woelen
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Synthesis of polychromates
The following experiment is quite nice. Besides the dichromates and chromates, there also exist trichromates. These trichromates are easily made by
recrystallizing dichromates from concentrated nitric acid.
http://woelen.homescience.net/science/chem/exps/polychromate...
These trichromates also are more potent oxidizers than dichromates and they have deeper colors.
In the experiment, I describe a single crystallization. They can, however, easily be purified by means of another recrystallization from conc. nitric
acid, this recrystallization, however, introduces a fairly large loss (appr. 50%) and for most experiments with these compounds the added purity is
not really necessary.
Edit(woelen): Made link work again.
[Edited on 25-9-17 by woelen]
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chloric1
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Splendid!
I don't know where you get the time or how you find your material but this is great! You have taken a well known material and made an obscure
curiousity with it! This is what madscience is about!
Fellow molecular manipulator
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Zinc
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Very nice!! I would like to try that but I don't have any HNO3.
A little OT but how do ammonium chromate and ammonium trichromate decompose? I know how ammonium dichromate decomposes (a very nice volcano like
reaction)
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woelen
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Decomposition of ammonium chromate is similar to decomposition of ammonium dichromate. It first looses water and ammonia, leaving ammonium dichromate
behind, which in turn decomposes in the well known way. The reaction is somewhat less violent, due to the consumption of heat during the first
decomposition stage.
The decomposition of ammonium trichromate is spectacular. It forms 'snakes' when decomposing and the size of these snakes is amazing  given the small amount of starting material! More will follow on that . The webpage will be extended.
[Edited on 11-1-08 by woelen]
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woelen
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As promised, an update of the webpage. It now contains 4 videos of the decomposition of ammonium dichromate and of ammonium trichromate. These
decompositions really are nice and spectacular.
http://woelen.homescience.net/science/chem/exps/polychromate...
[Edited on 25-9-17 by woelen]
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Jor
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| Quote: | Originally posted by Zinc
Very nice!! I would like to try that but I don't have any HNO3.
A little OT but how do ammonium chromate and ammonium trichromate decompose? I know how ammonium dichromate decomposes (a very nice volcano like
reaction) |
I would guess you can also use sulphuric acid instead of nitric. However in concentration range of 80-85%, because potassium dichromate will be
dehydrated to CrO3 (at least this is what I think) by conc. sulphuric acid, and will not dissolve. Im not sure about this tho.
Maybe even hydrochloric acid would be suitable to lower the pH sufficiently. But I dont know if the chloride ion is interfering (It might form
chlorine with trichromate, or it might be coordinating?).
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woelen
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With conc. hydrochloric acid at not too high temperatures, the chlorochromate ion is formed, CrO3Cl(-). With potassium dichromate, one could make the
salt KCrO3Cl. That is another one, which I intend to try one time. But this synth is more critical. When the temperature becomes too high, then
chlorine is formed, and trivalent chromium goes in solution.
With sulphuric acid you could be right, but I just followed literature, which mentions nitric acid as the acid of choice for making these
trichromates.
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S.C. Wack
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A very old HNO3-less route that I noticed while going through my papers...probably have this in English somewhere but oh well...and yeah I know this
isn't that great...
In einem 50 ml Becherglas lösen wir bei 60° 11 g pulverförmiges Kaliumdichromat und 17.4 g Chrom(VI)-oxid in 22 ml Wasser. Die erhaltene Lösung
ist bei der Temperatur von 60° eben gesättigt. Das Becherglas mit der Lösung stellen wir in ein Wasserbad und dicken die Lösung bei 60°
vorsichtig ein. Wenn etwa 13 ml Wasser abgedampft sind, trennen wir noch in der Wärme die Kristalle von der Mutterlauge durch Dekantieren, oder wir
saugen sie auf einer kleinen Glasfritte scharf ab. Die gewonnenen Kristalle trocknen wir zwischen Filterpapier. Die Ausbeute beträgt etwa 8 g.
- Einführung in die präparative anorganische Chemie, 1963.
