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Author: Subject: Write-up on synthesis of K3CrO8, a very energetic peroxochromate
woelen
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Write-up on synthesis of K3CrO8, a very energetic peroxochromate

I again made some K3CrO8, now carefully controlling and writing down the experimental conditions, amounts used, etc. This results in a recipe (ideal for nodrugs cooks ), which can be followed by other home chemists, in the hope that the compound K3CrO8 becomes more widespread among home chemists. It is a very interesting compound, which keeps well (I already have a sample, 17 years old, and now I have a new sample as well).

http://woelen.homescience.net/science/chem/exps/K3CrO8_synth...

The compound looks like very dark brown crystals:

Its aqueous solution is red/brown. It is very energetic. It detonates (yes! detonation) with powdered red P when heated, and it even explodes on itself, when it is heated in the dry state above a flame.

Edit(woelen): Made links work again.

[Edited on 6-4-15 by woelen]

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vulture
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I'm not such a fan of washing this with organic solvents, especially ethanol, which could be readily oxidized.

Other than that, nice experiment!

One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
woelen
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The idea of washing with ethanol, and then ether, comes from a book 'Chemische preparaten, derde druk', written by Edel. This is a Dutch book from just after WW II and it contains a procedure for making K3CrO8 (with a procedure more involved than the one I propose in my webpage). The book suggests rinsing with ethanol and optionally with ether.

If Edel does such a suggestion, then one can fairly safely assume that it is OK

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Jor
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I want to make some as well, but I only have sodium dichomate and sodium hydroxide. Does this synthesis also work for the sodium salt? I dont think I want to buy KOH and K2CrO7 just for making one compound. Potassium dichromate is nastier than sodium dichromate as well,a s it more easily forms dust in the air.
I could try making the sodium-salt ofcourse, but I first wanted to ask if you have experience with that, because otherwise it would be quite a waste of dichromate and hydrogen peroxide.

You should also try making the barium-salt (I see on your website you have bariumhydroxide.) Test the interaction of the barium-salt with red phosphorus. Maybe that mix has other properties, and besides I expect the flame produced to be green.
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Are you sure it's not some polymer? Otherwise where does the electron come from?

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not_important
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 Quote: Originally posted by Jor I want to make some as well, but I only have sodium dichomate and sodium hydroxide. Does this synthesis also work for the sodium salt...

There might be a problem with the sodium salt being too soluble, just as sodium dichromate and permanganate are much more soluble that the potassium salts.
woelen
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@Jor: I agree with not_important, I am afraid it won't work with the sodium salts. You certainly will get such a very dark brown solution, but I have severe doubts whether a crystalline solid settles at the bottom. And if you have such a solid, then cleaning it may be another difficult issue. The first two rinses with cold water might redissolve a lot of the solid.
But you could give it a try on a smaller scale. Dissolve a big spatula of sodium dichromate and a big spatula full of sodium hydroxide in just a few drops of water. You'll get a very concentrated yellow solution of sodium chromate with excess NaOH. Now you have some volume V of yellow solution. Take a volume approximately equal to 2V of 30% H2O2. Chill both test tubes in a refrigerator and make them as cold as possible. Even better would be a really cold ice-bath, made of pre-frozen salt, crushed ice from the refrigerator at -15 C or so and a bit of pre-chilled ether (or acetone if you don't have ether). The colder you can get the better. With this ice-bath you should be able to go well below -20 C. In this ice bath you stick the test tube with yellow solution (also pre-chilled) and you carefully drop the pre-chilled H2O2 in it. At such a small scale you have one thing working at your advance. The smaller amount results in a much lower heat production to surface area ratio. You more easily can keep the mix ice-cold. It REALLY is important to keep the stuff ice-cold, otherwise all H2O2 will decompose.

@Nerro: This is not some polymer. According to literature, K3CrO8 is a peroxo compound. It has chromium in the +5 oxidation state and four peroxo ligands. So, a better formula would be K3Cr(O2)4. The ion is [Cr(O2)4](3-).
A similar type of anion exists for acidic peroxochromates. At low pH, an anion Cr2O12(2-) can be formed, which is very dark blue. Its structure is [(O2)2OCrOOCrO(O2)2](2-). This is a mixed peroxo/oxo anion, with a bridging peroxo-group between the two Cr's, both Cr's being in oxidation state +6. I did not try to make this yet, this is a project for the (near) future. The acidic variation is less stable though.

