, which gave me a bubbling salt melt. A lot of oxygen seems to be given off, for a
process that's supposed to make perchlorate in reasonable yield. I added a few scoops of Cr2O3, which resulted in the exothermic decomposition
(reaching up to about red heat, 700C or so) and probably quantitative reaction of what little chrome I added. The melt turned yellow to orange on the
surface, while the rest frothed up with prodigious vigor, giving off choking fumes and, presumably, a lot of oxygen. When it finished, there was a
dull red glow (around 700C in daylight), but all material was solid, so it was all decomposed to KCl.
So, this reaction must be done as a combination. That said, I then blended the remaining 250g Cr2O3 with 450g KClO3 and began heating it slowly.
Careful heat control is required to keep the reaction from running away, especially on this scale (I went cautiously; I didn't intend to find out what
happens when a whole pound of this stuff cooks off all its chlorine and oxygen at once!). At one point, the entire mass became a beet red,
crystalline semisolid sort of consistency, which I suspect was total conversion. I proceeded to melt the material and pour it out. The resulting
slab remained a drab color on cooling. I don't know if I simply couldn't see the unreacted Cr2O3 at that beet-red stage, or if more had formed (due
to oxidation of the crucible? decomposition without bubbling? I don't know).
I am presently hot filtering and recrystallizing the material, which appears to have left a pretty good bit of Cr2O3 on the filter. It's going
slowly, so I'll be at it some more, reheating the liquor, dissolving crude K2Cr2O7 and filtering it. So far, I already have more consistently
orange-red product sitting in the filtrate than I have collected to date (e.g. see above), so in an absolute sense, yield was good.
Tim
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How do you separate potassium dichromate from potassium perchlorate? KClO4 is far less soluble than dichromate.
Can you test your product for presence of perchlorate?
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12AX7
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Chlorate and perchlorate both decompose at the final temperature I brought it to, but I could seperately try fusing KClO3 to produce KClO4.
Tim
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Yes, and preferably not in a metal container, since metals catalyse evolution of oxygen. A porcelain or quartz crucible would be good, a small beaker
would do the job as well. You also need patience.
I havent tried the decomposition of KClO3 since I found NaClO3 to be a better starting material due to its lower melting point, at which it doesnt
already start to decompose.
Whats wrong with Cr2O3 + NaOH + KNO3? I found that to be a very good method of (di)chromate synthesis, completely converting all Cr2O3 if heated and
stirred enough, with only water vapor as byproduct.
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12AX7
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...And N2?
I have neither KNO3, nor NaOH at the moment. KClO3 gives off chlorine, but it's just two things and some heat. Besides which, KClO3 can be made
indefinitely in one's own lab.
Tim
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