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Bedlasky
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I tried this simple experiment on my own. I was little bit curious how WFNA would react with dichromate, so I set up two test tubes with 1,3 g of
K2Cr2O7, in the first test tube I added little bit of 65% HNO3 and in to the second one little bit of 99% HNO3. Dichromate immediately turned red in
cotanct with 99% HNO3, on contact with 65% HNO3 it remained orange. I put both test tubes in to the hot water bath. Dichromate in the test tube with
65% HNO3 turned red after short time. I added 65% HNO3 in to the test tube until all solid dissolved. Polychromate was clumped together in to a single
clump, I must broke it with a glass rod. Due to this I overshot the amount of HNO3 which resulted in low yield (more about it later). I put the test
tube in to the cold water bath and than in to the fridge (5°C) for a 2 hours.
In to the second test tube I added 99% HNO3 but red solid refuse to dissolve (I suspected that it may be CrO3 and I was right as my analysis showed
later). Solid was free floting powder, unlike polychromate in 65% acid. 3 ml of HNO3 and it still didn't dissolve. I didn't want to add water in to
the hot acid due to exothermic reaction between water and pure nitric acid, so I instead add 0,5 ml of 65% HNO3. I hoped that small amount of water
will help with dissolving. Nothing. I added another 0,5 ml of 65% HNO3, but still nothing. But all solid apparently reacted with HNO3 in a few
seconds, there weren't any orange crystals. So I brought concentration of the acid back at >90% by adding 2,5 ml of 99% HNO3 and than I cooled it
in cold water bath. I filtered it through glass filter and put it in a small beaker in a dessicator with KOH.
After two hours of cooling all polychromate from the first test tube crystallized and I filter it through a glass filter. But there was only little
bit of solid on the filter (much less than from the second test tube). So I repeated experiment with 2 g of K2Cr2O7, I added little bit of 65% HNO3,
wormed it in hot water bath, broke the lump and then mixed it and slowly adding just enough of 65% HNO3 to dissolve all solids. Cooling in the freezer
for next 1,5 hours, filtration. Know I had decent amount of solid. I put it in to the small beaker in the dessicator with KOH. I left both compounds
in the dessicator for a weekend (after which they were nice dry dark red solids) and today I performed some titrations to find out what compounds I
made.
I used ferrometric titration (0,1 M FeSO4) with potentiometric end-point determination (Pt electrode). I did it in my job, I was lazy and there is a
titrator with platinum electrode and standardized FeSO4, so why not use it?
3Fe2+ + CrVI --> 3Fe3+ + Cr3+
I calculated percentage of Cr in a sample to find what polychromate I made.
K2Cr2O7 - wCr = 35,35 %
K2Cr3O10 - wCr = 39,57 %
K2Cr4O13 - wCr = 42,09 %
CrO3 - wCr = 52 %
I weighed cca 0,04 g of sample (on four decimal places of course). I also did titration of pure potassium dichromate, just to check that everything
with equipment is alright. General formula, sample weights and FeSO4 volumes below:
wCr = cFe2+VFe2+MCr/(3msample)
mK2Cr2O7 = 0,0410 g
m65 = 0,0410 g
m99 = 0,0418 g
VK2Cr2O7 = 9,04 ml
V65 = 10,94 ml
V99 = 13,45 ml
"65" and "99" refering to HNO3 which was used during the preparation of polychromate.
According to control titration pure K2Cr2O7 contain 35,78 % of Cr, which is in good agreement with theory.
So compound which was made from potassium dichromate and 99% HNO3 contain 52,21 % of Cr, which corresponds to CrO3 (as initially suspected).
Compound which was made from potassium dichromate and 65% HNO3 contain 43,23 % of Cr, which corresponds with K2Cr4O13. Slightly higher result was most
likely due to some residual HNO3 and H2O contamination (tetrachromate crystallized in relatively large crystals, which probably retained some traces
of solvent, know they are crushed to powder and sit in dessicator with KOH which should remove rest of nitric acid. Similar problem didn't occur with
CrO3 because it was in the form of a fine powder).
My further plans are to investigate what conditions lead to formation of potassium trichromate (I'll plan to try some crystallizations from 30-50%
HNO3) and than try to make their ammonium analogues and investigate their decomposition.
[Edited on 10-4-2023 by Bedlasky]
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