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EDTA - A Molecule with a
Complex Story
Scott A.
Sinex
Prince George's Community College -- Reprinted from
http://www.chm.bris.ac.uk/motm/edta/edtah.htm
Note from Dr. Kulisz: This is
an excellent article demystifying and explaining principles of the synthesis of
and the chelation with the ethylenediaminetetraacetic acid (EDTA).
We thank Dr. Sinex for this excellent article.
What do beer, the special sauce on a
McDonald's Big Mac, and blue colored shampoos such as Aloe Vera or Revlon
Aquamarine have in common?
The EDTA Molecule
EDTA or ethylenediaminetetraacetic
acid is a novel molecule for complexing metal ions. It is a polyprotic acid
containing four carboxylic acid groups (acidic hydrogens are
red) and two amine groups with lone pair electrons
(green dots). The classic structural formula is
given below. EDTA is synthesized on an industrial scale from ethylenediamine,
formaldehyde, and a
source of cyanide (HCN or NaCN).
Click here for the industrial reactions to open in a new window. On a
worldwide basis over 100,000 metric tons are produced annually.
Besides the four carboxylic group
hydrogens, EDTA can add two more hydrogens onto the amine groups. The
structures of the fully protonated form (left), the typical form found in many
textbook (center, matching the 2D structure above), and the fully deprotonated
(all acidic H's removed) form (right) are given below.
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forms at very low pH or
very acidic condition
(fully protonated)
H6Y+2 |
classic form
H4Y |
forms at very high pH
or
alkaline conditions
(fully deprotonated)
Y-4 |
Complexing Metals
The unusual property of EDTA is its
ability to chelate or complex metal ions in 1:1 metal-to-EDTA complexes. The
fully deprotonated form (all acidic hydrogens removed) of EDTA binds to the
metal ion. The equilibrium or formation constants for most metals, especially
the transition metals, are very large, hence the reactions are shifted to the
complex. Many of the reactions are pH dependent, especially the weaker forming
complexes with Ca+2 or Mg+2.
M+n + Y-4
 MYn-4
Kf = (MYn-4)/(M+n)(Y-4)
Metal analysis can be done by
titration with EDTA and the use of a metal ion indicator. The pioneering work
with EDTA was done by Gerold Schwarzenbach in the 1940's. The common reagent
for making EDTA solutions is Na2H2Y.2H2O.
Click here for a
list of formation constant (Kf) values. The values of Kf
increase with the charge on the metal ion and as ionic radius decreases with
constant charge. Water hardness, mostly from dissolved Ca+2 and Mg+2,
is determined by EDTA titration at pH = 10 -
click here for a
typical experiment.
The structure of a classical complex
of Fe+3 with EDTA is shown below. This is EDTA acting as a
hexadentate ligand or all six sites on the ETDA bind to the metal ion. How
would you describe the geometry of the octahedral Fe+3 in the
complex?
The octahedral coordination of the
Fe-EDTA (left) and many other such complexes are very strained. The space-fill
version of the structure (right) shows how crowded the structure appears.
Consider the structure of Cr+3 with EDTA shown below. What is
different for the Cr-EDTA complex compared to the Fe-EDTA complex?
A number of metal-EDTA complexes have
been reported to have the EDTA acting as a pentadentate ligand (only five sites
on EDTA bind, one carboxylic group does NOT). A water molecule or another
ligand is in the sixth site, so the complexes are still octahedral in geometry.
So let's examine the formation of the
complex. The animation below was produced in Spartan '04 by placing an Fe+3
(green) next to the EDTA with acidic H's removed (fully deprotonated as Y-4)
and then minimizing the energy. Note that EDTA forms only five bonds to the Fe+3,
as the complex forms (center image is complex at end of animation). The image
to the right has a molecule of water added, plus the EDTA has picked up a
hydrogen ion on the carboxylic acid group not bound to the iron, and energy
minimized to form the six coordinate complex.
