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]

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

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.

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]

Excellent thank you

I`m 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 doesn`t decide to form crystals in the fridge as it seems pretty saturated and it`s 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, I`m wondering now if it maybe Impact sensitive

Done:



it`s not as Good a yield as your 5g but I`m 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).
it`s worked MUCH better than my prior attempt and I`m 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!

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]

Edit: shrunk image slightly (no attachments > 800 pixels wide, please)

[Edited on 3-16-2008 by Polverone]

Some photos...

A closer look at (NH4)3CrO8

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.

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?

I still think that I have K3CrO8, but the crystals are larger. If the material is crushed, then it becomes brown. This also is what YT2095 has noticed already. I have another sample, with smaller crystals and that indeed is somewhat more brown:

The compound, from a previous batch:


The last batch:


I also noticed the difference between my two samples, but on further testing I can only conclude that they are the same compound. Both give a red/brown solution, both give a blue solution on acidification, and both violently explode on heating, giving exactly the same kind of yellow smoke. When mixed with S it behaves like flash, with red P it detonates.

So, I am quite confident that they are the same compound, but either in a different crystal shape, or a different purity. Actually, I have the impression that the black material is more pure. These crystals really are nice and glittering, while the brown material has more dust-like stuff around the crystals. Actually, the crystals are not really black, I already wrote in my first post in this thread that they are very dark brown.

A similar thing I know from anhydrous copper(II) chloride. This stuff always is described as yellow/brown. I once made this, and I obtained dark chocolate brown crystals. On adding them to water they nicely gave a clear green/blue solution and on crushing them, I obtained a yellow/brown powder. So, again I think that the situation here is similar. Large crystals look darker.

The experiment with ammonium peroxochromate looks very interesting. I knew of the ammine complex and never dared making more than a few mg of that, but the ammonium salt of the peroxo chromate ion sound very good. I'll definitely try that one myself.

I do not regret that I made this compound. It actually adds a nice chemical to my set of chemicals. And yes, you have to be careful. I also mentioned it on my web page, you really have to be careful for the spraying droplets of solution. Chromates, and also perchromates are toxic carcinogens. But there are nastier chems.


EDIT: There is one difference between both of my samples and the sample of Taoiseach. My samples explode on heating, even when not confined and not mixed with any other chemical. I never could get it to burn, it explodes, as shown on my website.
YT2095, could you put a tiny(!!) amount in a test tube and heat that above a flame? Does it burn, or does it explode with a POP sound? Don't do this test in open air, use a wide test tube. It still is unconfined in that case, but no K2CrO4-smoke is spread in the air.

This is another page on K3CrO8 I have written some time ago.
http://woelen.scheikunde.net/science/chem/exps/raw_material/...
It also contains a synth method, much like the one, presented here. I have done that synth two times, but one time I had a runaway, even from ice cold material. Hence my new method which uses much more dilute H2O2 (it is based a method from Edel, who uses even more dilute H2O2).

As you see, interesting material! I certainly will go more into it and the contrinution of Taoiseach is appreciated very much . Good to see that this stuff raises so much interest!



[Edited on 17-3-08 by woelen]

[Edited on 17-3-08 by woelen]

Potassium tetraperoxochromate (V)

UnintentionalChaos, very good and deep correction!

UnintentionalChaos, you are absolutely right

I have regarded the bubbling of oxygen throughout the reaction simply as a side reaction of decomposition of hydrogen peroxide. During the entire reaction, bubbling continued and I simply was thinking it was decomposition of hydrogen peroxide, nothing more. Stupid me .
I already was wondering why this happended, even at -15 C in the refirigerator.

I certainly will change this in my webpage. The nasty thing about peroxide reactions is that stoichiometry of these reactions cannot simply uniquely be determined by balancing elements and charges, left and right. A similar problem occurs when one is writing down the equations for reduction of permanganate by hydrogen peroxide.

This indeed is the correct equation: 2CrO4(2-) + 9H2O2 + 2OH(-) ---> 2[Cr(O2)4](3-) + 10H2O +O2

The nasty thing of this equation is that it is the addition of the following:

2CrO4(2-) + 7H2O2 + 2OH(-) ---> 2[Cr(O2)4](3-) + 8H2O
2H2O2 --> 2H2O + O2

The latter is simple decomposition of hydrogen peroxide. And that is where I made a mistake.

