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> > Again, for EDTA, that is about 2.0, 2.7, 6.2 and 10.0 for the four
> > carboxylic acid groups.
Greg Morin wrote:-
> Ok, looking at this again I see that these are probably in fact the pKas
> for the acids, but I'm still at a loss as to why my reference gave values
> of 0.2 and 0.9 as well...
I think I would read the reference pretty carefully. Those pKa's
are for stronger acids than carboxylic.
> maybe so weirdo gas phase kinetic values...
pKa's are for aqueous solutions, and are _equilibrium_, not kinetic
> any case, I think we're still in agreement that as the pH goes up the EDTA
> complex becomes more stabile (increased acid ionization and decreased amine
There are two parts to deciding whether a complex is "stable" or not.
One has to look at how easily it comes apart, and how readily the products
do something else (so it doesn't reform). The pKa's of the acids and the
pKb's of the amines (protonation, not ionization) tell you a bit about
how readily the EDTA will do something else, and it appears from the pKa's
that two of the acid groups will be essentially competely ionized, and a third
will be at least partly so (at aquarium pH). The amine groups will be
mostly protonated. I haven't been able to find the pKb's, but I would
expect them to be about 8-9. (Does anyone have the numbers?)
Far stronger bases than this form stable compexes with transition
metal ions, because the +N-H bond in the protonated amine is in competion
with a +N-M bond that can form. The point about a chelating agent, is that once
it starts to latch on to a metal ion, it is likely to latch on with more
of its parts, because they can't get very far away. Once it is completely
latched on, getting off is difficult, because even when one bit comes off,
it can't get away in the same way that a separate ion could, so is likely
to reattach before another bit comes away.
This means that a chelate complex is more stable than one would
predict from just the bond energies involved. A complex the flies into
many pieces is more likely to fall apart and less likely to get together.
In thermodynamic terms, a chelate is more stable because of the entropy
term in the equations concerning stability.
Paul Sears Ottawa, Canada