Bedlasky
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Nitrite in acidic solution
Hi.
We had in work today acidic sample which contains lots of nitrite. I was surprised because nitrites are unstable under acidic conditions. Sample
contain nitric and sulfuric acids. Nitrites was determined spectrophotometrically as an azo dye. Sample had yellow colour so I suppose there was some
nitrogen oxides interference (due to equilibrium NO + NO2 <--> N2O3). But I am not sure about that because NO isn't very soluble in water. There
are possible another interferences?
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AJKOER
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Any light especially lab light richer in UV?
HNO3 + UV --> •OH + •NO2 (see p. 12 at http://cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05_L7.... )
•NO2 + •NO2 = N2O4
N2O4 + H2O = HNO2 + HNO3
indicating a possible equilibrium source of nitrite. There is also a 2008 study making the comment:
"We propose that these complexes are potentially important in the thermal and photochemical production of HONO observed in previous laboratory and
field studies."
apparently through newly reported complex formation active in visible light, to quote:
"The electronic states of (HNO(3)).(N(2)O(4)) and (NO(3)(-)).(N(2)O(4)) were investigated using an excited state method and it was determined that
both complexes possess one low-lying excited state that is accessible through absorption of visible radiation. "
Reference link: https://www.ncbi.nlm.nih.gov/pubmed/18825290 .
Also, you mentioned azo dyes, where many are natural light photocatalyst introducing electrons (e-) and electron holes (h+) to promote further radical
creation. The latter could also serve as an initiator (via •NO2 formation) for the referenced visible light photocatalyst complex HNO3.NO2 cited
above.
Lastly, per the first cited reference, the presence of O2 (from air) may further promote a radical chain.
[Edited on 12-11-2019 by AJKOER]
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woelen
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A solution of acidified nitrite can be yellow, due to dissolved NO2, especially if there is a large concentration of HNO3. When you add nitrite to
HNO3, then it gets oxidized. You basically have the following reaction:
HNO2 + HNO3 <---> 2NO2 + H2O
When the concentration of water is not too high (e.g. in a concentrated H2O/HNO3/H2SO4 mix), then quite some NO2 can remain dissolved in the liquid.
If nitrite is added to dilute acid (e.g. 10% HNO3 or 10% H2SO4), then the liquid becomes blue, due to dissolved N2O3:
2 HNO2 <---> H2O + N2O3
At the same time, the liquid bubbles, due to formation of NO and escaping of that gas. The NO2, which remains behind under these conditions
disproportionates, basically, the reverse of the reaction I wrote at the beginning of this post.
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AJKOER
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Actually technically, the formation of HONO from the action of water on NO2 may be somewhat complex and discussed in recent literature. To quote a
2018 source (https://www.pnas.org/content/pnas/early/2018/06/20/180771911... ):
"Various chemical routes have been reported for HONO production, including direct emission from the combustion of fossil fuels or biomass (11, 12),
gas-phase homogeneous reactions (13, 14), the reaction of NO2 on heterogeneous surfaces (15, 16), and photolysis reactions of HNO3 or NO2 (17, 18).
One major source of HONO is the surface hydrolysis of NO2 through NO2 dimerization, as proposed by Finlayson-Pitts et al. (19):
2NO2 + H2O → HONO + HNO3
.....Additionally, NO2 hydrolysis on a heterogeneous surface is too slow (20–60 times) to be responsible for the unexpected high concentration of
HONO observed in atmospheric due to its high activation energy... The low reaction rate for NO2 hydrolysis is largely due to its high activation
energy. Chou et al. (20) reported that direct formation of HONO from N2O4 hydrolysis requires overcoming an energy barrier as high as 30 kcal/mol.
Chen and coworker (21) found that a barrier of ∼17.1 kcal/mol limits HONO direct formation"
Also to quote:
"Since first reported by Finlayson-Pitts et al. (19), the asymmetric isomer ONONO2 has been considered as an intermediate in the hydrolysis of NO2.
Recent spectroscopic evidence reported by Stanton and coworkers (34) confirms the formation of asymmetric ONONO2 from the dimerization of NO2 in the
gas phase. It has also been observed that symmetric N2O4 converts to asymmetric ONONO2 with a very short lifetime, even on an ice...."
My limited understanding is that the radical NO2 does NOT directly react with H2O, but with itself forming N2O4 and an associated asymmetric isomer
ONONO2, playing a role to overcome an energy barrier issue. The presence of select compounds (like NH3) can promote HONO formation as does photolysis.
Also, in the current context of acid surfaces, per this source ("Heterogeneous NO2 conversion processes on acid surfaces: possible atmospheric
implications, link: https://www.sciencedirect.com/science/article/abs/pii/S13522... ) cited above in the 2018 paper, to quote:
"The heterogeneous conversion of NO2 on water/sulphuric acid surfaces was studied in a quartz reactor and a bubbler system. The NO2 decay and the HONO
formation are first order in [NO2] and are limited by an uptake coefficient γ≈10-6. It was observed that HONO formation on acid/water surfaces of
moderate acidity only occurs via the reaction 2NO2+H2O→HONO+HNO3. The involvement of NO on the HONO formation is of minor importance"
[Edited on 12-11-2019 by AJKOER]
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Ubya
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what λ are you using for your spectrophotometric determination? if you use a band that doesn't overlap on the absorbtion peak/band for NO2 you
shouldn't have any problems.
what azo dye are you using?
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feel free to correct my grammar, or any mistakes i make
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Bedlasky
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Reagents are aminonaphtalenesulfonic acid and sulfanilic acid.
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