The problem can be resolved by assuming preservation of ligands for part of the H2O2:

H2L, L = [OO], L is preserved and we write CrL4(3-) for the peroxochromate ion and H2L for the part of the hydrogen peroxide which goes into ligand transfer.

The other assumption needed is pure reduction and no decomposition of H2O2. Another preservation of groups needs to be introduced:

H2Ox, Ox = [O2], Ox is preserved in reduction of hydrogen peroxide: H2Ox --> 2H(+) + Ox + 2e. We write H2Ox for the part of hydrogen peroxide, which is reduced.

Now we have to solve the equation

CrO4(2-) + H2L + H2Ox + OH(-) --> CrL4(3-) + H2O + Ox

Now, using preservation of L and Ox, the equation can be uniquely determined and this is exactly your equation:

2CrO4(2-) + 8H2L + H2Ox + 2OH(-) --> CrL4(3-) + Ox + 10H2O

So, the trick is group preservation of L and Ox.


UnintentionalChaos, thanks for your deep correction. Without this it would have remained erroneous forever! Tonight I'll change this and I'll also change some other webpages, which deal with the K3CrO8 compound.




[Edited on 17-3-08 by woelen]

The reaction also was quite well behaved for me, but it sure did bubble. I had many tiny bubbles during the entire period that the reaction was running. So, for anybody doing the reaction, PLEASE COVER UP THE BEAKER by filter paper, a piece of glass or use a loosely capped bottle. I used filter paper, and the paper was brown at the entire circle, where it covered the beaker.
Twenty years ago, I did the same reaction, but at that time I simply dumped 40ml of 30% H2O2 in the mix. A dark red/brown fountain of hot foam was spewn out of the beaker, almost 1 meter high and there was a lot of steam. Wooo that was frightening. Hence my modification to use dilute H2O2.

the reason I didn`t use Alu is that a "thermit" type reaction could take place doing that.
and that could give a false reading of sensitivity.

the paper I used was the VERY thin cigarette rolling papers (Rizla Silver pack).

edit: I think the bad burn you had on your finger some time back is testamony to it`s reactivity with alu

[Edited on 17-3-2008 by YT2095]

Again a few hours in the lab

So in the UK, people exposed to the normal environment die at 37% (1/3rd) from cancer, whilst only 24% of [Cr6+] -exposed die from cancer?
Does this mean Cr6+ is an anti-cancer drug, saving an estimated 13 % of potential cancer patients?

Let's not get too excited about Cr6+!

Also let's not forget the benzene we happily fuel every few days, straight into our cars! Noone bats an eyelid there!

<back on topic>

Peroxochromates

I'm still quite positive I do have the right stuff, simply because I have found a lot of references in literature which describe peroxychromates as red-brown, yet not a single one describes them as black.

Nevertheless woelens hypothesis of a chromate-peroxychromate double salt sounds very plausible. I will test my compounds with BaCl2. I figure the hypothetical double salt would dissociate into CrO8(3-) and CrO4(2-) in aequous solution, the latter would precipate as light yellow insoluble barium chromate on addition of Ba(2+).

I'm very curious about the outcome because it could as well be the other way around! It is not unusual that mixed-valency compounds look black, so woelens stuff might actually contain Cr(V) and Cr(VI). Possibly due to decomposition of the peroxychromate at temperatures close to 0°C, according to the reaction

2[Cr(O2)4] (3-) + H2O ===> 2CrO4(2-) + 2OH(-) + 3,5O2

The difference between woelens and my synthesis is that I used much lower temperatures and (I think) less alkaline solutions.

There's a very interesting paper "EPR Spectroscopic Studies on the Formation of Chromium(V) Peroxo Complexes in the Reaction of Chromium(VI) with Hydrogen Peroxide" which proves that there at least 2 more species are formed upon the addition of H2O2 to alkaline chromate solutions:

[Cr(O)(O2)2(OH2)] (-)
[Cr(O2)3(OH)] (2-)

The formation of [Cr(O2)4] (3-) is favored by excess of H2O2. It is actually proven to be formed over a wide range of pH if excess H2O2 is present.

