Sciencemadness Discussion Board

Ammonia Oxidation-Failure

Agari - 20-12-2015 at 20:37

I will try to explain the entire event in an unnecessarily complicated way: I tried the oxidation of ammonia gas to nitrogen monoxide and have clearly failed. I have attempted the oxidation reaction as follows:
Inside a reaction tube, a copper coil was secured. The tube was then connected to a 3-neck round-bottom flask on one end,while the other end was connected to an 500 milliliter Erlenmeyer flask. To one of the necks of the 3-neck flask was connected a test tube containing dry ice,this was to help pressurize the apparatus. To the remaining neck of the round-bottom flask was connected to an Erlenmeyer flask containing a 30% solution of ammonia,while the reaction tube itself was connected to an oxygen generator that used household bleach and 3% hydrogen peroxide(The "flask end" used a 2-hole stopper). After I set up the entire apparatus,I submerged the test tube with the dry ice into a water bath,hoping that the CO2 would pressurize the apparatus. The ammonia solution was then heated to 50 degrees Celsius(In hopes that the ammonia gas would bubble out),and the bleach in the oxygen generator was added to the hydrogen peroxide via addition funnel. I then heated the reaction tube using a blowtorch(Wasn't my first time,the first try ended in failure when a different tube ruptured,I had to buy a new one). In the end,no nitric oxide was produced,at least I don't think any adequate amount was generated. I know that nitric oxide is oxidized to nitrogen dioxide simply in the presence of oxygen,yet I did not notice any brown gas in the collecting flask.Either I didn't get any nitric oxide and nitrogen dioxide generation,or they were generated only in trace amounts,neither of which is good. Is there any way for me to improve the setup to get noticeable amounts of NO,and subsequently NO2?

battoussai114 - 21-12-2015 at 06:49

You'd be better of heating the copper instead of the flask (heat the copper till red hot). You're still not likely to get very good yields since a side reaction consumes NO and Ammonia producing water and N2, very short residence times are necessary in the industrial Ostwald process to minimize the losses caused by this.

Also, remember this reaction is exothermic -even though the activation energy is high-... You don't want to overheat ammonia oxygen mixtures or your glassware might become a very cool model rocket.

[Edited on 21-12-2015 by battoussai114]

Zyklon-A - 21-12-2015 at 07:10

Also, even if nitrogen oxides were present, they'd quickly combine with excess ammonia, forming nitrites and/or nitrates.
A better way would be to first dry the vapours, then mix with (dry) oxygen/air. Pass this mixture through a red-hot copper tube with copper mesh inside. After the reaction has started, IIRC, the heat produced will be localized on the catalyst, eliminating the need for further heating.
Also, I'd use air rather than oxygen.

phlogiston - 21-12-2015 at 11:47

I once got essentially the same setup to work, with air, ammonia (generated the same way) and copper wire. The method works, but you need to heat the copper to get it started.

However, you will most likely run into another problem: The heat produced readily melts the copper. It is difficult to prevent this if you cannot carefully control the rate at which you feed ammonia/oxygen. It is nearly impossible with such a crude methode for generating ammonia gas.

Also, I got only a tiny yield. I am not sure whether that was because I could only run it for a short period of time before melting the copper and the glass tube, or because the reaction proceeded to N2, rather than NOx.

[Edited on 21-12-2015 by phlogiston]