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Re: iron gluconate

>EDTA-chelated iron (as well as DTPA chelates and a few others) have

>in horticultural use for decades with a record of apparent

Does anyone know where I might find the documentation that supports the
record of apparent effectiveness? I just want to be able to take a look
at the studies first hand...

>It sounds to me like the precipitate you're describing might be found

>the bottle.  Ferrous carbonate is the natural mineral siderite, which

>usually brown, not green; maybe a fresh precipitate would be green. 

>doubt that you would see iron carbonate in aquarium use, where the

>would be too dilute and the pH too low to precipitate the carbonate.

Well we haven't observed any precipitation in the bottle. The precipt I
was referring to was seen in a container of fresh water from a planted
tank here, after a day or so there was a _very_ light dusting of light
green powder on the bottom which we presumed might be the carbonate
(since the other counter anions in the water would produce only soluble
Fe+2 salts. The Merck Index (12ed, pg 4087#4089) states that ferrous
carbonate is practically insoluble in water. Since we know that Fe+2 is
rapidly oxidized in the water by itself (within seconds) this would
suggest that the gluconate is keeping the iron in the Fe+2 state  long
enough for an anion exchange (carbonate for gluconate) process to yield
the precipitate we're seeing; if it is around long enough that the
precipt is not noticeable until a day later, then this would suggest
the ferrous gluconate is around long enough to be utilized in the Fe+2
state (i.e. at least a day).

>How is ferrous gluconate "targeted" to the leaves and stems?

By keeping the iron solubilized the leaves and stems have a greater
opportunity to absorb the iron than do the roots. This "opportunity" is
based on two factors: (1) The leaves and stems represent a much higher
percentage of the plant's surface area than do the roots and (2) over a
given unit of time a greater volume of water passes by the leaves and
stem (unless an UGF is employed, in which case the water flow
discrepancy is not as great). 

At this point I would like to publicly retract/modify my initial
statement concerning the near inability of plants to utilize EDTA-Iron.
Based on the responses I've received to my posting and further reading,
I now suggest the following mechanisms. 

The success the horticulture industry has seen with EDTA-Iron has been
in its capacity as a fertilizer/root supplement. Based on what we know
about plant roots and their ability to release chelates
(phytosiderophores) and extract iron from their chelates it would be
reasonable to assume the plants can extract the iron from the EDTA-Iron
chelate as well. Also, since we know that the iron in the soil is by
far mainly in the Fe+3 state, the fact that EDTA-Iron is in the Fe+3
state as well has little impact on the benefit of EDTA-Iron vs normal
soil extraction. Although I think we all agree that the plant would
much "prefer" to use Fe+2 if available.

 In an aquatic environment the same rules apply, the benefit that
people see from using EDTA-Iron supplements is from the capacity of
those plant's roots to extract the iron from the EDTA. The leaves and
stems do not have the chemical machinery to extract iron from a
chelate. This last statement is based on conjecture on my part in that
(a) I have seen no evidence to the contrary and (b) there would be no
evolutionary advantage to leaves and stems producing siderophores to
extract iron as those siderophores would simply be washed away. If they
don't produce siderophores then they have no need for chemical
machinery to readsorb chelates and extract the metal. 

The advantage to Flourish Iron is that it supplies stabilized ferrous
iron. The leaves and stems _can_ use the ferrous iron and I suppose
they can use ferric iron as well, just not nearly as well as ferrous.
So if you have plants that have a great capacity to adsorb iron from
their stems and leaves, Flourish Iron is what you would want to use. If
that capacity is not so great or you're satisfied with the level of
iron absorbed through the roots in relation to the plants growth and
appearance, then EDTA-Iron will be suitable (although gluconate iron is
still going to be _better_ than EDTA-Iron with respect to charge and
ease of absorption). So the short summary is: EDTA-Iron suitable for
roots, gluconate iron good for leaves, stems and roots.

>> Actually, pH plays no role (in the aquarium) with either EDTA or

>> The pKa's of the 4 carboxylic acids of EDTA are 0.26, 0.96, 2.00 and

>> and for gluconic acid it is 3.86 (source Handbook of Biochemistry

>> Molecular Biology, 3ed, Vol 1, p. 310 Fasman, Gerald D. Ed, CRC
Press 1976) 

>> Since plant tank pH is going to be well above pH 4 all the acids are

>> ionized.


>My reference puts the pKa of EDTA at 2.008, 2.683, 6.098 and 10.181.

>values would indicate considerable importance for pH on EDTA

>The values I sited are listed by James Plambeck on the University of

>Alberta web site at:



<excerpt>Virtually every source I've read on iron chelates indicates
that the

stability of the complex is pH dependent.  For EDTA, in particular the

complex starts to break down at pH over about 6 and it's generally not

useful at pH well over 7.  Other chelates are more stable at higher pH



Well, we're both right. I listed the pKa's for the carboxylic acids
only. The 6.0 and 10.1 pKa values are for the amines.  However, I think
you got the pH importance backwards ;-) As the pH goes below 6 the
interaction of one of the amines becomes less and less important (as it
becomes increasingly ionized). Going above 6 decreases the ionization
of the first amine resulting in a stronger interaction of the amine
with whatever metal is chelated. You would need to go above a pH of 10
for maximum chelation (so that both amines are unionized) but there is
really no need to do that. (So I guess I just talked you out of trying
a ferrous gluconate product! ;-) 

>Uncomplexed Fe+3 iron is virtually non-existent under aquarium

Why? If there is no EDTA present in the aquarium then what happens to
the Fe+3 if it is non-existant? I initially had thought this too... but
I see in the Merck Index that there are several soluble ferric
compounds, so that would imply the Fe+3 remains solubulized. 

>> (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

>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

>cation(Skoog and West. Fundamentals of Analytical Chemistry)."

Based on the known properties of EDTA, it's not unreasonable to have
_assumed_ EDTA can complex Fe+2... however, based on everything I've
read this complex simply does NOT form. I don't know _WHY_ it doesn't
form, but the fact that remains is that it just doesn't. If anyone has
any reference that  specifically refers to an Iron(II) EDTA complex I
would be very interested to see it

>In a biologically active medium saturated with oxygen, ferrous iron is

>simply not stable for any length of time (minutes or less).

Even if that ferrous iron is complexed by a reducing agent and there is
a suitable amount of reducing compounds that have been added to the
water? Are you saying that these have NO EFFECT at all on the average
length of time that the Fe+2 will remain in the ferrous state before
being oxidized? Based on what we've seen, Flourish Iron will keep the
iron in the ferrous state for at least a day.

>No, in countless experiments over many years, research has clearly

>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

>75 to 300 (1 to 4 day doubling time).

This sounds very interesting... could you provide references to the
research you cite? Please tell me this was a root supplement only? ;-)

>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.

Again, I would be very interested in being pointed to some of the
relevant literature here...


-Greg Morin

Gregory Morin, Ph.D.  ~Research Director~~~~~~~~~~~~~~~~~~~~

Seachem Laboratories, Inc.      www.seachem.com     888-SEACHEM