Peroxo chromate

Just accidently, I discovered a very energetic mix, which is easy to ignite and explodes violently, even when not confined, at quantities of less than 10 mg

Some time ago, I made the brown salt K3CrO8, which has a very high oxygen content. This can be made from an ice-cold (-10 C or so) lemon yellow concentrated solution of K2Cr2O7 and KOH. Take 1 mol of K2Cr2O7 for 3 mols of KOH and dissolve in as little as possible water.

Add SLOWLY, drop by drop, 30% H2O2 (also ice cold) to this yellow solution. If this is done slowly and temperature is not allowed to go over 0 C, then brown crystals of K3CrO8 separate from the liquid. For each mol of K2Cr2O7 take appr. 6 mols of H2O2.

The crystals can be rinsed with a mix of 50% water and denatured alcohol, also ice cold. A final rinse with ice-water makes them clean. Then let them dry in contact with air. The salt is not hygroscopic at all, and it crystallizes well.

Now, how to make the explosive follows here:
Take 40 mg of this salt, crunch the crystals to a fine powder, and mix it with 10 mg of finely powdered red P. When 10 mg of this mix is ignited with the flame of a candle, then a very loud BANG is obtained. I really was impressed.
Things become even more impressive if some of the red P is replaced by Al-powder.

This is an insanely powerful explosive. No need to confine the stuff, just a tiny heap of solid gives a loud bang.

I do not encourage anyone to make this mix in larger quantities, but it is fun to play with it in mg quantities. I have no idea about the stability of the mix on storage, so please don't keep it around. Just make it and use it immediately.

The K3CrO8 looks as follows:



It is a peroxo compound of chromium in the +5 oxidation state. A better formula is K3[Cr(O2)4]. This peroxo compound keeps indefinitely, provided it is kept in a dry place.

[Edited on 2-2-07 by woelen]

I'd think that dichromate with red P would already give a nice bang...Care for a reference experiment?

Woelen,

Do you get the impression that a true detonation takes place or is it simply a fast deflagration?

Perhaps, you could press a gram of it in a cap with a loose ignition charge and see if it has any potential as an initiator of a secondary explosive.

Have you tried hitting it with a hammer to see if it is sensitive to impact?

"Things become even more impressive if some of the red P is replaced by Al-powder."

Does that mean it even makes a bang with flake aluminium powder in 10 mg quantaties?! That would be really something!

The fact that it makes a sharp bang means it goes supersonic, so in that sense is would be detonation. Though for powder mixtures it is usually called deflageration, even when it goes supersonic. Aluminium/chlorate flashpowder can reach 3200 m/s under ideal circumstances though it is generally not called detonation. The quantities in which it can go supersonic unconfined is amazing though for K2Cr2O8/P, I don't know any other pyrotechnical mixture that can do this, with the possible exception of chlorates or permanganates + 1-3 micron Mg. The fact that it can detonate in these quantities can only mean that it must be a very sensitive mixture, since the speed and acceleration of the reaction are greatly determined by the speed by which the oxidator can release it's oxygen (decompose). So I wouldn't recommend making a gram of it anyway...

You could test the brisancy by lighting a 10 mg amount on an empty can of beer and see if it would penetrate the can... (safely behind protection with ignition source on a stick)

[Edited on 3-2-2007 by nitro-genes]

Looks like the decomposition of K3[Cr(O2)4] is quite exothermic or else such a rapid deflagration seems impossible. Woelen, have you tried to ignite K3[Cr(O2)4] without any reducent? If truly exothermic the decomposition should propagate without additional heat. After all, even the decomposition of H2O2 is quite exothermic, it is just the high activation energy that prevents it from explosively decomposing by itself. The complex with Cr looks like it brings the activation energy of the peroxide decomposition down very dramatically (probably by the same SET mechanism as the Fe metal ions do).

Nice and interesting discovery Woelen. Your experiments are always nice to read about.

I could be totaly wrong, but "Dragon eggs" springs to mind here, that has a very similar report in minute quantities also.

Just a thought

Accident with K3CrO8/Al + very small amount of P

Heating K3CrO8 without reducing agent results in quick decomposition (soft puffs, with slight hissing noise), giving yellow smoke and oxygen gas. Not particularly violent. The yellow smoke most likely is K2CrO4. Be careful not to inhale that smoke, it probably is carcinogenic.

I had an accident with my second experiment

I replaced 80% of the red P with Al-powder (German black) in order to make it more energetic (red P makes things easier to ignite, but also somewhat less energetic and certainly less hot). I mixed this stuff in a thick Al-foil folded "cup" in order to avoid static discharge. But, despite of that, suddenly it ignited with an intense white flame and a soft bang, while I had the Al-foil with the powder in my hand. After this, the Al-foil stuck to my middle finger and now I have a deep burn of appr. 1½ cm² on my left middle finger. This is amazingly painful and I hope to have a reasonable sleep tonight.
Fortunately, the mix did not burn through the thick Al-foil, so I do not have chromate contamination in my finger, I "only" have a burn. I also think that I was lucky that the chems were not yet mixed very well, otherwise the burn would have been even hotter or I would have had a stronger explosion. In total now I had appr. 100 mg of chems.