So could it be that woelens and my compounds differ by the number of peroxo-ligands, because they get partially replaced by oxo- and hydroxo-ligands depending on temperature & pH?

Anyways we can already conclude from that paper that a strong excess of H2O2 is more important than high pH. A final writeup on the synthesis should pay attention to that. I would rather use a huge excess of H2O2 and hamper decomposition of peroxychromate by very low temperatures rather than highly alkaline solution.

Btw I have found a much better photo from my perchromate synthesis which shows the half-frozen mass just after the H2O2 has been added. You can see the thermo isulation box (I made from styrofoam pieces) which was filled with 3 parts ice + 1 part table salt. This made sure the whole mix never completely melted and temperature was well below 0°C all the time. Of course this could have been just the wrong decision and cause chromate to be included in the crystal lattice... we will see!

Peroxychromate impact test

diperoxotriamminechromium(IV)

woelen was right about diperoxotriamminechromium(IV); its forumla is [Cr(NH3)3(O2)2] and not [Cr(NH3)3(O2)4].

There is a synthesis for this compound in Brauer's Handbook of Preparative Inorganic Chemistry, vol 3.:

Cool a mixture of 25 ml 10% ammonia and 5ml 50% solution of CrO3 to 0°C, then add 5ml 30% H2O2. Let is stand in an ice bath for 1h. Then warm to 50°C until almost all has redissolved and the vigorous bubbling has ceased. Filter to remove precipated (NH4)2Cr2O7 and cool the clear liquid to 0°C. Yield: 0,3g.

(Can anyone compute the right amount of ammonium chromate from these details? I have no clue how much CrO3 there is in 5ml of a 50% solution)

Properties: Light-brown needles, soluble in dilute ammonia, slightly soluble in water under decomposition, insoluble in other solvents. Explodes on heating.

I start to believe that my putative [Cr(NH3)3(O2)2] is actually not a pure compound but a mixture of perchromate and the ammine complex. It does contain light-brown needles, but also some other stuff more dark in color. I did not warm the solution to 50°C when I made it, so that might be the reason not all perchromate was converted.

To add to the confusion, there's a paper "Some Decomposition Reactions of diperoxotriamminechromium(IV)" by R.G. HUGHES, E.A.V. EBSWORTH and C.S.GARNER. It describes the compound as lustrous black needles. I once obtained such black needles from K2Cr2O7 and H2O2 in ammoniacal solution at room temperature. However, when I tried the experiment again, I obtaind a mixture of black needles and fine red crystals. It might have been a change in temperature and/or concentration of ammonia which caused the different outcome. The red stuff is said to decompose in ammonia water in the K. A. Hofmann und H. Hiendlmaier paper. In their diperoxotriamminechromium(IV) syntheses, they always produce a mixture of peroxychromate and the ammine complex, and then wash away the peroxychromate with ammonia water at room temperature. At least, this is consistent with Brauer's synthesis.

The original paper on peroxychromates by K. A. Hofmann und H. Hiendlmaier says that the reaction depends on both temperature and concentration of NH3 and either yields ammonium perchromate or the triammine complex. However, that paper dates back to 1905. There is another statement from ERNSTH . RIESENFELD, WILLIAAM. KUTSCH and HERMAN OHL which is of great interest: "Contrary to Hofmann and Hiendlmaier’s statement (Abstr., 1905,ii, 716), the action of hydrogen peroxide on ammonium chromate, whether
containing 2.5 per cent. of free ammonia or saturated with ammonia,leads to the formation of ammonium perchroinnte, (NH,),CrOs, unless insufficient peroxide is used or the temperature is allowed to rise above 0°C, when a mixture of ammonium perchromate and Weide’s triaminine chromium tetroxide. Cr04 3NH3 (Abstr., 1898, ii, as), or the latter alone, is obtained. When treated with 10 per cent. ammonia, ammonium perchromate changes immediately at 40°, but
only slowly at the laboratory temperature, into Weide’s compound, which separates in needles and rectangular and rhombic plates. The three forms have the sp. gr. 1.964 at 1 5-S0, and when examined crystallographically appear probably to be identical, and do not constitute two isomeric substances as stated by Hofmann and Hiendlmaier (Zoc.
c i t . ) . The action of dilute sulphuric acid on Weide’s cornponnd leads to the formation of chromium sulphate and hydrogen peroxide, but not of chromic acid ; the amount of oxygen evolved varies from 3.54 equivalents with 1 per cent. to 2.24 equivalents with 20 per cent.
acid."

