AJKOER
Radically Dubious
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Registered: 7-5-2011
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Kinetics and Mechanism of Oxidation of Hydroxylamine by Fe(lll)
A 2002 work “The kinetics and mechanism of oxidation of hydroxylamine by iron(III)” at http://pubs.rsc.org/en/content/articlelanding/2002/dt/b20160... suggests the following reaction path:.
Fe3+ + NH2OH = Fe(NH2OH)3+
Fe(NH2OH)3+ = Fe2+ + H2NO• + H+
Fe3+ + H2NO• = Fe2+ + HNO• + H+
2 H2NO• → N2 + 2 H2O
2 HNO• → N2O + H2O
My expanded take on the mechanics of the above (as I have only access to the abstract) is as follows:
NH2OH = H+ + NH2O-
NH2O- + Stimulus → NH2O-*
NH2O-* = e- + H2NO• (as apparently hydoxylamine decomposes easily under ordinary conditions from air contact, slight warming, high pH, see https://books.google.com/books?id=Mtth5g59dEIC&pg=PA658&...)
Fe3+ + e- = Fe2+
Net reaction in accord with above: NH2OH + Fe3+ = Fe2+ + H2NO• + H+
And also: 2 H2NO• → N2 + 2 H2O
Similarly:
H2NO• = H+ + NHO•- (equilibrium moving to the right at higher pH, see source below)
NHO•- = e- + HNO•
Fe3+ + e- = Fe2+
Net reaction in accord with cited path: H2NO• + Fe3+ = Fe2+ + HNO• + H+
And also: 2 HNO• → N2O + H2O
The implied combined net reaction is:
NH2OH + 2 Fe3+ = 2 Fe2+ + ½ N2O + ½ H2O + 2 H+
Which is in agreement to what is commonly cited on the internet, for example:
2 NH2OH + 4 Fe3+→N2O(g) ↑ + H2O + 4 Fe2+ + 4H+ (Source: see http://www.askiitians.com/results/?utm_source=homepage&u...)
However, there are also additional possible paths to nitrogen formation:
N2O + e- --> .OH + OH- + N2
Source: THE APPLICATION OF THE ELECTRON BEAM PROCESS IN WATER AND WASTEWATER TREATMENT: FUNDAMENTAL AND APPLIED STUDIES,
Link: https://www.google.com/url?sa=t&source=web&rct=j&...
And also, at acidic pH likely more nitrogen gas:
H+ + e- = •H
•H + N2O → •OH + N2
Source: “Reaction of the hydrogen atom with nitrous oxide in aqueous solution – pulse radiolysis and theoretical study”, by Lukasz Kazmierczak,
Dorota Swiatla-Wojcik and Marian Wolszczak, RSC Adv., 2017, 7, 8800, DOI: 10.1039/c6ra27793d, link: http://pubs.rsc.org/-/content/articlepdf/2017/ra/c6ra27793d
Note, as a dated reference, see “The Reduction of Silver Ions by Hydroxylamine”, a 1939 paper available at http://pubs.acs.org/doi/abs/10.1021/ja01878a033 ), the amount of N2O in case of NH2OH acting on AgNO3 is reported to increase at higher pH and
more N2 at lower pH.
Finally, it may be possible that NH2OH, as an amine, shares the cited ability of amines to promote the stabilization of solvated electrons (see, for
example, http://pubs.acs.org/doi/abs/10.1021/j100168a009), which could foster an electron transfer reactions cited above.
Comments welcomed, especially if you have read the entire article.
[Edited on 4-1-2018 by AJKOER]
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AJKOER
Radically Dubious
Posts: 3026
Registered: 7-5-2011
Member Is Offline
Mood: No Mood
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Found a short paper, "Reaction pathways of hydroxylamine decomposition in the presence of acid/base", with more details on radical reactions flowing
from NH2OH (or HA in Table 1). See http://folk.ntnu.no/skoge/prost/proceedings/aiche-2004/pdffi...
Note, I wish at least researchers could agree on naming conventions, as, for example, H2NO• is shown in Table I as NH2O.
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I should mention why the reduction of Fe(lll) to Fe(ll) by NH2OH is more important post 2011. The answer, see "Strong Enhancement on Fenton Oxidation
by Addition of Hydroxylamine to Accelerate the Ferric and Ferrous Iron Cycles", by Liwei. Chen†, et al, in Environ. Sci. Technol., 2011, 45 (9), pp
3925–3930, DOI: 10.1021/es2002748, link: http://pubs.acs.org/doi/abs/10.1021/es2002748?journalCode=es... . To quote from the extract:
"The HO• mechanism was confirmed to be dominating in the Fenton-HA system, and the generation of HO• was much faster and the amount of HO•
formed was higher than that in the classical Fenton system."
which likely is due to NH2OH assisting in the recycling of the iron ions.
[Edited on 4-1-2018 by AJKOER]
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