Ethylenediaminetetraacetic acid ( EDTA), also called EDTA acid, is an aminopolycarboxylic acid with the formula . This white, slightly water-soluble solid is widely used to bind to iron (Fe2+/Fe3+) and calcium ions (Ca2+), forming water-soluble complexes even at neutral pH. It is thus used to dissolve Fe- and Ca-containing scale as well as to deliver iron ions under conditions where its oxides are insoluble. EDTA is available as several salts, notably disodium EDTA, sodium calcium edetate, and tetrasodium EDTA, but these all function similarly.
Uses
EDTA is widely used in industry. It also has applications in food preservation, medicine, cosmetics, water softening, in laboratories, and other fields.
Industrial
EDTA is mainly used to
Sequestrant (bind or confine) metal ions in aqueous solution. In the
Textile, it prevents metal ion impurities from modifying colours of dyed products. In the pulp and paper industry, EDTA inhibits the ability of metal ions, especially
manganese, from catalysing the disproportionation of hydrogen peroxide, which is used in chlorine-free bleaching.
Gas scrubbing
Aqueous Fe(EDTA)
− is used for removing ("
Sulfur scrubbing")
hydrogen sulfide from gas streams. This conversion is achieved by oxidising the hydrogen sulfide to elemental sulfur, which is non-volatile:
In this application, the iron(III) centre is
Redox to its iron(II) derivative, which can then be reoxidised by air. In a similar manner,
are removed from gas streams using .
Food
In a similar manner, EDTA is added to some food as a
preservative or stabiliser to prevent catalytic oxidative decolouration, which is catalysed by metal ions.
Water softener
The reduction of water hardness in laundry applications and the dissolution of scale in boilers both rely on EDTA and related complexants to bind
calcium,
magnesium, as well as other metal ions. Once bound to EDTA, these metal complexes are less likely to form precipitates or to interfere with the action of the
and
. For similar reasons, cleaning solutions often contain EDTA. In a similar manner EDTA is used in the cement industry for the determination of free
Limestone and free
Magnesium oxide in cement and clinkers.
The solvation of iron ions at or below near neutral pH can be accomplished using EDTA. This property is useful in agriculture including hydroponics. However, given the pH dependence of ligand formation, EDTA is not helpful for improving iron solubility in above neutral soils.
Ion-exchange chromatography
EDTA was used in separation of the
by ion-exchange chromatography. Perfected by F. H. Spedding
et al. in 1954, the method relies on the steady increase in stability constant of the lanthanide EDTA complexes with
atomic number.
Using
polystyrene beads and
copper as a retaining ion, EDTA causes the lanthanides to migrate down the column of resin while separating into bands of pure lanthanides. The lanthanides elute in order of decreasing atomic number. Due to the expense of this method, relative to countercurrent solvent extraction, ion exchange is now used only to obtain the highest purities of lanthanides (typically greater than 99.99%).
Medicine
Sodium calcium edetate, an EDTA derivative, is used to bind metal ions in the practice of chelation therapy, such as for treating mercury and
lead poisoning.
It is used in a similar manner to remove excess
iron from the body. This therapy is used to treat the complication of repeated blood transfusions, as would be applied to treat
thalassaemia.
In testing
In medical diagnosis and organ function tests (here,
kidney function test), the
chromium complex Cr(EDTA)
− (as radioactive chromium-51 (
51Cr)) is administered
intravenously and its filtration into the
urine is monitored. This method is useful for evaluating glomerular filtration rate (GFR) in
nuclear medicine.
EDTA is used extensively in the analysis of blood. It is an anticoagulant for blood samples for CBC/FBCs, where the EDTA chelates the calcium present in the blood specimen, arresting the coagulation process and preserving blood cell morphology.
EDTA is a slime dispersant, and has been found to be highly effective in reducing bacterial growth during implantation of (IOLs).
Dentistry
and
use EDTA solutions to remove inorganic debris (
smear layer) and lubricate the
in endodontics. This procedure helps prepare root canals for
obturation. Furthermore, EDTA solutions with the addition of a
surfactant loosen up
inside a root canal and allow instrumentation (canal shaping) and facilitate apical advancement of a file in a tight or calcified root canal towards the apex.
