EDTA-Magnesium relationship

Wolfgang Schechinger Wolfgang.Schechinger at med.uni-tuebingen.de
Fri Mar 31 11:15:51 EST 2000


I'm not 100% sure, but I'd say the number is between 1 and 2. 
It depends on how much EDTA is ionized. 
Theoretically, 1 molecule may bind 2 ions Mg (four negative charges 
will complex 2 * 2 positive charges), but since EDTA is a weak acid, 
the molecule will not be 100% ionized in average.

In the end, one would have to do some calculations using the 4
different pK values for EDTA assuming that Mg2+ comes from 100% 
dissociated MgCl2. Of course all depends from the solution's pH.

Happy calculating. Maybe a nice problem for next weeks freshmen's 
anorganic course? Is there any?

It's probably easier to make some Mg2+/EDTA mixtures and add some 
other chelator that will form a coloured complex with free Mg, and 
put the stuff into the photometer.

Have a nice weekend!

Wolfgang




> think about it:- the ED-tetra-acetic-acid has 4x 0minus on the end
> and no direct ionic bond. Therefore WRT divalent cations; it cant be
> 2 and it cant be more, therefore it must be one or none, but it cant
> be none so it must be one. :-)))))
> 
> I must get out more.
> > Only one. I remember that Merck catalog had a picture showing how
> > EDTA binds divalent cations.(Sorry, no catalog at hand).
> > 
> > Michael Allen wrote:
> > 
> > > Hello Bionetters
> > >
> > > I was wondering if any of you could tell me how many divalent
> > > cations (Mg2+ specifically) a single EDTA
> > > (ethylenediaminetetraacetic acid) molecule can bind at one time.
> now this is the tricky bit. you have to compare the preferential
> relationship between Mg2+ and Ca2+ and decide which percentage of
> EDTA will have Mg bound and Ca bound. Then you must consider the
> concentrations of Mg and Ca in the system and then do a calculation.
> 
> > >
> > > I have searched high and low for this, but could not find a good
> > > answer.
> > >
> > > Thanks in advance
> 
> cheers
> bob
> 
> -- 
> Robert Hartley,
> Centre for Cell Engineering,University of Glasgow,UK.
> mail: rh at mblab.gla.ac.uk, Tel: ++44 (0)141 330 4756
> Web : http://www.gla.ac.uk/Inter/CellEngineering
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Dr. Wolfgang Schechinger, Dept. of Pathobiochemistry
University of Tuebingen, Germany
email: wolfgang.schechinger at med.uni-tuebingen.de 
wwWait: http://www.medizin.uni-tuebingen.de/~wgschech/start.htm
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