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> (1) EDTA-Iron is in the Fe+3 state, not Fe+2.
You can complex either ferrous or ferric iron with EDTA since this
chelator is a sexadentate ligand (six charged sites). Chelated iron
will not precipitate out of solution but yes, the Fe:chelator is a CLEAR
yellow-brown color. "One of the valuable properties of EDTA is that it
combines with metal ions in a 1/1 ratio regardless of the charge of the
cation(Skoog and West. Fundamentals of Analytical Chemistry)."
> (2) With regard to this statement: "In an oxic environment "reducing
> agents" will not prevent the oxidation of ferrous iron." I think the answer
> in (1) above with the little experiment shows that reducing agents not only
> prevent the oxidation of ferrous iron, but actually will reduce ferric back
> to ferrous, even with a relatively mild reducing agent such as ascorbic
In a biologically active medium saturated with oxygen, ferrous iron is
simply not stable for any length of time (minutes or less).
> (3) With respect to this statement: "The charge on chelated iron is
> irrelevant to iron uptake." It may be irrelevant in that both _can_ be
> adsorbed, however it is relevant in that Fe+2 is much more easily taken up.
No, in countless experiments over many years, research has clearly shown
that chelated Fe:EDTA can deliver adequate amounts of iron for maximal
growth rates. In my own experiments I have found that 0.2 to 0.9 uM
FE:EDTA (0.1 to 0.5 ppm) will suppport relative growth rates of between
75 to 300 (1 to 4 day doubling time).
> This is the crux of the matter. If EDTA
> sucks nutrients (cations) out of plants, how likely is it that if one adds
> EDTA with a nutrient (Iron) that the plant is going to pluck it right back?
This is an interesting paradox that I spent many sleepless nights
thinking about during my PhD years. The paradox, simply stated, is that
EDTA can remove nutrients from solution and cause deficiency, while at
the same time it can provide Fe to the plants. The answer lies in the
relative solubilities of the ions in question. Soluble cations such as Ca
or Mg are taken up as cations and EDTA will prevent this uptake. Fe, on
the other hand, is insoluble in water so the EDTA will prevent its
precipitation. Uptake of Ca:EDTA or Fe:EDTA is about the same, but for Ca
it is much too slow relative to Ca+2 uptake, while for Fe it is much
faster than uptake of the free Fe ion.
Since Fe never occurs unchelated in natural, oxygenated systems, plants are
perfectly capable of absorbing chelated Iron.
> (all ferrous compounds have a green coloring).
Ferrous hydroxide is green and quickly turns to Ferric oxyhydroxide in an
> All I can say is this statement is in line with the known chemical and
> biological facts concerning EDTA as I've outlined above. If anyone has any
> evidence otherwise, I would be more than happy to take a look at it.
Well, theres about 50 years of hydroponics and aquatic plant research
that clearly shows that Fe:EDTA delivers adequate amounts of Fe for
sustained maximal growth.
> Actually, pH plays no role (in the aquarium) with either EDTA or gluconate,
> The pKa's of the 4 carboxylic acids of EDTA are 0.26, 0.96, 2.00 and 2.67
> and for gluconic acid it is 3.86 (source Handbook of Biochemistry and
> Molecular Biology, 3ed, Vol 1, p. 310 Fasman, Gerald D. Ed, CRC Press 1976)
Pka values for EDTA are approximately 2.0, 2.7, 6.2, and 10.3.
> >While it may be true that only Fe+2 is taken up by plants, the charge on
> >the chelated Fe is irrelevant to its uptake.
> I'm sorry, I don't follow here. If as you're saying plants only uptake Fe+2
> and the chelated iron is Fe+3, then how is the plant going to uptake the
Current thinking suggests that plants attach the Fe:EDTA complex onto
the membrane and remove the Fe ion. How this occurs is not fully
understood but it is certainly an effective uptake system.