Sciencemadness Discussion Board

Ammonium Chromate experiment

wotaen - 22-9-2016 at 00:08

Hi All,

I wanted to make the volcano demo. I don't have a fume hood so I wanted to perform this outside. I noticed that when the "volcano is erupting" it throws off bits, which I think will be combination of unburned ammonium chromate and chromium oxide. It'll be one time, with 10 or so grams. My idea was to burn it on a sheet of metal and then dispose the rest.

Neither amm. chromate, nor oxide are particularly healthy so I'm hesitant whether I want to do this demo. Any suggestions?

Thanks

j_sum1 - 22-9-2016 at 01:04

Hmmm.
Firstly, "burning" is probably the wrong word here. The chromium is actually being reduced not oxidised. Technically it is a decomposition -- initiated by high temperature. There is no additional oxygen needed.

(NH4)2Cr2O7 (s) → Cr2O3 (s) + N2 (g) + 4 H2O (g) (ΔH = −429.1 ± 3 kcal/mol)

So, if matter is being ejected it is probably undergoing decomposition -- you would not normally have any significant amount of ammonium dichromate left over -- unless somehow you failed to get the temperature high enough. The dichromate is readily distinguished from the chromium oxide by colour.

So all you have left is Cr2O3. It is not highly hazardous. Compared to your starting material which is carcinogenic and toxic. I doubt you have much to worry about.

If you keep your experiment clean you can collect the Cr2O3 -- there are some other experiments that can be done with it. Here are a couple of examples: https://www.youtube.com/watch?v=12Z_VeDtnuM

wotaen - 22-9-2016 at 01:16

Quote: Originally posted by j_sum1  
Hmmm.
unless somehow you failed to get the temperature high enough. The dichromate is readily distinguished from the chromium oxide by colour.


My plan is to set it off using a propane torch, or put a few drops of acetone (seen this in some of the videos) + match. How can I fail to get the temperature high enough if doing it this way?

Also, should I be worried about the vapour coming off? I guess that it's going to contain some ammonium dichromate + oxide.

I'm leaning towards not doing this, especially outside when winds can change. Or are the amounts inhaled/ingested this way negligible even for something like ammonium dichromate?

Thanks

j_sum1 - 22-9-2016 at 01:27

If the compound is in contact with a heat sink of some kind and/or is too spread out, that will quench the reaction.
If you take a look at the products you will have nitrogen gas and water vapour. The chromium oxide is light and fluffy with these gases entrapped but does not form particles fine enough to form a smoke -- it falls to the ground quite readily. You are not likely to breathe anything in.

There is a reason why this is a classic demo. It is reliable and spectacular and does not present much in the way of hazards. The only concern is the handling of your starting reagent. Wear gloves. Wash our hands and spatula. Don't lick your fingers -- all standard practice.

wotaen - 22-9-2016 at 01:39

Quote: Originally posted by j_sum1  
If the compound is in contact with a heat sink of some kind and/or is too spread out, that will quench the reaction.


Good point, my plan was to set this off on a large sheet of thick aluminium that I have...kind of a heatsink.

Thanks for your advise, will do it during non-windy days, just to be sure

j_sum1 - 22-9-2016 at 01:42

Just watch a bunch of videos so you know what to expect. It doesn't usually go out, you generally get 100% covnersion and it generally stays pretty well in one place.