Perfect confusion then...

To make things even worse, the R.G. HUGHES, E.A.V. EBSWORTH and C.S.GARNER paper indicates that the ammine-complex forms NH3 upon addition of NaOH and warming, so that might not be a good way to distunguish it from ammonium perchromate. Damn

[Edited on 21-3-2008 by Taoiseach]

Btw check this out:

CALCIUM PERCHROMATE. A NEW TYPE OF RED
PERCHROMATE

Its the first google hit that turns up. Full pdf can be downloaded FOR FREE.

It describes that addition of H2O2 to freezing cold chromium hydroxide leads to formation of what they call "red perchromic acid". Addition of calcium chromate or acetate leads to the formation of calcium peroxychromate. I think they are wrong about structural formula and valency tough. The paper is from the early 30s so at that time they might not have fully understand the complex coordinative chemistry behind peroxo-chromium compounds.

Anyways this compound might be useful to prepare other perchromates by double displacement reaction with metal sulfates. Also, barium peroxychromate could be prepared conveniently from this "red perchromic acid" as well.

One might even dream about nickel hydrazine peroxychromate - a compound right out of devil's kitchen

I have papers on Rb and Cs peroxychromates (including synthesis), but they are worthless for practical experiments. The Cs compound explodes upon seperation from the solution and the Rb compound is unstable at room temperature.

[Edited on 21-3-2008 by Taoiseach]

I made a webpage about the triammine peroxo complex of chromium(IV):

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


This compound has properties, close to those, described by Taoiseach. It burns very fast, when heated.

Next project will be the making of the "red perchromic acid". Just making Cr(OH)3, filtering/rinsing and adding H2O2 does not sound very difficult. The element chromium, combined with peroxide, is good for MANY interesting experiments.

Whoever tries to repeat this experiment: be VERY careful, avoid inhaling of fine droplets of the fizzling solution, and do not scale up. I am not sure about the stability, and if it deflagrates, then the damage from 100 mg can be overcome, but the damage of 10 grams may be a disaster!! You are warned.

The picture below shows my K3CrO8 and the Cr(NH3)3(O2)2:




Edit(woelen): Made links work again

[Edited on 6-4-15 by woelen]

There are papers which say that the potassium salt decomposes at room temperature as well - but it obviously doesn't! woelen has kept his sample for many years already.

There is a dissertation "Preparation, Structure and Vibrational Spectroscopy of Tetraperoxo Complexes of CrV+, VV+, NbV+ and TaV+" which gives a detailed synthesis procedure for the ammonium salt. They stored it at -4°C yes. But they did the same with the potassium salt to "prevent possible oxidation to chromate!". Altought we know that this is unnecessary because the salt IS stable at room temperature (given its completely dry, that is). It also says that "All these compounds are very unstable as they decompose very rapidly at room temperature" - but this is not true, at least not for the potassium salt. In fact the only decomposition at room temperature the author actually observed was that of ammonium tetraperoxovanadate, (NH4)3[V(O2)4]. It seems like he was too lazy to investigate decomposition for all other tetraperoxo-compounds and stated that they would be similarly unstable.

He has also recorded x-ray diffraction patterns which show the ammonium and potassium salt to be of the same structure, so it is very unlikely he has mistaken the triammine complex for the ammonium salt. Thus I'm quite sure the ammonium salt DOES exist and it CAN be prepared - altough it is little known in literature.

Btw in "Ignition of Explosives by radiation" ammonium tetraperoxychromate is mentioned as a "red-brown" powder with a detonation temperature of 90°C.

Stability might have to do with water of hydration. Hydrated peroxychromates such as Li3[Cr(O2)4]*10H2O, Na3[Cr(O2)4]*14H2O, and Cs3[Cr(O2)4]*3H2O are known. The hydrated potassium salt is said to loose water of crystalliation if it is washed with ethanol or ether (I'm quite sure that acetone also works). The water stored in the crystal lattice might be detrimental to stability if it is lost easily and then accumulates to take some of the compound into solution where it decomposes. That might be the reason why the "wet peroxychromates" are said to be so unstable.

