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Re: EDTA chelation

> From: Greg Morin <greg at seachem_com>
> Subject: Re: iron gluconate
> 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! 
> ;-)

	EDTA complexes generally have both amine nitrogens and three of the
carboxylates attached to the metal atom.  The sixth co-ordination position
on the metal atom is associated with a water molecule.  The pH does _not_
have to be very high for that.  Quite strongly basic amines co-ordinate
well with transition metal ions at moderate pH, because the bond formed
between the nitrogen lone pair and the metal atom is strong.  

Paul Sears        Ottawa, Canada