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Animation
of Y-4 complexing Fe+3
This is an avi file, so you'll need to have Real
Player or Windows Media Player configured accordingly in order to
see it. |
complex with EDTA as a
pentadentate ligand |
octehedral complex with
EDTA and water |
Uses of EDTA
Because of its strong complexing
ability for most metal ions, it is used in the food industry as a sequestering
agent. The complexing of the metal ion may prevent further reactions, such as
binding metals that are cofactors for enzymes, or just remove a metallic taste,
such as metal contamination added during processing. See the Dow Chemical site
on the use of their commercial product -
Versene. Some
typical examples are given below.
Recent studies have shown that
NaFeEDTA and Na2EDTA added to typical iron fortification compounds in
cereals increased the absorption of iron in adult humans.
This same property allows EDTA use
for incidents of lead poisoning by the medical profession. The formation
constant for Pb-EDTA complex is 1018. Intravenous injection of Na2CaEDTA
solution is given at 25 mg/kg body mass/day over 6 hours for 5 days when blood
lead levels go over 45 mg/dL.
The Pb+2 ion replaces the Ca+2 ion in the complex because
the formation constant for the lead complex is greater than the calcium complex.
Pb+2 + CaY-2
PbY-2
+ Ca+2 K ~ 108
The five day limit is there to
prevent Zn+2 depletion, since the Zn+2 ion replaces the Ca+2
ion in the complex too. EDTA is added to stored blood in blood banks as an
anticoagulant to bind Ca+2 ion.
Other reported uses of EDTA in medicine do not have a proven clinical basis.
Click here for more information.
Another major use of EDTA has been in
detergents to act as a builder (chelates metals) especially as a replacement for
phosphates, a major nutrient in wastewater. However, a problem with EDTA is its
inability to biodegrade in the environment. EDTA is found in many natural
waters and occurs at higher levels in wastewater effluents. Western European
countries have banned the use of EDTA in detergents. Recently a ban was adopted
in
Australia. EDDS (S, S'-ethylenediaminedisuccinic
acid), a structural isomer of EDTA, has been used as a biodegradable
substitute. EDDS is a good complexing agent and is broken down during
wastewater treatment processes. This is a contribution to green chemistry by
making a minor structural change. The cumulative production of EDDS by
Octel surpassed 10,000 metric tons in 2002.
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| EDDS
structure showing chiral carbons in red
- both must be S. l-Aspartic acid in green
box. |
biodegradable S, S'- EDDS molecule |
octahedral
FeEDDS- complex |
EDTA is added to many commercial
beers to stabilize foaming, taking advantage of the surfactant properties of
EDTA, and used to remove scale by complexing calcium from calcium carbonate that
forms on the processing equipment.
The blue color of the Cu-EDTA complex
is used in many shampoos. See the
FDA use of disodium
EDTA-copper in cosmetics.
References
Ethylenediaminetetraacetic Acid and
Related Chelating Agents in Ullmann's Encyclopedia of Industrial Chemistry
Vol. A10
What's that stuff: Food
Preservatives, C&EN, 11 November 2002
http://pubs.acs.org/cen/science/8045/8045sci2.html
CSPI's
Guide to Food Additives
http://www.cspinet.org/reports/chemcuisine.htm
The Brewer's Handbook
http://www.beer-brewing.com/index.htm
Poisoning in The Merck Manual
(scroll down to lead poisoning)
http://www.merck.com/mrkshared/mmanual/section19/chapter263/263b.jsp
UK Awards for Green Chemical
Technology to Octel Performance Chemicals
http://www.chemsoc.org/pdf/gcn/Octel_Award.pdf
A Brief History of Inorganic
Classical Analysis
http://www.cstl.nist.gov/nist839/839.01/beckhist.pdf
Storing Up Trouble
http://www.chemsoc.org/chembytes/ezine/1998/store.htm
EDTA: The Chelating Agent under
Environmental Scrutiny
http://www.scielo.br/pdf/qn/v26n6/a20v26n6.pdf
Chemical Speciation of EDDS and its
metal complexes
http://www.scilet.com/Papers/csb/csb113/CBSWhitburn.pdf
An Evaluation of EDTA Compounds for
Iron Fortification of Cereal-based Foods
Br. J. Nutr. 84 (6), 903-910 (2000)
Emerging Chemical
Drinking Water Contaminants in Identifying Future Drinking Water Contaminants
http://www.nap.edu/books/0309064325/html/112.html
Just for FUN: See the EDTA song -
click here.
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