So to summarize:

-A newer dissertation from 2003 proves that the ammonium salt does exist and is isomorph to K3CrO8 (no ammine!)
-It is not yet proven that the ammonium salt is unstable at room temperature. Overanxious handling of other peroxychromates, such as storing K3CrO8 at freezing temperatures altough its not necessary, is commonly found in literature
-Water of crystallization might be a major factor in decomposition. Thorough washing with ethanol/ether seems to be a very good idea!

I have done some calculations on the [Cr(O2)2(NH3)3] synthesis given in Brauer. It uses 3,75g CrO3, 25ml 10% ammonia and 5ml 30% H2O2. There is a striking shortage of H2O2!

Considering that the reaction should go something like

CrO4(2-) + 3H2O2 + 2OH(-) + 3NH3 ---> [Cr(O2)2(NH3)3] + 2H2O + O2 + 2O(2-)

(the 2O(2-) would be protonated to 2H2O in aequous solution)

there should be 11,6ml of H2O2 - twice as much as used in Brauers synthesis!

Now FINALLY something falls into place This is consistent with the ERNSTH . RIESENFELD statement I cited earlier, stating that the triammine complex is formed only if

1. the temperature is allowed to rise above 0°C
and/or
2. insufficient peroxide is used (!)

otherwise, ammonium perchromate is said to be formed.

Given that the Brauer synthesis settles for such a shitty yield as 0,3g of the triammine complex from 4g of CrO3 and goes through the hassle of filtering out unreacted (NH4)2CrO4, we now have a strong hint that the peroxide shortage is essential for the formation of [Cr(O2)2(NH3)3]!

This might be due to NH3 and O-O competing for ligand places around the Cr(+IV). A shortage of H2O2 likely prevents formation of peroxychromate. Also, temperatures well above 0°C destroy any peroxychromate and leave the triammine complex only, which is said to be stable in strong ammonia water. In one paper they replaced the NH3 by CN by heating the triammine in ammoniacal solution with KCN for 5 hours, so dont't be shy to turn up the heat!

-----

Summary:

-The triammine complex is much more stable at elevated temperatures than CrO8(3-)
-Formation of the triammine complex is favored by shortage of H2O2
-Any peroxychromate can be turned into the triammine by heating with ammonia water to 50°C

If the above is correct, [Cr(NH3)3(O2)2] can be made conveniently from potassium dichromate and H2O2! No need to use fancy ammonium dichromate which is hard to come by. Reaction with potassium dichromate would certainly go though an peroxychromate intermediate which can be restrained/destroyed by shortage of H2O2 and heating.

Will try that soon. Btw Diperoxoaquoethylenediaminechromium(IV) also sounds interesting, but is said to change color within a few days at room temperature. It explodes violently when heated.

Bad news :(

I can only say WOW!

I did a hammer test with the K3CrO8 and with a mix of K3CrO8 and sulphur.

Pure K3CrO8 could easily be decomposed, by simply hitting with a hammer. No hard banging was needed. The decomposition was accompanied with formation of some smoke and some noise. Not really impressive, but it was very instructive to see how easily I could get it decomposed, simply by hitting with a hammer. I did the test with some of the solid, wrapped in a small piece of Al-foil.

Next, I did a test with K3CrO8 + S. No measured amounts, just mixing appr. 2 parts of K3CrO8 (coarsely crushed crystals) with 1 parts of flowers of sulphur, amounts estimated by the eye. Again, wrapped in Al-foil and carefully tapped with a hammer, on a concrete tile. The result was really impressive , given the small amount used for this experiment. There was a very loud bang, at first hit!

I can confirm the results of Taoiseach, the material is very energetic, and also impact sensitive, more so, than I expected.

Yes woelen it needs little energy to decompose, even without fuel. I got scared when I realized how easily it goes off - I had made way too much of this stuff, in retrospect.

NHCrO8 + Fe was even more impressive. I'm not impressed easily as I've synthesized a bunch of primaries and tested them before. It was incredibly loud and gave a very long-lasting shockwave. Its a pitty this stuff decomposes.