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Jor
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Ok, Im gonna try the experiment you just told me soon. I will post results when Im done. Are you going to try make the barium-salt or not? I dont have Ba(OH)2, but it would be an interesting compound just like the potassium-salt and you could compair their ' eigenschappen' (Dutch word, I dont remember the English word, woelen do you know?).
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If you perform the experiment with sodium, and don't get crystals, try adding a cold saturated solution of a barium salt - nitrate would be best - and see if that gives a precipitate. I am just guessing, but the alkaline earth chromates are much less soluble than those of the alkalies, so the barium peroxochromate might drop out of solution.

The barium, and silver, peroxochromates appear to exist; there are terse references to them to be found:

 Quote: Silver peroxochromate and sulfuric acid: In attempts to prepare "perchromic acid," a mixture of silver (or barium) peroxochromate and 50% sulfuric acid prepared at -80 deg C reacted explosively on slow warming to about -30 deg C. [Bretherick, L. Handbook of Reactive Chemical Hazards. 4th ed. Boston, MA: Butterworth-Heinemann Ltd., 1990 15]

The sodium salt is mentioned in several patents as a peroxide source for etching copper surfaces. It is possible that it could be forced from solution by the slow addition of cold alcohol or acetone; if that worked washing would have to be done with cold alcohol and acetone or ether used to dry the solid. Try it on a very small scale to start with.
woelen
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eigenschappen = properties
'compair' must be 'compare'

I don't think that experiment with Ba(OH)2 will work. As soon as a barium salt is added to either a chromate, or a dichromate, the highly insoluble BaCrO4 is precipitated. Even in an acidic solution of a dichromate, a precipitate of BaCrO4 is formed. Even the tiny amount of chromate, present in a dichromate solution (by means of equilibrium) is sufficient to precipitate BaCrO4. The solution becomes even more acidic in this way.

Btw, the K3CrO8/P mix does not show a flame. If this mix is ignited, then there hardly is any light output, just an increadibly powerful BANG.

[Edited on 12-3-08 by woelen]

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PHILOU Zrealone
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I'm more than eager to see what happens with (NH4)3CrO8...

(NH4)2Cr2O7 is already famous for its green volcano experiment...

Ammonium peroxochromate might be a high initiating explosive on itself...
2 (NH4)3CrO8(s) --> 3N2(g) + Cr2O3(s) + 12H2O(g) +1/2 O2(g)

Almost perfect OB
Slight oxygen excess, a lot of gases.

PH Z (PHILOU Zrealone)

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woelen
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Of course I tried this as well, but this compound is not made from ammonia, ammonium dichromate and hydrogen peroxide. If these compounds are mixed, then another complex is formed: Cr(NH3)3(O2)2. This complex contains chromium in the +4 oxidation state and it is said to be quite unstable. Not something you want to have around in gram-quantities.

Just for fun at test tube scale: dissolve some ammonium dichromate in concentrated ammonia solution and add some 10% H2O2. After some time, very small black crystals are formed, which stick to the glass en settle at the bottom. Use pre-chilled reagents. I think that this also works with sodium dichromate or potassium dichromate. The complex is a neutral complex and does not contain any ionic species. It is not an ammonium-salt, but an ammine complex. If you decide to do this experiment, don't make more than 100 mg or so!

I don't know of a way of making (NH4)3CrO8. This compound requires very high pH for its formation, but at such a high pH, ammonium ion hardly exists (virtually all is NH3 at that pH). At lower pH, where ammonium ion is present, the formation of the CrO8(3-) ion is inhibited, and other peroxo compounds are formed (such as the one mentioned above, or even Cr2O12(2-)).

[Edited on 12-3-08 by woelen]

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PHILOU Zrealone
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Another brainstorming thing, the structure of K3CrO8 ...
and the structure of the free acid that turns spontaneously in CrO5 a complete apart world (aparte wereld )

Does K3CrO8 display a strong alkalinity when dissolved in cold water?

For the (NH4)3CrO8 maybe via methatesis from Ba3(CrO8)2 or a soluble salt from wich the metal could be precipitated by the counter anion joined to the ammonium.
Ba3(CrO8)2 (aq) + 3(NH4)2SO4(aq) --> 3BaSO4(s) + 2(NH4)3CrO8(aq)
It is possible that the sulfate anion would be oxydised to peroxosulfate or peroxodisulfate...

NH2-CO-NH2 could also be tested, there is an adduct of NH2-CO-NH2 and H2O2 that is crystaline NH2-CO-NH2.H2O2 ; urea might dus witdraw the oxydative power of the peroxochromate.

[Edited on 13-3-2008 by PHILOU Zrealone]

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Jor
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EDIT: stupid post

[Edited on 13-3-2008 by Jor]
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the last time I made this a year or so ago, the yield was quite low, but seeing the Exact instructions now (and still faced with the same problem of only having 9% H2O2) I see that you made the Chromate and Hydroxide soln up to 30ml, if I were to leave it at the 20ml, would that make up for the extra water in the dilute H2O2 that I have?