From this you can conclude, that this mix is not useful for the real work and I want to advice everyone to be VERY VERY careful with this, especially if Al-powder is used as well. The hammer test is not needed anymore, even simple friction by mixing can ignite the material.

K3CrO8 is dangerous stuff!

[Edited on 3-2-07 by woelen]

Wow. Very interesting! You should look into getting a patent for that if it isn't already patented. I'm sorry to hear about your accident but I'd love to see a picture of the burn Care to post one?

Very interesting!
From the first post onwards, I was convinced that peroxo chrome compounds are supposed to be blue!
I looked it up, and indeed, I remembered something different.
Cr2O7(2-), in the presence of H2O2 and *acid*, forms CrO(O)2, which is blue, and is still formally Cr(VI) (Jander & Blasius) .
So this is CrO5, so quite differnt to the compound that Woelen described
Since I am at it, it is made by
mixing cold HNO3 (or H2SO4) with dichromate, and ether is layered on top. To this, 2.5 mol/l H2oO2 is added and the mix is shaken. A blue colour appears in the etheric phase, which is CrO5.
With pyridine and the evaporation of the ether, one obtains the solid pyCrO(O2)2. This reacts with Ag2O to pyridine and O2.
Looks like this compound cannot be isolated other than some Pi complex.
Anyway, this is a peroxo compound of chromium that always fascinated me. It would be way cool to get it into its pure state!

Regardless... it'd be really good to get a proper comparison to other primaries. Perhaps some members more astute in these matters would be willing to do a little experimentation? Safely of course

Well, I stored the compound in a glass vial for many years already. Part of my K3CrO8 is 17 years old and still as active as when I made it. Recently I made another batch and it looks exactly the same. This stuff can be made reproducibly and can be stored safely. At the time of making this material, I of course studied its properties and also its stability. That is why I only isolated the K3CrO8 peroxochromate and not the blue K2Cr2O12, the latter being too unstable for safe storage.

So, the peroxochromate itself is fairly safe, but a mix of this with strong reductors is dangerous, as I have experienced the hard way

@garage chemist: CrO5 in ether decomposes to a chromium (III) compound. I actually did try this. The solution at first is blue, but many hours later it turns green, the typical color of chromium (III).
Also, making a concentrated solution is remarkably difficult. Just try it in water. Take 1% H2O2 with 1% HCl or H2SO4 and also add a 1% solution of K2Cr2O7. You'll see an intense blue solution, but it only lasts for seconds.
Change all concentration to 0.1%. You still get quite a deep blue color, but the solution now remains blue for a much longer time.
Again change concentrations, now to 0.01%. You still obtain a clearly visible blue solution, and now it lasts even longer.
With somewhat higher concentration you even don't get the chance to have a lot of it absorbed in the ether layer.
Maybe if you work at -15 C, but at 0 C or so, it is not worth the effort.

[Edited on 4-2-07 by woelen]

Hmmm... strange, they add non-oxidizing stuff to the solid? Why would you want that? Is this useful as rocket fuel?

I did a test with only sulphur added to the K3CrO8 as reductor, instead of red P and/or Al. This mix still burns amazingly fast. It does not give a bang, but it burns very fast and makes a WHOOP noise when ignited. It burns fast like flash powder, however, with much less light (the light output actually is quite low).
The K3CrO8/S mix also can be ignited very easily. Just keep it a few cm above the flame and it goes WHOOP. I did not make a really fine powder, I only crunched the crystals of K3CrO8 with a little spatula, added some flowers of sulphur and swirled a little in a big pan in order to get the chems mixed. I do no more crunching and mixing, while I have the material in my hands.

I also did a smoke analysis of the K3CrO8/S mix, by igniting some of this mix under a big beaker, capturing some smoke. The smoke, gives a yellow tint to water, and on addition of acidified H2O2, the color is deep blue. From this it can be concluded, that the smoke contains chromate, which is NOT good.
I also attempted to do a smoke test with the K3CrO8/P mix, but I only did this with 10 mg, due to the explosive nature (I don't want glass shattered around). I could capture some smoke, but no chromate could be detected, not even with the very sensitive H2O2/acid test. So, with red P it seems that (provided sufficient P is used) all chromium is converted to the +3 oxidation state and that is a good thing.

Altogether, the red P mix is more interesting than the S mix.


It is a pity that I do not have LiAlH4. I really would love to see what happens if powdered K3CrO8 is mixed with powdered LiAlH4 and then ignited. That must be really spectacular . Of course only in mg quantities.

[Edited on 5-2-07 by woelen]

Seems like a worse idea than ammonium dichromate!

Tim

well it seems my patience paid off, the liquid became less Bromine brown and went very much lighter and there was a crop of crystals along the bottom side of the test tube and at the base of it also, not a fantastic yield but a good milligram or 2, and certainly enough to experiment with

I entirely forgot to conc some of the H2O2, but that can wait until the morning.

currently this compound is still in the test tube after a good rinse and is drying on its side under the lamp heater (I dry all my glassware that way).

I`m looking forwards to tomorrow, I have plenty work and projects to complete, also my copper acetate should be ready for drying, already the crystals are large and almost black looking, truly beautiful! I shall take some pics if the cam will do it justice.

Some research on peroxchromates