The Amateur Chemist

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Synthesis of Potassium Tetraperoxochromate(V)



In this experiment we will make some potassium tetraperoxochromate, an interesting +5 oxidation state chromium compound.  This chemical is quite easy to make, but you must be extremely careful. It is highly toxic, as much as potassium dichromate, and will explode violently when heated. The compound is a peroxo compound, which is best written as K3Cr(O2)4. When you mix the potassium dichromate with the potassium hydroxide, potassium chromate - yellow solution - will form. This potassium chromate then reacts with the hydrogen peroxide at a low tempature to produce the potassium tetraperoxochromate. 


   2K2CrO4 + 9H2O2 + 2KOH  2K3CrO8 + 10H2O + O2



Ingredients

5 grams potassium dichromate

5 grams potassium hydroxide

40 ml of 15% hydrogen peroxide (20 ml 30% + 20 ml water)

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Procedure


Take the 5 grams of potassium dichromate and the 5 grams of potassium hydroxide and dissolve them in 20 ml of cold distilled water. 

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The potassium hydroxide will dissolve quickly, but the dichromate will take several minutes to dissolve, even with heavy stirring. The solution will become quite warm due to the potassium hydroxide dissolving and the solution will turn a deep yellow.  When all the solid is dissolved, add enough water until the total volume is 30 ml. You probably won't have to add much.

 

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You should now have two solutions - the potassium chromate and the hydrogen peroxide. Put them both in the freezer or an ice bath until they are both extremely cold. The colder the better. When both solutions are ice cold, take them out and carefully mix them together. The solution will turn dark red, and then black as you finish adding the hydrogen peroxide. Stir with a glass rod as you mix them together.

 

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At this point the solution will start to bubble. You must immedietly put it back into the freezer or ice bath then. Make sure you cover it first, since it can spray tiny droplets into the air. My experience with this is that when I put it back in the freezer right away, I have no problem with the bubbling at all. If you don't do this though, or if you handle it too hard, the solution can bubble over the sides of the beaker. Quite a lot of gas is produced, so leave it in the freezer or ice bath for at least two hours. 

 

It is good to check up on it every few minutes at first, and then perhaps every 20-30 minutes after that just to make sure nothing is going wrong. Make sure it doesn't warm up to much or that it doesn't freeze over. After two hours of sitting, quite a few crystals will have deposited out into the bottom of the beaker. Decant the above solution off carefully and rinse the crystals 2 times with 7 ml each of cold distilled water. Swirl, then decant. Then rinse 4-5 times with 7 ml each of ethanol (or pure/clear denatured alcohol.  At first, the liquids will be quite dark, but towards the last 2-3 alcohol rinsings, it should be much clearer. The last rinse should be crystal clear. If possible, rinse again with some diethyl ether - it's not nessesary, just makes the product purer.

 

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You can then let it sit on a clean, dry filter paper to get rid of the last of liquid and to let it dry.  If possible, put it in a petri dish or similar to dry fully. It should be fully dry within 30 minutes to several hours. It is a very dry, non-hygroscopic solid, so it should be easy to bottle up and store. It appears like a black - perhaps dark brown - solid with small white flecks throughout. It looks quite cool.

  

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I got 6-7 grams which is about 70% of theory.