Logically is seems to be a Sound idea, but in Chem, sometimes Logic is not the case!

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With your 9% H2O2 I expect that you will have a reasonably good yield. Try to make a solution in only 15 ml of water and if not all k2Cr2O7 dissolves, then add 1 ml more. Increment in steps of 1 ml and really do your best to dissolve it in as little as possible of water. Then you can add 60 ml of your 9% H2O2. It is a little bit less than 40 ml of 15% H2O2, but if your solution is chilled VERY well and kept this way, then the lower amount of H2O2 does not need to be a problem. My recipe already has a built in 50% excess of hydrogen peroxide. I expect you to get a nice crop of K3CrO8, maybe even 5 grams or better.

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Excellent thank you

Im actually making some more now, and yes I had to do the watter addition in increments, now I have a very strong soln and just hope it doesnt decide to form crystals in the fridge as it seems pretty saturated and its quite hot from the KOH dissolving.

just out of Curiosity, have you tried an Impact test on this stuff, like a hammer drop?
I only did the RP reaction, and never tried just direct heat, Im wondering now if it maybe Impact sensitive

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Done:

its not as Good a yield as your 5g but Im quite pleased with it, and the Purity is Very good, I lost quite a bit washing it (used 50ml washes rather than 7ml, and the same amount for the Alc washes too).
its worked MUCH better than my prior attempt and Im very happy with 3.5g as I have no Real use for it, but it makes a great sample size for future ref.

Cheerz Woelen!

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woelen
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Nice that you also have some of this material now. A few nice experiments:
1) Heat a small amount in a test tube, as dry crystals
2) Crunch a small amount (a really small amount, something like at the tip of a small screw driver) and mix this carefully with half the amount of finely powdered red P and ignite. You WILL be impressed!
3) Dissolve some in water, and add a small amount of dilute HCl or H2SO4. Very nice change of color! Extract with ether.
4) Do the same as (2), but replace part of the red P with fine Al-powder, preferrably german dark en throw a small amount of this mix in a strong test tube, filled with Cl2. Again, you WILL be impressed.

With exps (2) and (4), be very careful not to apply friction on the mix. I once burnt one of my fingers with this (you may remember, well over 1 year ago now, I posted that on sciencemadness as well).

Next weekend I will try the hammer test and a friction test.

PS: Why did you use 50 ml washes? Isn't that a waste of chems (esp. the alcohol and ether)? 5 rinses of 10 ml clean better than 1 rinse of 50 ml and in this case, the losses also most likely are somewhat less.

Just some math: Suppose you have a glass, filled with 1 ml of dirty liquid and you rinse this with 10 ml by good mixing and swirling. After rinsing and decanting, 1 ml of liquid is left. This left-over liquid has 1/11 the impurity as the original one. If you do the same rinse again, then the liquid has 1/121 of the impurity left. For 5 rinses, the liquid only has appr. 1/150000 of impurity left. (assuming that no impurity sticks in holes, cracks etc.).
If you take 50 ml in a single rinse, then you dilute 51 times.
So, with 50 ml you can cleanup much better than simply appr. 50 times. That is why I always use small rinses, repeated. Only if a compound is VERY soluble, then you may want to use a single large (and quick) rinse in order to avoid excessive losses.

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YT2095
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Quite simple I was using a 400ml beaker to do it all in (I dont have any smaller) and 7ml would have been totaly lost in the bottom of it, so 50ml washes with ice water and done quickly worked nicely.

I just tested for impact sensitivity and I could get it do anything, it was in Thin paper (in case metal had an effect) and then well and truly smashed hard, nothing...

interesting to note though, the Crushed crystals are actually Yellow/Orange in color and not the brown looking as the larger crystals appear.

I lit the paper after to make sure it Hadnt "gone off" quietly, and got a purplish flash, so Im satisfied that its in no may dangerous with any impact I can provide.
I cant do a proper friction test (I dont know how its done correctly) but grinding it up on frosted glass with a frosted glass bobby had no effect (also makes a yellow/orange powder).

Additional:
since we Know RP with a good many Oxidisers is never a good idea as its too harsh, I tried with Sulpher this time, it doesnt explode in the way that it does with RP, but closer to a Flash powder, and Potassium benzoate as the reductor isnt really anything great at all.

Edit 2:
I did the HCl and got a green color with bubbles (I assume O2)
but then I tried GAA, I figured not only is it an Acid, but Also a Fuel (reductor to a degree) and That was VERY nice change, to a beautiful Purple color, the bubble release is Much slower however.