Anyone dare to try hydrazine perchromate?

Quote:

Originally posted by Taoiseach Anyone dare to try hydrazine perchromate?

Fun for the whole family, right?

No mention of the hydrazine but here's a rundown of the idea of explosibility of some other peroxychromates (from Gmelin Cr [B] p. 750-2 and Cr [C] 410-8).

trimethylammonium peroxochromate, black solid, when the apparently very pure substance was on a scale to be analyzed until the very small amount exploded with a deafening bang.

tetramethylammonium peroxochromate. [(CH3)4N].CrO5. brown-violet solid, hisses from strong heating giving off O2 and forming a bright yellow powder, which if heated further burns giving off sparks. Alkali hydroxides, acids, BaCl2- or AgNO3- solution decompose it.

piperidinium peroxochromate (C5H12N)CrO5. in direct light black, and partial light violet solid (from cold dark brown oil). Pretty stable when in the cold. Upon heating creates a bright explosion.

anilinium (C6H8N)CrO5. After its preparation, as it is being dried shortly on clay, and then KOH for 4 hours in the desiccator, it is recommended to surround this with ice, since the compound explodes far more violently than the pyridinium compound. The compound decomposes in 1 to 2 days, decomposes by heating and with conc. H2SO4 or alkalies under explosion. With dilute acids or alkali hydroxides releases O2.

chinolium.CrO5. little crystaline plates, not stable for long. Decomposes from heat with the evolution of light and forming chromium oxide.

guanidinium peroxochromate (CN3H6)3CrO8.H2O. yellow-brown powder. In the dry state can be stored for several weeks. The compound is not explosive, does not ignite when grinded in a porcelain bowl, but hisses when heated giving off white smoke.

1,10-phenanthroline.CrO5: light-blue powder. Stable at room temperature. Non-explosive. Conc. H2SO4 liberates O2 and forms Cr2(SO4)3. With boiling alkali solution liberates O2 and forms chromate.

ammonium peroxochromates: (NH4)2CrO5, (NH4)3CrO8, and (NH4)2Cr2O12.2H2O have been identified.

(NH4)2CrO5 is very unstable and the brown solid will ignite spontaneously a little above room temperature, with H2O it forms a yellow orange solution.
(NH4)3CrO8, small red-brown, octahedral crystals with a reddish glance. This compound must be analyzed in the wet condition, since when completley dry it decomposes. It is storeable for a longer period in an atmosphere saturated with H2O-vapors. At about 40 deg.C. it converts to the chromate, at 50 deg.C. it decomposes explosively forming chromium oxide. Pouring conc. H2SO4 over the salt gives a violent reaction with the evolution of light and forms Cr2O3 which vaporizes in green flakes. The same reaction results when bumped or shocked but here is a strong detonation. It is decomposed noticeably from H2O even at 0 degrees.
(NH4)2Cr2O12.2H2O: violet-black powder. The dry compound is storeable in the dry state for a few days, but in air decomposes within 24 h completley to (NH4)2Cr2O7. Burns with a loud hiss when heated strongly, forming chromium oxide and red-brown nitrogen oxides vapors. The conc. aq. solution decomposes pretty quickly to (NH4)2Cr2O7 and liberating oxygen.

[Edited on 10-4-2008 by Hammerl]

I tried to make the barium salt of CrO8(3-), but this does not work.

I have tried the following: Dissolve some K3CrO8 in a solution of NaOH. The reason of doing this is that dissolving in plain water results in visible decomposition (very small bubbles of oxygen appear). In a solution of NaOH this does not occur. Problem I have, is that K3CrO8 is only VERY sparingly soluble. In a test tube, half filled with a solution of NaOH, I only can dissolve a tiny spatula of solid K3CrO8. Heating does not work, if I do that, then I again see bubbles of oxygen.

In another test tube I dissolved Ba(OH)2.8H2O. Next, I mixed the two liquids. This results in formation of a yellow/brown precipitate, which is amazingly hard to separate from the liquid. It is very flocculent and hardly settles.

I used pre-cooked water, in order to get rid of carbon dioxide (otherwise a precipitate of BaCO3 is formed as well) and flooded both test tubes with butane gas (my cheap-ass method of creating an 'inert' atmosphere ).

