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Ferrous Gluconate

   I recently responded to some e-mail correspondences from James Purchase 
and Christopher Coleman concerning a discussion on this list about our 
Flourish Iron product. I thought it would be helpful to join the list and 
keep an eye out for any other questions concerning our products that might 
pop up. I realize there is a strong anti-commercial posting clause in the 
list charter so let me just say from the outset I intend to abide by that 
in the strictest sense.

With that said, I would like to respond to the responses to my posted 
letter to Christopher:

(1) EDTA-Iron is in the Fe+3 state, not Fe+2. Every source I have 
investigated shows this to be the case (Merck Index, 12th ed, pg 684#4076, 
Aldrich Catalog, 1998-99 pg#764 Item # 35,961-0 and others). It is actually 
a Fe+3 and Na+ chelate which gives an overall charge neutral species. I 
realize one could argue that you could have 2 Fe+2 to one EDTA to give the 
same net charge, but apparently this compound does not form (or does not 
form readily). There are a lot of compounds that I can draw that are 
chemically correct, but that doesn't mean they can be made. Actually there 
is an experiment you can do to prove this yourself. I'll even provide 
anyone with some samples of these chemicals if you can't get access to 
them. To understand this little experiment you need to know that ferrous 
(Fe+2) compounds give greenish compounds and that ferrous iron in solution 
gives solutions with a greenish cast to it. Ferric (Fe+3) compounds are 
reddish to brown (like rust) and give brownish solutions. Okay, in a glass 
container, add some ferrous sulfate... as it dissolves the solution turns 
from greenish to more of a light brown. It is being oxidized to Fe+3 in the 
water.  Then add some Prime (or another reducing agent, such as ascorbic 
acid (Vitamin C)), about 10 drops or so. The solution then turns back from 
brown to a light green. Now add some tetrasodium EDTA, the solution turns 
back to brown! This is because the EDTA binds up the iron and at some point 
in the process the iron is oxidized to Fe+3. BTW, the tan/brown color of 
Flourish Iron is from the gluconic acid, so, no, Flourish Iron is not all 
oxidized in the bottle.

(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 
acid. Also, the complexation of ferrous iron will change the oxidation 
potential of the iron. From Shriver, Inorganic Chemistry 2ed, 1994, WH 
Freeman & Co., pg 308 "The formation of a more stable complex when the 
metal has the lower oxidation number favors reduction and the reduction 
potential becomes more positive." In this case the comparison is with the 
hexahydrate of Fe+2 vs the gluconate of Fe+2. The gluconate complex is more 
stable and therefore favors remaining in the reduced state. This does not 
mean that the Fe+2 will _never_ be oxidized while complexed to gluconate, 
only that it is going to take longer than if the Fe+2 were not complexed. 
The presence of additional reducing agents (in Flourish Iron) makes that 
process take just that much longer. Keeping it in the reduced form longer 
gives the plants that much more opportunity to absorb it.

(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. 
The evidence I offer for this is mainly empirical. When humans are iron 
deficient the form the iron supplements that they are given is the ferrous 
state. You will never find such supplements in the ferric state. One could 
argue that plants are different from humans. Actually in this respect they 
are not, but, Ok. To this I cite the Sigma Bio Sciences 1996 Plant Culture 
Catalogue pg 68-73 which lists all of the chemical parameters of their 
Plant Culture Basal Salt Mixtures and Media. This is basically a list of 
different culture media for the hydroponic growth of plants in research. So 
it is assumed here the researchers _want_ the plants to grow well :-). Of 
the 50 different mixtures listed 86% listed exclusively a ferrous source of 
iron (ferrous sulfate€7H2O), only 12% listed exclusively a ferric source of 
iron (2% had no iron source). Again, I want to reiterate, this does not 
mean that plants _can't_ use ferric iron, only that they can use ferrous 
much more easily. When a researcher is not interested in evaluating the 
iron parameter he/she uses the most readily accessible source, ferrous. The 
12% and 2% cited are most likely special mixtures where they do want to 
evaluate ferric uptake and utilization. Also, here I will finally cite 
something that most people on this list do have available to them and that 
is the Aquarium Atlas Volume 2 by Hans Baensch, 1ed, 1993, pg 150.  Here it 
is stated that "... as a rule, only ferrous (Fe+2) is available for aquatic 
plants." This leads into my next response...

