AJKOER - 6-8-2015 at 06:59
Here is a paper https://www.google.com/url?sa=t&source=web&rct=j&... "Stability of Hydrogen Peroxide in Sodium Carbonate Solutions" that does not
resolve adequately, in my opinion, the mystery of why H2O2 is nine times more unstable in a Na2CO3 solution versus a NaOH adjusted solution for
various factors. The authors make the comment, to quote:
"Peroxide decomposition in carbonate liquor, which is probably caused by catalytic metal ions, is slowed by conventional alkaline peroxide-stabilizing
agents (MgSO4, Na2SiO3, and DTPA). However, the carbonate (or bicarbonate) anion per se also appears to have a role in this system, which
differentiates carbonate from caustic liquors in pulp brightening and bleaching systems.'
My proposed explanation, which could serve as the basis for a paper, relating to an acceleration in decomposition of H2O2 (which also happens to
explain a similar tendency observed in the presence of CO2, as mentioned in the article) is the formation of various radicals including, in
particular, the carbonate radical anion in the presence of light or transition metal impurities. The respective chemistry starts with the formation of
the hydroxyl radical:
H2O2 + uv → HO• + HO•
And, to a limited extent, the reverse reaction:
HO• + HO• + M → H2O2
And also, in the presence of any Iron (or Copper) impurities:
H2O2 + Fe2+ → HO• + OH- + Fe3+
Fe3+ + O2•- → Fe2+ + O2
Now, as to the formation of the carbonate radical ion itself:
"The carbonate radical ion, CO3. •- , can be prepared by the reaction of carbonate or bicarbonate ions with the hydroxyl radical:
HO• + CO3(2-) → •CO3- + OH-
Reference: https://www.google.com/url?sa=t&source=web&rct=j&... Note also, the discussion on the formation of the CO2 radical anion as a basis
for possibly explaining the instability in the presence of carbon dioxide gas as well.
Another source reference is the following doctoral thesis, "Carbonate Radical in Natural Waters" by Jiping Huang at the University of Toronto, link:
https://www.google.com/url?sa=t&source=web&rct=j&... I extracted the following pertinent comments:
Per page 14, "The photooxidants include hydroxyl radical (Draper and Crosby, 1984), alkylperoxy radical (Mill et al., 1980), singlet oxygen (Haag and
Hoigne, 1986), triplet state (Zepp et al, 1985), &nitrate radical (Larson and Zepp. 1988), dibromide ion radical, (Zafhiou et al., l984), solvated
electrons (Zepp et al., 1987a) and superoxide (Petasne and Zika, 1987). Hydrogen peroxide, a widely distributed oxidant in natural waters that is
produced via the intermediacy of superoxide radicals (Cooper and Zika, l983), may be involved in the oxidation of these compounds by the Fenton
reaction (Zepp et al., 1992) or by peroxidase-catalyzed oxidation (Cooper and Zepp, 1990)."
Per page 15, "Production of hydroxyl by nonphotochemical pathways proceed via Fenton-type reactions between reduced metals (Fe, Cu, Mn) and hydrogen
peroxide "
And, finally, per page 21, " However, a quenching effect (on the hydroxyl radical) by carbonate and a much smaller effect by bicarbonate was
observed.."
Note also, a possible doube whammy, the formation reaction for the carbonate radical anion also forms OH- which can raise the pH. Commercial solutions
of H2O2 at maintained at a pH 3 and are known to be increasingly unstable as the pH rises (see, for example, https://www.google.com/url?sa=t&source=web&rct=j&...
So what I am proposing, as the basis of the paper, is that the known scavanger properties of the carbonate radical anion on the hydroxyl radical as
sourced from the hydrogen peroxide and possible effect on pH upon formation, is accelerating the decomposition of H2O2 in the presence of light and/or
transition metal impurities.