So, I did get some brown precipitate (actually quite light, yellow brown), but the amount was very small, and it was a pain to isolate. Because of the very small amount, I could not separate it from the filter paper, it was just a thin layer on the filter paper, half-way inside the paper.

After a lot of hassle I gave up. Probably, the only method of isolating this stuff is using a centrifuge, but I don't have one.

The main problem is the very poor solubility of K3CrO8. I see no other option for making the barium salt. Direct reaction with barium salts does not work, because of the very low solubility of barium chromate. Working wit Ba(OH)2 and CrO3 or working with a sodium chromate solution and adding barium hydroxide to that do not work.

Check out the pic - this is what I obtained by adding BaCl2 to a solution of ammonium perchromate (which must have been at least partly decomposed already at that time). A slimy flocculent precipate appeared that did not look exactly like the bright-yellow dust-like precipate formed by addition of Ba2+ to a solution containing CrO4(2-). Maybe a mixture of barium chromate and barium perchromate? The stuff was so light and fluffy it could not be filtered.

[Edited on 17-4-2008 by Taoiseach]

Check out attachment

This is the most interesting part. Check out the lower left part of the graph, starting from CrO4((2-)

Interesting dark coloured solution

Carried out the synthesis of potassium tetraperoxochromate(V) the other day, and it went pretty well. I obtained a nice quantity of very dark red-brown (almost black) crystals. This was my second time performing the synthesis; the first time I got a fine crystalline red-brown powder, which I assume to be the exact same compound, but with smaller particle size. Below is a pic of the solid from the first synthesis: (the second is still drying)



I then did some experimentation with the solid in aqueous solution, such as the standard decomposition in acidic environment forming the unstable blue CrO(O₂)₂·H₂O species (CrO₅) which promptly decomposes to Cr(III).

However, I also found that adding 30% hydrogen peroxide to a solution of the tetraperoxochromate produced a dark, maroon coloured solution that slowly evolved oxygen gas. This solution seemed stable for quite a length of time, but after I left it for a while I found that it had decomposed back into a solution with the same colour as the original tetraperoxochromate solution. I then added some HCl to the resulting solution and found that it followed the same behaviour as the original tetraperoxochromate solution in the formation of the blue peroxo complex. (yet this did seem to linger around longer before decomposing. Not exactly sure what happened, but I'll be sure to repeat it when I get the time)

Below are pics of the maroon solution generated by the addition of hydrogen peroxide to the tetraperoxochromate and the original tetraperoxochromate solution for comparison



I am just wondering what the nature of this solution could be. Looking at the diagram posted by Taoiseach on pg 3 of this thread, I guess this could be the species [CrO(O₂)₂OH]^- in the middle of the diagram. The hydrogen peroxide might act to oxidize the Cr(V) to the Cr(VI) of the second species (as well as providing the slightly acidic environment needed as per the diagram), which slowly decomposes back to the original tetraperoxochromate solution.

Still, this is all given that some other weird complex is not the explanation for the maroon colour.


Also, I am wondering if anyone has had any luck in the synthesis of the so-called violet perchromates, such as K₂Cr₂O₁₂, or the explosive tetraperoxochromate(VI) suggested by the aforementioned diagram.

Upon dilution the red colour seems to be maintained.

Below are thee pics; the original tetraperoxochromate solution, the concentrated dark red solution from hydrogen peroxide addition, and the dilute solution.



The concentrated solution evolved oxygen at a much quicker rate with respect to the more dilute solution.

I also added a solution of barium nitrate to a concentrated solution of the unknown complex and it immediately got lighter and cloudy. Slowly, a precipitate was formed with an off-white coloration.



I have doubts of whether this is actually the barium salt of the complex in question due to the relatively large amount of precipitate in comparison with the concentration of the original tetraperoxochromate solution. I have a feeling that this could just be barium peroxide; however, I had no time to separate the solid from the solution and will be sure to do that soon in order to get a better idea of what it might be.

I will also attempt adding the barium nitrate solution to a dilute solution of the red complex to see if that makes a difference.

Apologies for the wacky picture sizes!

A mixture of crystals and powder.

Small crystals


Also mine:
https://upload.wikimedia.org/wikipedia/ka/e/e5/%E1%83%99%E1%...