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Re: Iron: a factoid and two questions

ac554 at freenet_carleton.ca (David Whittaker) commented: 
> >... 
> One wishes to measure only the iron available to the plants. An acid 
> digestion might catch iron which is normally not available.

True.  This raises my other question: Does anyone know what _is_
available to the plants?  With a proper lab, a person might try, for
instance growing plants in a well-aerated hydroponic setup with good
supplies of all trace elements, but iron available only from a chelate
of Fe+++. Or unavailable, as it might prove to be.  Sounds like the sort
of thing somebody would have done, but we don't know the right graduate
student to make a project of finding out.

> Several 
> people have claimed that the test they use requires an extended wait 
> to obtain results with EDTA. Check the Archives.

I saw some of these.  I gave my LaMotte kit much longer than the
recommended time to spot that 10 ppm of chelated iron, but it still
didn't work.

> Personally I think 
> that the Emperor has no clothes, but then I'm not a fashion expert.

Heck, I'm not even sure which Emperor to look at.
> Craig Bingham suggested using one of the Hach kits with a potassium 
> persulfate digestion to oxidize the chelating agent. It would be best 
> to read his post on this. I'd love to test for iron, but like you, I 
> am unsure of what each is measuring.

But we know that whatever Dupla is using is measurable and works.  If
only I jnew where to go from there.
> The following are found in various micronutrient mixes. 
> EDTA(ethylene-diamine tetra acetic acid) 
> DTPA(diethylene triamine pentaacetic acid) 
> EDDHA(ethylene-diamine dihydroxyphenyl acetic acid) 
> Iron gluconate 
> Iron sulfate 
> lignosulfonic chelants of iron 



And Shawn Keslar <keslar at wvunrcce_nrcce.wvu.edu>  says,
> Subject: Iron 
> I believe that I have seen iron chelated with NTA (nitrilotriacetic 
> acid) and DTPA (diethylenetriaminepentaacetic acid - say THAT fast five 
> times) in commercial mixes.

> ...  We adjust our iron conc. by bringing an acidified sample 
> here to work, where we have a plasma sectrometer that's usually running. 
> One of the luxuries of being a chemist

Great solution!  Wish I could do that.  However, see above, do we know
what's available iron and what isn't?
> ....with respect to the ferrous/ferric question, I have seen some 
> references (on the Krib, or on Steve Pushak's page, maybe) to some 
> plants having an enzyme named ferric reductase.  This enzyme reduces 
> ferric iron to ferrous iron that the plant can use. 

Now that makes sense.  You'd expect plant life to be smart enough to do
that.  Raises another matter, though: where would they get ferric ion to
work with, given its low solubility at reasonable pH?  Maybe by breaking
down complexes?  Like some humic-acid equivalent of EDTA?  

[He asked his hairy uncle the baboon, and he spanked him with his hairy
paw.  He asked -- but I've forgotten who the Elephant Child's other
uncles and aunts are.]

(But since you're an actual chemist -- Have I slipped a cog when I look
at the solubility product for Fe(OH)3, which is about 1e-36 (10 the minus
36) and conclude that at pH 7 you can have up to 1e-15 molar Fe+++ in
solution, and not more?)

Dan Drake
dd at dandrake_com