AJKOER - 4-11-2015 at 11:19
I recently performed a reaction between Copper metal in dilute H2O2 and Citric acid with MgSO4 to serve as an electrolyte in lieu of the usual NaCl.
This basically is an electrochemical reaction with formation of Copper citrate which occur here ostensibly as well. But with time, there become
evident the formation of a fine white precipitate (which I suspect was Mg(OH)2 ) and the solution's color started to change becoming eventually the
aqua blue of CuSO4.
I suspect a Fenton-like reaction producing OH- (the cause of the Magnesium hydroxide formation) and the associated hydroxyl radical. The latter OH.
radical appears here to have destroyed the citrate ion.
My research has since come upon an article "Hydroxyl radical formation from the auto-reduction of a ferric citrate complex" by Gutteridge, link: http://www.ncbi.nlm.nih.gov/pubmed/1665838 .
To quote the abstract:
"When a ferric citrate complex is prepared from citric acid and ferric chloride, and the pH value left unchanged, a reduction of the iron moiety takes
place. Within several hours a substantial yield of ferrous ions can be detected in the solution. When placed in a phosphate buffer pH 7.0 with a
suitable detector molecule, oxidative damage to the detector molecule can be observed. Thus, deoxyribose is degraded with the release of
thiobarbituric acid-reactive material and benzoate is hydroxylated to form fluorescent dihydroxy products. Damage can be prevented by scavengers of
the hydroxyl radical such as mannitol, formate the thiourea, by catalase and by the protein caeruloplasmin, suggesting that Fenton chemistry occurs
leading to the formation of hydroxyl radicals."
Someone may be interested in exploring whether the suggested auto-reduction of citrate for Fe and my apparent observation for Cu, applies to other
active transition metals, especially Co, Mn and Ni.
If someone is wondering why citrates (or, copper) see, for example, full text of "Generation of Hydroxyl Radicals from Dissolved Transition Metals in
Surrogate Lung Fluid Solutions", by Edgar Vidrio, Heejung Jung, and Cort Anastasio at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2626252/ .
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[Edit] Found a paper "Surface-Induced Nickel Hydroxide Precipitation in the Presence of Citrate and Salicylate" by Noriko U. Yamaguchi, et. al., at
https://www.google.com/url?sa=t&source=web&rct=j&... . To quote from the paper:
"Figure 2b compares band positions of all three sorbents with and without citrate. Without citrate, LSgibbsite and pyrophyllite induced the formation
of Ni–Al LDH. Atcit/Ni51, are d shift to a-Ni hydroxide was observed in the presence of both HS-and LS-gibbsite, while the precipitate formation was
almost completely suppressed for pyrophyllite (the n2 band was too weak to be fitted). However, a pyrophyllite sample with a lower citrate/Ni ratio of
0.3 confirms the general trend that increasing citrate causes a red shift of n2 positions towards that of a-Ni hydroxide (Fig. 2b)."
I did not find a possible explanation by the authors, as to why, with time, there was such an observed increase in the Ni hydroxide formation, but I
would argue it is consistent with my hypothesis relating to a possible Fenton-like auto-reduction reaction based on nickel.
[Edited on 4-11-2015 by AJKOER]
morsagh - 9-12-2015 at 11:58
I don´t think so, as i have read (but from old book so i can be wrong) fenton mechanism have intermediate of Fe-OOH, i know this complexes with Co,Mn
and Ni too but haven´t heard about Cu in something like this. You must always think about electroxidation or reduction (don ´t know what electrodes
you used). Magnesium hydroxide can be just produced at cathode as normal electrolysis of magnesium sulfate to produce sulfuric acid and magnesium
hydroxide (if you didn´t stir)