Eyedrops
It serves as a
preservative (usually to enhance the action of another preservative such as benzalkonium chloride or
thiomersal) in ocular preparations and
eyedrops.
Alternative medicine
Some alternative practitioners believe EDTA acts as an
antioxidant, preventing
from injuring
blood vessel walls, therefore reducing
atherosclerosis.
These ideas are unsupported by scientific studies, and seem to contradict some currently accepted principles.
The U.S. FDA has not approved it for the treatment of atherosclerosis.
Cosmetics
In
, cleaners, and other personal care products, EDTA salts are used as a sequestering agent to improve their stability in air.
Laboratory applications
In the laboratory, EDTA is widely used for scavenging metal ions: In
biochemistry and molecular biology, ion depletion is commonly used to deactivate
metalloenzyme, either as an assay for their reactivity or to suppress damage to
DNA,
, and
.
EDTA also acts as a selective
Enzyme inhibitor against dNTP hydrolyzing enzymes (
Taq polymerase, dUTPase, MutT),
liver
arginase and horseradish peroxidase
independently of metal ion
chelation. These findings urge the rethinking of the utilisation of EDTA as a biochemically inactive metal ion scavenger in enzymatic experiments. In analytical chemistry, EDTA is used in complexometric titrations and analysis of
water hardness or as a
masking agent to sequester metal ions that would interfere with the analyses.
EDTA finds many specialised uses in the biomedical labs, such as in veterinary ophthalmology as an collagenase to prevent the worsening of corneal ulcers in animals. In tissue culture, EDTA is used as a chelating agent that binds to calcium and prevents joining of cadherins between cells, preventing clumping of cells grown in liquid suspension, or detaching adherent cells for passaging. In histopathology, EDTA can be used as a decalcifying agent making it possible to cut sections using a microtome once the tissue sample is demineralised.
EDTA is also known to inhibit a range of metallopeptidases, the method of inhibition occurs via the chelation of the metal ion required for catalytic activity.
Other
The oxidising properties of Fe(EDTA)
− are used in
photography to solubilise
silver particles.
EDTA is also used to remove crud (corroded metals) from fuel rods in nuclear reactors.
Side effects
EDTA exhibits low acute toxicity with (rat) of 2.0 g/kg to 2.2 g/kg.
It has been found to be both
Cytotoxicity and weakly
Genotoxicity in laboratory animals. Oral exposures have been noted to cause reproductive and developmental effects.
The same study
also found that both dermal exposure to EDTA in most cosmetic formulations and inhalation exposure to EDTA in
cosmetic formulations would produce exposure levels below those seen to be toxic in oral dosing studies.
Synthesis
The compound was first described in 1935 by Ferdinand Münz,
who prepared the compound from
ethylenediamine and chloroacetic acid.
[. Also ] Today, EDTA is mainly synthesised from
ethylenediamine (1,2-diaminoethane),
formaldehyde, and
sodium cyanide.
This route yields the tetrasodium EDTA, which is converted in a subsequent step into the acid forms:
This process is used to produce about 80,000 tonnes of EDTA each year. Impurities cogenerated by this route include glycine and nitrilotriacetic acid; they arise from reactions of the ammonia coproduct.
Nomenclature
To describe EDTA and its various
Protonation, chemists distinguish between , the
conjugate base that is the
ligand, and H
4EDTA, the precursor to that ligand. At very low pH (very acidic conditions) the fully protonated H
6EDTA
2+ form predominates, whereas at very high pH or very basic condition, the fully deprotonated form is prevalent. In this article, the term EDTA is used to mean H
4− xEDTA
x−, whereas in its complexes stands for the tetraanion ligand.
Coordination chemistry principles
In coordination chemistry, is a member of the aminopolycarboxylic acid family of ligands. usually binds to a metal cation through its two amines and four carboxylates, i.e., it is a
hexadentate ("six-toothed")
chelating agent. Many of the resulting coordination compounds adopt octahedral geometry. Although of little consequence for its applications, these octahedral complexes are chiral. The
cobalt anion Co(EDTA)
− has been resolved into
.