[Edited on 7-8-2015 by AJKOER]
AJKOER - 7-8-2015 at 07:16
Here are extracts on the behavior associated with alkaline H2O2 solutions in the presence of CO2, to quote from the cited paper bottom of page 2 to 3:
"Previous workers (14) have shown that the decomposition of hydrogen peroxide in alkaline solutions (pH about 12, 303 K, ionic strength ca. 1–2M) is
accelerated by the presence of carbon dioxide, and they have postulated the involvement of peroxycarbonate in the decomposition process. Others have
shown that this effect of carbon dioxide disappears in purified solutions, and they have invoked transition metal carbonato-complexes as catalysts for
peroxide decomposition (13, 15). The presence of carbonate may also affect the free-radical mechanisms of peroxide decomposition (9, 10, 11), but
there are no studies of this matter reported in the literature."
Now, as to how CO2 accelerates the decomposition of say aqueous H2O2/NaOH, my view is that it reacts with water forming Carbonic acid which
dissociates weakly creating the bicarbonate anion (and to a lesser extent depending on pH, the carbonate anion also, please see Wikipedia's
discussion and data table at https://en.m.wikipedia.org/wiki/Carbon_dioxide ) . As noted in the opening thread either carbonate or bicarbonate anion can react with the
hydroxyl radical to create the carbonate radical anion. As such, I would expect the latter reaction to proceed as:
HO• + HCO3- → •CO3- + H2O
which is interesting as there would be no/limited pH effect (as the starting solution is already alkaline) and also, as the bicarbonate anion is a
weaker scavenger of the hydroxyl radical, the general decomposition effect on H2O2 should be more muted than with Na2CO3.
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I am thinking of the following test to show the possible impact of increased hydroxyl radical presence. Create two equal H2O2 solutions except that
one has been infused with N2O. Add equal amounts of Na2CO3 and place both in sunlight in an appropriate vessels permitting uv exposure. After a few
hours, remove both solutions and let stand over night. Next day, add dropwise NaOCl to each solution until no more oxygen evolution occurs.
As the N2O infused solution with uv produces more hydroyxl radicals, there should be less NaOCl needed to react with available H2O2 if my hypothesis
is accurate on the role of the added OH radicals in decomposing the H2O2.
[Edited on 7-8-2015 by AJKOER]
AJKOER - 9-8-2015 at 05:55
OK, came across an excellent and must read in my opinion discussion that ties together my assertion and the reference opening paper plausible
explanation relating to the presence of peroxymonocarbonate ( see "Oxygen radicals and related species" by Augusto et. al. at
link: https://www.google.com/url?url=http://scholar.google.com/sch... ). To quote from page 13:
"The most recognized biological source of CO3●- is the reaction between peroxynitrite and CO2 (Reaction 8) (Figure 4). CO3●- is also produced by
the peroxidase activity of SOD, by the turnover of XO and by transition metal-ion catalyzed decomposition of peroxymonocarbonate (HCO4-) [13, 28].
The latter is an oxidant found in equilibrium in aqueous solutions of H2O2 and bicarbonate (HCO3) (Reaction 17), whose potential participation in
biological processes has been recently discussed [13, 28].
H2O2 + HCO3- = HCO4- + H2O (Reaction 17)
Most CO3●- reactions are oxidations by both electron transfer and hydrogen abstraction mechanisms to produce radicals from the oxidized targets.
Addition reactions of the carbonate radical to produce stable target adducts are virtually unknown in the literature [13, 28]. As a charged species,
CO3●- is an important oxidizing agent in aqueous environments...."
Now, as I previously noted:
HO• + HCO3- → •CO3- + H2O
So, if we were to add hydroxyl radicals to the both sides of equilibrium Reaction 17 :
H2O2 + HO• + HCO3- = HCO4- + H2O + HO•
And then substitute per the above, I am, however, uncertain as to the actual reaction direction (that is, the reverse reaction may be solely
appropriate which would be consistent with the transition metal-ion catalyzed decomposition of the HCO4- producing carbonate radical anion):
H2O2 + •CO3- + H2O = HCO4- + H2O + HO•
But the reaction as written, also does plausibly imply a reaction with an oxygen source (like hydrogen peroxide, per previously cited source to quote:
"Certain reactions of this radical were suggested to involve oxygen atom or oxide transfer") and the carbonate radical anion, but nevertheless, there
is at least an apparent linkage between the authors' explanation (tentatively citing decomposition of peroxymonocarbonate) and my proposition
involving •CO3- .
