Chemistry Forum

Hi guys, can someone please help me with a balanced equation of the oxidation of ammonia to nitrate in water?
Many thanks.

There is no such direct method. Ammonia catalytically oxidized by air first to nitric oxide.
NH₃ + O₂ –> NO + H₂O
Nitric oxide simultaneously oxidizes by air to nitric dioxide
NO + O₂ –> NO₂

Nitric dioxide reacts with water producing nitric acid and nitric oxide:
NO₂ + H₂O –> HNO₃ + NO

Now try to equilibrate each equation. It’s easy.

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Actually, I just came across a reference in "A dictionary of chemistry and the allied branches of other sciences, Volume 1 By Henry Watts", page 183 (an online Google book) that states NH₄OH will react with finely divided Cu (or platinum) in the presence of oxygen or air. One of the byproducts is stated to be "nitride of ammonia". This compound is subsequently given the formula NH₄NO₂.

Thus, if we can further oxide the NH₄NO₂ to a nitrate, the mission is accomplished.

However, other than the reference, I have not performed this oxidation myself (although substituting H₂O₂ for air might be interesting way to speed things up).

Copper simply oxidizes by air since the oxide is easily soluble in ammonia (blue solution). This refreshes the surface of copper and accelerates further corrosion.
Cu + O₂ --> CuO
CuO + 4NH₃ (aq) ----> [Cu(NH₃)₄]₂(OH)₂

Formation of NH₄NO₂ in these conditions? I doubt.

I agree, the reference refers to ammonia (aq) which maybe liquid NH₃ and not NH₄OH.

If it is NH₃, then I have just added some catalysts (the platinum catalysts is well know) to your equations. In fact, a platinum catalysts was employed in the first ammonia fueled cars.

A point of confusion (at least for fish owners), is that there are bacteria that can apparently convert NH₄+ in a fish tank to nitrate (which kills the fish!).

Ammonia (aq) means aqueous ammonia that can be represented as NH₄OH. At the same time it’s not ammonia but oxygen that is responsible for corrosion. Ammonia just removes protecting oxide layer and refreshes the surface for further attack. Liquid ammonia is also corrosive to copper
http://www.environmentwriter.org/resour ... mmonia.htm
although I couldn’t find quickly the mechanism. Possibly it’s still air and carbon dioxide that maybe dissolved in liquid ammonia that is a cold or compressed liquid.
Certainly platinum is not something that is related to corrosion. Ammonia driven cars can use liquid ammonia directly as fuel. No platinum required.
http://www.hydrogencarsnow.com/blog2/in ... -vehicles/

In other process platinum may catalyze degradation of ammonia to nitrogen and hydrogen, and hydrogen in turn can be used in hydrogen fuel cells. Degradation of ammonia on platinum is apparently an opposite process to production of ammonia from hydrogen and nitrogen. This is a reversible reaction, so basically it can be used for preservation of hydrogen in a more compact ammonia form:

3 H₂ + N₂ 2 NH₃

The downside of liquid ammonia is that it is toxic and suffocating. Also it is a controlled substance, and therefore broad usage may complicate prevention of illegal methamphetamine production.

As for natural production of nitrates from ammonia, yes, this is very common.
http://www.thetropicaltank.co.uk/cycling2.htm
http://en.wikipedia.org/wiki/Nitrogen_cycle

Per http://en.wikipedia.org/wiki/Ammonia
"The combustion of ammonia in air is very difficult in the absence of a catalyst (such as platinum gauze), as the temperature of the flame is usually lower than the ignition temperature of the ammonia-air mixture." Thus, my comments on the first ammonia engine is valid.

Also, the combustion of ammonia produces nitrogen and water and not nitrogen oxides unless supported by catalyst.

4 NH₃ + 3 O₂ → 2 N₂ + 6 H₂O

Per the same source, "However, nitrogen oxides can be formed as kinetic products in the presence of appropriate catalysts, a reaction of great industrial importance in the production of nitric acid:"

4 NH₃ + 5 O₂ → 4 NO + 6 H₂O

You are right about the reactions: ammonia burns nicely without catalysts (green flame) into N₂ and H₂O. In the presence of platinum it can oxidize into NO.
Again, no need for a catalist to burn ammonia on air.