(4) The biological relevance of EDTA-Iron vs Gluconate-Iron. From the above 
Baensch reference on page 151 is where the idea must be coming from that 
EDTA complexed iron is available to plants (he says exactly that). Here I 
must disagree with Mr. Baensch. The best way to look at this is to simply 
back up and look at what EDTA actually is and does. EDTA is a non-natural 
compound that is used to _sequester_ metal ions in living organisms. For 
example EDTA is used in severe cases of lead or heavy metal poisoning 
[Merck Index 12th ed, pg 593#3559]... i.e. it is so strong it can chelate 
these toxic metals and render them _inert_ . Here I would like to quote 
from a message I received from James Purchase which is relevant: "On the 
topic of E.D.T.A and its dangers, I came across just such a reference in 
the literature - but it was dealing with E.D.T.A. being added to an 
aquarium by itself (in attempt to bind nutrient ions already present). 
Caution was urged as overdosing E.D.T.A could lead to the nutrients 
actually being sucked out of the plants, leading to collapse. Most of us, 
when dealing with E.D.T.A., use it in a form where it has already been 
associated with the nutrient ions." 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? 
If the EDTA can pull the nutrients out, then clearly the EDTA is the 
stronger of the two in this tug of war. Now on the other side of the 
coin... Gluconate is a naturally occurring, thus biological systems are 
going to be equipped with the tools necessary to metabolize/interact with 
it; they are not equipped to metabolize/interact with EDTA.The natural 
result of such interactions will be the release of its complexed iron. And 
again, I come back to the medical approach: What do physicians give 
patients that are low in iron, do they give them EDTA-Iron? No. They give 
them ferrous gluconate (which I believe Beverly Erlebacher was eluding to 
in her message regarding ferrous gluconate as being a cheap substitute for 
aquarium chemicals).

(5) With respect to the statement that Flourish Iron simply "oxidizes and 
precipitates as rust": although I can't rule out the possibility that some 
of it is doing just that, the only precipitates we have observed are 
ferrous compounds (probably carbonate), this is based on their green 
coloring (all ferrous compounds have a green coloring). Even though 
Flourish Iron is targeted as a supplement for the stems and leaves, 
whatever material that does fall out before being adsorbed by the stems and 
leaves can still be adsorbed by the root system of the plants.

(6) With respect to Mortimer Snerd's query about massive algae outbreaks 
after using Flourish and Flourish Iron with each water change: The Flourish 
Iron definitely is not the source of the problem here. It is formulated 
such that constant dosing will not lead to unwanted organic build up that 
can contribute to algae growth. This product was actually an outgrowth of a 
difficulty many people had with the initial release of Flourish. Flourish 
has the same gluconate iron and to keep their iron up, people would dose 
quite often. The problem was that they were adding too much of all of the 
other stuff in Flourish that isn't consumed as rapidly as the iron. This 
led to a build up of too much organics. Flourish still has the iron in it, 
but you wouldn't want to use Flourish exclusively to maintain an iron 
level. We then spun off Flourish Iron so it could be dosed on a separate 
schedule without leading to organic buildup. The dosing for Flourish is 
based on an assumed average plant load, and it may be that you should cut 
back the amount of Flourish you are using because your plant load is either 
lower than average or the variety of plants you have don't utilize the 
vitamins and amino acids as rapidly as other plants.

>I think there is this opinion on Seachems part that EDTA is
>two strong ( actually they commented that "The amount of EDTA-iron
>that the plants are able to use is so small as to not really be useable"  )
>This is a pretty strong statement,

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.

>And I believe there are pH dependencies here clouding any
>blanket statements about either product.

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

>It also seems to me that if the gluconate form does get into the water
>more readily, that it has the potential to preferentily benefit algae.

I'm afraid I don't understand the reasoning behind this statement. Are you 
saying plants are better able to "extract" iron from EDTA-iron when 
compared to algae? And that if the iron is more easily accessible then the 
algae has the upper hand?

>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 

>What is more, gluconate will not be an effective reducing agent
>for metal ions.  Glucose itself can be, but the aldehyde group on
>glucose has already been oxidised when we get to the acid.  The OH
>groups won't reduce metal ions.

Well, actually the gluconate is a reducing agent in the same manner as 
ascorbic acid is a reducing agent (although not as strong as ascorbate). 
This question is also answer in question (2) above with respect to the 
change in oxidation potential of complexed iron. The additional reducing 
agents we employ in Flourish Iron also help to keep the iron in the reduced 
state. But as I said above, this only prolongs the inevitable, giving the 
plants more opportunity to to adsorb the iron.

> The idea of the chelating agent is to keep the iron in solution, not to
>maintain a II oxidation state.

But if you can do both, then wouldn't that be better? Also, keeping it in 
the water doesn't matter if the plants can't sufficiently utilize it. Of 
course that depends on one's definition of "sufficient."

>The main advantage of gluconic acid is,
>I suspect, that it is very cheap.  It is used in cleaning solutions
>because (under alkaline conditions, at least) it will hang on to metal
>ions.  EDTA will be a lot more effective.

Actually gluconic acid is more expensive that EDTA. I'm curious, what 
cleaning solutions employ gluconic acid? Why would they use it for the 
metal chelating ability... EDTA would be a much better choice? ;-)

I'm sorry for the length of this message, but I wanted to be complete and 
thorough and to try and answer everyone's questions regarding gluconate 
iron as best I could. And if anyone has any cited references that 
contradicts anything I've said, I would very much like to take a look at 
them. If I find any of my points are in error based on such references I 
will of course retract or clarify such points. If anyone can not gain 
access to a reference I've cited, I'd be happy to photocopy and mail it to 
you. I can't post this kind of stuff on the web due to copyright issues 
(with the respective publisher of each source).

-Greg Morin
Gregory Morin, Ph.D.  ~Research Director~~~~~~~~~~~~~~~~~~~~
Seachem Laboratories, Inc.      www.seachem.com     888-SEACHEM