--------------------------------------------------------------
Results are in of my comparative experiment of two equal H2O2/Na2CO3 solutions constructed from 50 ml 3% over the counter H2O2 plus 3.2 grams of
Na2CO3, where one H2O2 solution was infused with N2O gas from an 8 gram N2O cartridge. Both solutions were in identical thin plastic bottles and
placed in sunlight with occasional shaking for some 3 hours. The bottles were adjacent to each other and their right/left positioning was rotated.
I tested 5 ml from each solution and added drops of 8.25% aqueous NaOCl to detect ongoing oxygen presence. The N2O free solution required 23 drops,
while the N2O infused solution just 11 drops. On opening, both vessels showed considerable gas buildup, with the N2O vessel producing the largest
noise.
As the action of aqueous N2O in sunlight is a known source of hydroxyl radicals, this experiment certainly suggests a positive role of radical
formations in inducing H2O2 decomposition in the presence of sodium carbonate (which was prepared by heating food grade NaHCO3 and may contain
transition metal impurities).
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[Edit] My assumption of the direct action of the carbonate radical anion on H2O2 is apparently correct. Here is an extract from the literature (see
"...the role of copper-and carbonate radical anion-mediated oxidations" by Dario C. Ramirez, et al, available at https://www.google.com/url?url=http://scholar.google.com/sch... ):
"The greater inactivation of SOD1 and oxygen evolution that we observed in (bi)carbonate buffer compared with phosphate buffer (Fig.9) could be
explained by direct H2O2 consumption by CO3●- [11,28]:
CO3●- + H2O2 ---) HCO3- + HO2●
this reaction produces more HO2● and consequently, by reaction(4) [45], more oxygen evolution and enhanced H2O2 consumption,.."
Interestingly, adding hydroxyl radical to each side of the above reaction and substituting as before:
HO• + HCO3- → CO3●- + H2O
yields:
CO3●- + H2O2 + HO• ---) (HO• + HCO3- ) + HO2● = (CO3●- + H2O ) + HO2●
Upon noting the required presence of the carbonate radical anion which appears on both sides of the equation:
H2O2 + HO• ---CO3●- --) H2O + HO2●
As: HO2● + HO2● = H2O2 + O2, we have:
1/2 H2O2 + HO• ---CO3●- --) H2O + 1/2 O2
which implies that the hydrogen peroxide is decomposed by the hydroxyl radical (and all possible paths to HO•) to oxygen and water in the presence
of the carbonic radical anion (which will form by the action of the hydroxyl radical on either bicarbonate or carbonate).
I do not believe this theoretically derived result has been so stated in this form anywhere previously.
[Edited on 9-8-2015 by AJKOER]
AJKOER - 10-8-2015 at 10:17
A few finer points based on this source (see "SUPPORTING INFORMATION Production of Gas Phase NO2 and Halogens from the Photolysis of Thin Water Films
Containing Nitrate, Chloride and Bromide Ions at Room Temperature" available at https://www.google.com/url?sa=t&source=web&rct=j&... ), the quoted reaction below does proceed (albeit I strongly suspect more rapidly
as it corresponds to a pH unadjusted solution of hydrogen peroxide in sunlight forming hydroxyl radicals) absence the presence of carbonate or
bicarbonate:
"G24 HO• + H2O2 = HO2 + H2O "
"A32 HO• + H2O2 = HO2 + H2O "
Also, per the same source, the self decomposition reaction of HO2 computed under different conditions:
"A49 HO2 + HO2 = H2O2 + O2 "
"G33 HO2 + HO2 = H2O2 + O2 "
There is also a reaction by way of the hydroxyl radical:
"G27 HO• + HO2 = H2O + O2 "
"A29 HO• + HO2 = H2O + O2 "
and apparently, via the carbonate radical anion also:
"A53 •CO3- + HO2 = HCO3- + O2 "
[Edited on 10-8-2015 by AJKOER]