Edit 3: interestingly if you add water to the now purple GAA (ass I did while washing the test tube out), it totally destroys the color towards a gray and then Plenty of bubbles are given off.

[Edited on 15-3-2008 by YT2095]

[Edited on 15-3-2008 by YT2095]

[Edited on 15-3-2008 by YT2095]

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These compounds are NASTY!

Well I have made K3CrO8, (NH4)3CrO8 and [Cr(NH3)3(O2)4] as well. I obtained decent yields (>70%) and was rewarded with 3 very intriguing compounds. Yet I DON'T recommend making this stuff! Peroxychromates are about the most evil compounds I have ever made. They are corrosive and cause a stinging paing on the skin, like sodium or hydrogen peroxid. They are EXPLOSIVE! They burn when ignited, even without fuel. They explode when ignited under confinement - again, even without fuel! They are impact and friction sensitive. Plus they have been shown to induce DNA cleavage. Making them requires thorough temperature control and one needs to work very quickly, because these compounds have to be made in aequous solution yet water quickly decomposes them. The synthesis I will describe hereby requires to add ice-cold H2O2 to a pre-chilled chromate solution in an ice-salt bath, while stirring vigorously and monitoring temperature all the time so that you can pause the reaction whenever temperature goes above -5°C. The reaction will release O2 and thus cause considerable bubbling, producing ultra-fine droplets. Knowing that the beaker your looking at produces an invisible aerosole of a substance that has been shown to cut human DNA into pieces doesn't make this synthesis a very enjoyable one. Subsequent cleaning of glassware and anything else that came into contact with the peroxychromate was grueling and produced a huge amount of contaminated water.
So in retrospect, I regret making this nasty stuff. Anyways I want to share my findings. Especially because they are quite different from woelen's. The first time I made K3CrO8 I closely followed woelens instructions, and I obtained black glistering crystals that closely resembled KMnO4. However the "red perchromates" are said to be red or brown, not black. I don't know if woelens compound is just a modification or an entirely different compound. But I found a way to make peroxychromates that look exactly like the chemistry book description!

SYNTHESIS OF K3CrO8 AND (NH4)3CrO8
============================

It is absolutely essential to keep the temperature well below 0°C throughout the whole reaction! In my case I made sure it never climbed above -5°C. It is absolutely NO PROBLEM AT ALL to have a partially frozen solution. The reaction will take place even in a completely frozen solution. I could observe red-brown perchromate crystals growing on the sides of the beaker altough everything was frozen rock-hard. It just takes a long time. I kept the mixture in the refrigator for 24 hours and obtained >70% yield. It was striking that the solution NEVER TURNED BLACK, unlike woelens one. First the solution becomes red-brown and then it darkens a little, but at no point it was black. I found it impossible to keep the temperature at -5°C without an ice-salt bath, so that might be the reason why woelens compound looks so different.

Temperature control is even more important with the ammonium salt! Ammonium tetraperoxychromate easily turns into [Cr(NH3)3(O2)4] ("Weide's compound") if
-temperature is allowed to get close to 0°C
or
-insufficient H2O2 is used
In these cases, some of the O-O ligands are replaced by NH3, forming the yellow-brown "chromium tetroxide triammine", which is also explosive.
Ammonium peroxychromate decomposes considerably faster in aequous solution than the potassium salt. However, the dry compound is actually much more stable!

So here's the recipe:

K3CrO8: Dissolve 10g K2CrO4 and 6g KOH in minimum amount of water. Chill it until most of it is frozen but make sure that nothing crystallizes again! I had to add small amounts of water several times to make sure all stays in solution even when freezing cold. Prepare an ice-salt (ice+NaCl) bath. Also chill 32ml H2O2 (30%) to 0°C.
Put the (mostly frozen) chromate solution into the ice bath and immediately start adding the H2O2 in small portions with vigorous stirring. Never allow the temperature to go above -5°C. I had to pause the additions several times because each addition causes a significant warming. Once all H2O2 is added, leave the solution in the ice bath until it is frozen. Then put it into the refrigator and keep it for 24 hours. Remove from refrigator and let it warm up. As soon as all ice is melted, quickly decant the liquid and wash the red-brown precipate with ice-cold water and cold ethanol. Press dry between filter paper. Let it dry in warm air. Yield: 11g.

(NH4)3CrO8: Disspolve 8g (NH4)2Cr2O7 in 15ml ammonia (25%). Add just enough water to make sure everything dissolves and stays in solution even if the liquid is ice cold. Chill it until most of it is frozen. Also chill 42ml of H2O2 (30%). Follow the instructions above. Yield: 12g.