The link below is a 2008 article on the oxidation of NH₃ with O₂ in the presence of a Bimetallic CuO/CeO₂ Nanoparticle Catalyst authored by Chang-Mao Hung, Taiwan, Republic of China:

http://www.aaqr.org/VOL₈_No₄_December₂₀ ... 47-458.pdf

To quote "The catalytic oxidation of ammonia has been reported to precede as follows the exothermic global reactions:

4NH₃ + 3O₂ → 2N₂ + 6H₂O +1266 KJ (1)
2NH₃ + 2O₂ → N₂O + 3H₂O +1102 KJ (2)
4NH₃ + 5O₂ → 4NO + 6H₂O +904 KJ (3)

The SCO process that involves ammonia should be selective for nitrogen (reaction 1), and prevent further oxidation of nitrogen (reactions 2 and 3)"

What I find fascinating is how the original reference in Watt's Dictionary of Chemistry, over a century old, has been updated from the oxidation of ammonia in the presence of "finely divided copper" to a bimetallic CuO/CeO₂ nanoparticle catalyst.

On the discussion as to whether the ammonia reference in Watt's is NH₃ or NH₄OH, there are some recent papers citing wet oxidation of ammonia, so the point appears to be mute. For example, the link below brings up a pdf dated April 2010 entitled "CATALYTIC WET AIR OXIDATION OF AQUEOUS SOLUTIONS OF AMMONIA IN A CONTINUOUS-FLOW TRICKLE-BED REACTOR OVER METAL SUPPORTED ON CARBON MATERIALS" by Chang-Mao Hung₁,* and Wei-Bang Lin₂

The metal referred to is Cu-activated carbon fiber (ACF) catalyst.

http://doc-0o-a0-docsviewer.googleuserc ... ci4fq4tm7j

Your link doesn't work. Maybe this?
http://www.ncbi.nlm.nih.gov/pubmed/19147285

I recommend using this site http://tinyurl.com/ to abbreviate your long links

OK, I just came across a series of papers in the processing of waste water. It turns out that my earlier idea of replacing the use of oxygen with a stronger oxiderizer was on track. See for example, the paper by Karlis Svanks, "Oxidation of Ammonia in Water by Ferrates(VI) and (IV)"
https://kb.osu.edu/dspace/bitstream/han ... sequence=1 .

My guess is that the general goal in waste managment is the efficient and economical removal of NH₃ by oxidation in a normal temperature band and, on occasion, the restoration of nitrates. It terms out that Ferrates are perhaps the strongest oxidizes around yielding mono-atomic oxygen and are capable of oxidizing NH₄OH at room temperature. One equation given by Svanks, with excess K₂FeO₄, for example, directly produces a nitrate:

8 K₂FeO₄ + 3 NH₃ + (n+2)H₂O --> 3 KNO₃ + 4 Fe₂O₃.nH₂O + 13 KOH

Another reference gives the optimal Ph between 7.5 to 11 and the temperature between 10C to 35C to produce nitrates.

The main issue is how to prepare K₂FeO₄ as it not commercially available and has a shelf life of about 8 - 9 hours. One cited method is as follow:

2Fe(NO₃)₃ + 3NaOCl + 10NaOH ->2Na₂FeO₄ + 5H₂O + 3NaCl + 6NaNO₃

Unfortunatley, the yield is low around 10-15%. However, replacing Na₂(FeO₄) with the insoluble Barium Ferrate is said to increase ferrate production. The use of Ferric Nitrate seems to be preferable over Ferric Chloride and Fe(OH)₃. The web and above work by Svanks give other possible ferrate preparations with better yields. There is a recent Iranian Chemical Society paper detailing improvements resulting in higher yields to the Ferric Nitrate approach. Link is:
http://www.ics-ir.org/jics/archive/v7/4 ... icle-3.pdf

Interestingly, the use of ferrates to oxide ammonia does not require a high temperature, an elevated pressure or even a catalyst. For laboratory preparations, I wouild speculate that a CuO catalyst may increase the rate of ammonia conversion.