Boffis
International Hazard
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Registered: 1-5-2011
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Ascorbic acid complexes
There are numerous posts on the, often colourful, salts and complexes formed from transition metals and organic acids and ligands. So I would like to
add another one, which like salicylic acid, aspirin and acetic acid is readily available; ascorbic acid or vitamin C.
I use ascorbic acid to remove manganese oxide staining from mineral specimens. It is extremely effective and selectives since it does not attack
hydrated ferric oxides or even most carbonates, particularly if buffered, but rapidly removed earthy black manganese oxide films.
A few days ago while disposing of some spent ascorbic acid solution I splashed some on the white ceramic tiles. The following day it had dried leaving
purple spots. The mechanism of dissolution involves reduction of the Mn3+ and Mn4+ hydrated oxides to Mn2+ and then presumably the Mn2+ dissolves in
either excess ascorbic acid or the dehydroascorbic acid produced by its oxidation. The purple coloured produced on drying suggests that the Mn2+
compound oxidises again but the fact that it is purple rather than brown-black suggests that it is a complex of Mn3+ or Mn4+. Mn3+ produces a purple
phosphato complex for instance. Other tests have shown that the spent solution contains a little cobalt (with tetrathiocyanatomercurate ions) is
present and traces of iron (with bipyridyl).
A literature searched revealed that iron forms a purplish brown complex with ascorbic acid and this is the cause of the purple colouration that forms
when cooked potatoes are left exposed to the air (1). I couldn't find anything specifically about Mn complexes but a similar chemistry is likely. The
colour only becomes apparent on drying and disappears again on solution so it may be the iron impurities that give the colour.
Has anyone else come across ascorbate or dehydroascorbate complexes of manganese or other metals? The literature search threw up information on copper
and zinc complexes too. I am not at home at present so I can't do much experimental chemistry but if anyone else would like to investigate something
this compound looks like another that might produce interesting and colourful complexes.
1) Paul Muneta and Fred Kaisaki: American Potato Journal; October 1985, Volume 62, Issue 10, pp 531-536; Ascorbic acid-ferrous iron (Fe++) complexes
and after cooking darkening of potatoes (Yep they have an entire journal dedicated to spuds!)
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AJKOER
Radically Dubious
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Search the environmental and biochem literature for Mn reactions. Apparently, transition metals can induced cancer, DNA stripping,... This includes
manganese salts which can engage in so called Fenton-type (or, Fenton-like) reactions generating hydroxyl radicals.
Then, ascorbate and citrates, for example, are known in a subsequent step(s) to reduce the Mn back to keep the chain reaction involving hydroxyl
radicals active.
See, as an example, "Review of iron-free Fenton-like systems for activating H2O2 in advanced oxidation processes", by Alok D. Bokare and Wonyong Choi.
To quote the abstract:
"Iron-catalyzed hydrogen peroxide decomposition for in situ generation of hydroxyl radicals (HO(•)) has been extensively developed as advanced
oxidation processes (AOPs) for environmental applications. A variety of catalytic iron species constituting metal salts (in Fe(2+) or Fe(3+) form),
metal oxides (e.g., Fe2O3, Fe3O4), and zero-valent metal (Fe(0)) have been exploited for chemical (classical Fenton), photochemical (photo-Fenton) and
electrochemical (electro-Fenton) degradation pathways. However, the requirement of strict acidic conditions to prevent iron precipitation still
remains the bottleneck for iron-based AOPs. In this article, we present a thorough review of alternative non-iron Fenton catalysts and their
reactivity towards hydrogen peroxide activation. Elements with multiple redox states (like chromium, cerium, copper, cobalt, manganese and ruthenium)
all directly decompose H2O2 into HO(•) through conventional Fenton-like pathways. The in situ formation of H2O2 and decomposition into HO(•) can
be also achieved using electron transfer mechanism in zero-valent aluminum/O2 system. Although these Fenton systems (except aluminum) work efficiently
even at neutral pH, the H2O2 activation mechanism is very specific to the nature of the catalyst and critically depends on its composition. This
review describes in detail the complex mechanisms and emphasizes on practical limitations influencing their environmental applications.
Source link: http://www.researchgate.net/publication/262451840_Review_of_...
See also, 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. To quote from the abstract:
"Epidemiological research has linked exposure to atmospheric particulate matter (PM) to several adverse health effects, including cardiovascular and
pulmonary morbidity and mortality. Despite these links, the mechanisms by which PM causes adverse health effects are poorly understood. The generation
of hydroxyl radical (·OH) and other reactive oxygen species (ROS) through transition metal-mediated pathways is one of the main hypotheses for PM
toxicity. In order to better understand the ability of particulate transition metals to produce ROS, we have quantified the amounts of ·OH produced
from dissolved iron and copper in a cell-free, surrogate lung fluid (SLF). We also examined how two important biological molecules, citrate and
ascorbate, affect the generation of ·OH by these metals. We have found that Fe(II) and Fe(III) produce little ·OH in the absence of ascorbate and
citrate, but that they efficiently make ·OH in the presence of ascorbate and this is further enhanced when citrate is also added. In the presence of
ascorbate, with or without citrate, the oxidation state of iron makes little difference on the amount of ·OH formed after 24 hours. In the case of
Cu(II), the production of ·OH is greatly enhanced in the presence of ascorbate, but is inhibited by the addition of citrate. The mechanism for this
effect is unclear, but appears to involve formation of a citrate-copper complex that is apparently less reactive than free, aquated copper in either
the generation of HOOH or in the Fenton-like reaction of copper with HOOH to make ·OH. By quantifying the amount of ·OH that Fe and Cu can produce
in surrogate lung fluid, we have provided a first step into being able to predict the amounts of ·OH that can be produced in the human lung from
exposure to PM containing known amounts of transition metals."
Some cited reactions focusing on iron and copper:
R1 Fe(II) + HOOH --) Fe(III) + .OH + OH-
R2/R3 Fe(III)/Cu(II) + Asc(n) --) Fe(II)/Cu(I) + Asc(n+1)
R4 Fe(II) + O2 --) Fe(III) + .O2-
R5 Fe(II) + .O2- +2 H+ --) Fe(III) + HOOH
Source link: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2626252/
[Edited on 19-10-2015 by AJKOER]
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