[Cr(NH3)3(O2)4]: I have not yet developed a good recipe with stochiometric amounts. I simply added H2O2 to an ammoniacal solution of (NH4)2Cr2O7 at room temperature. Upon cooling I obtained a yellow-brown powder. This compound theoretically can also be obtained by dissolving ammonium peroxychromate in a strong NH3-solution and heating it to 60°C. When I tried this the solution started bubbling (O2!) above 50°C, indicating that some of the O-O ligands were displaced by NH3. However nothing precipated upon cooling. It could be that the solution was too dilute, and I was too lazy to put it in the freezer so it was disposed of. The compound is stable in very alkaline solutions. It can be kept under conc. ammonia for weeks without decomposition.

PROPERTIES
=========
K3CrO8: Red-brown powder. Burns violently with a white flame and yellow smoke (chromate vapour) when ignited in the open. It doesnt need fuel to burn! However when mixed with sulfur, it forms a flash-powder like mixture. Even if its lazily mixed and only coarsely ground, it burns instantaneously with a whoosh! sound and a very bright flash. Quite fascinating! I dont know of any other oxidizer that can turn such a lously fuel into a flash-type composition, especially without proper grinding & mixing!
I wrapped some perchromate into several layers of aluminium foil and heated it (no sulfur added!) and it deflagrated violently. It requires quite some confinement to really detonate. Even when mixed with sulfur it doesnt DDT easily - unlike NHN for example. So most of the time I only got violent deflagrations, but one time it did explode from just heating it under confinement.
When struck with a hammer, it explodes. Again, no fuel needs to be added to accomplish this! Hitting it with a pestle in a mortar also causes small explosions, however it requires quite a lot of force to make it go boom. I mixed a small amount with sulfur and hit it with a hammer - and that gave a very very powerful detonation!

(NH4)3CrO8: Brown powder. Definetely brighter than the potassium salt. When mixed with KOH and wetted with warm water, a distinct smell of ammonia is observed. I dont know if this is a definite proof, but it indicates that this really is an ammonium salt and not an ammine!!! When ignited in the open, it burns much like ammonium bichromate. It is fascinating that the ammonium salt decomposes so much more calmly than the potassium salt! It rather smoulders than burns, producing a lot of sparks but without the bright flame that can be observed when K3CrO8 burns.
When hit with a hammer, it detonates with a loud report. Just like the potassium salt, it needs strong confinement to detonate from heat. I wrapped several miligram quantities in aluminium foil and heated it with a bunsen burner. I never got a real detonation, just very violent deflagrations and a dull report from the bursting foil.
HOWEVER I was curious how it would react with a fine metal powder. I only have Fe powder (need to buy some Al real soon!), so I tried that. I mixed some peroxychromate with a small amount of ultrafine Fe powder (no measurements were made), wrapped it into aluminium foil and hit it with a hammer. Boy THAT was impressive! The report was deafening. The aluminium foil completely disappeard. Plus, there were small sprinkles of molten aluminium burnt into the anvil.
I did no precise measurements, but I had the impression that the ammonium salt requires a stronger impact to be initiated than the potassium salt.

[Cr(NH3)3(O2)4]: Yellow-brown powder. Unlike ammonium peroxychromate, it burns instantaneously (like flash powder) when ignited in the open. This might be an easy way to distinguish the two, because they look quite similar! When struck with a hammer, it also explodes.
I only made a very small amount of it, and I did no further experiments yet.

--------

So to recap, these are very very dangerous yet intriguing compounds. The ammonium salt when mixed with Al powder will most certainly give an excellent primary. The Al powder would be able to reduce the Cr2O3 from decomposing perchromate, unlike the Fe I used. And even tough I only used Fe powder, I got an incredibly powerfull explosion.

BE CAREFUL!

[Edited on 16-3-2008 by Taoiseach]

[Edited on 16-3-2008 by Taoiseach]

[Edited on 16-3-2008 by Taoiseach]

[Edited on 16-3-2008 by Taoiseach]
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Edit: shrunk image slightly (no attachments > 800 pixels wide, please)

[Edited on 3-16-2008 by Polverone]

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Some photos...

A closer look at (NH4)3CrO8

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Some photos...

This is the frozen reaction mass after removed from the refrigator for melting. The whole reaction took place at -5°C, and the mixture was only allowed to reach 0°C just before decanting the liquid and washing the crystals with ice-cold water and ethanol.

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Some photos...

A small sample of the potassium salt. It is definetely darker than the ammonium salt and more reddish. However, unlike the compound that is formed by woelens synthesis, it is not black.

Is the black version a different compound or just a modification?

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