Many complexes of adopt more complex structures due to either the formation of an additional bond to water,
i.e. seven-coordinate complexes, or the displacement of one carboxylate arm by water. The
iron Ferric EDTA of EDTA is seven-coordinate.
Early work on the development of EDTA was undertaken by Gerold Schwarzenbach in the 1940s.
EDTA forms especially strong complexes with
manganese,
copper, Fe(III),
lead and Co(III).
Several features of EDTA's complexes are relevant to its applications. First, because of its high denticity, this ligand has a high affinity for metal cations:
- Keq = 1025.1
Written in this way, the equilibrium quotient shows that metal ions compete with protons for binding to EDTA. Because metal ions are extensively enveloped by EDTA, their catalysis are often suppressed. Finally, since complexes of are , they tend to be highly soluble in water. For this reason, EDTA is able to dissolve deposits of and .
The p Ka values of free EDTA are 0, 1.5, 2, 2.66 (deprotonation of the four ) and 6.16, 10.24 (deprotonation of the two ).
Environmental concerns
Abiotic degradation
EDTA is in such widespread use that questions have been raised whether it is a persistent organic pollutant. While EDTA serves many positive functions in different industrial, pharmaceutical and other avenues, the longevity of EDTA can pose serious issues in the environment. The degradation of EDTA is slow. It mainly occurs
in the presence of sunlight.
The most important process for the elimination of EDTA from surface waters is direct photolysis at wavelengths below 400 nm.
Biodegradation
In many industrial wastewater treatment plants, EDTA elimination can be achieved at about 80% using
microorganisms.
Resulting byproducts are ED3A and iminodiacetic acid (IDA) – suggesting that both the backbone and acetyl groups were attacked. Some microorganisms have even been discovered to form nitrates out of EDTA, but they function optimally at moderately alkaline conditions of pH 9.0–9.5.
Several bacterial strains isolated from sewage treatment plants efficiently degrade EDTA. Specific strains include Agrobacterium radiobacter ATCC 55002
Alternatives to EDTA
Interest in environmental safety has raised concerns about biodegradability of aminopolycarboxylates such as EDTA. These concerns incentivize the investigation of alternative aminopolycarboxylates.
[ Candidate chelating agents include nitrilotriacetic acid (NTA), iminodisuccinic acid (IDS), polyaspartic acid, EDDS, methylglycinediacetic acid (MGDA), and L-Glutamic acid N, N-diacetic acid, tetrasodium salt (GLDA).]
Iminodisuccinic acid (IDS)
Commercially used since 1998, iminodisuccinic acid (IDS) biodegrades by about 80% after only 7 days. IDS binds to calcium exceptionally well and forms stable compounds with other heavy metal ions. In addition to having a lower toxicity after chelation, IDS is degraded by Agrobacterium tumefaciens (BY6), which can be harvested on a large scale. The enzymes involved, IDS epimerase and C−N lyase, do not require any cofactors.
Polyaspartic acid
Polyaspartic acid, like IDS, binds to calcium and other heavy metal ions. It has many practical applications including corrosion inhibitors, wastewater additives, and agricultural polymers. A polyaspartic acid-based laundry detergent was the first laundry detergent in the world to receive the EU Ecolabel.
S,S-Ethylenediamine-N,N′-disuccinic acid (EDDS)
A structural isomer of EDTA, EDDS (EDDS) is readily biodegradable at high rate in its S, S form.
Methylglycinediacetic acid (MGDA)
Trisodium dicarboxymethyl alaninate, also known as methylglycinediacetic acid (MGDA), has a high rate of biodegradation at over 68%, but unlike many other chelating agents can degrade without the assistance of adapted bacteria. Additionally, unlike EDDS or IDS, MGDA can withstand higher temperatures while maintaining a high stability as well as the entire pH range. MGDA has been shown to be an effective chelating agent, with a capacity for mobilization comparable with that of nitrilotriacetic acid (NTA), with application to water for industrial use and for the removal of calcium oxalate from urine from patients with .
Methods of detection and analysis
The most sensitive method of detecting and measuring EDTA in biological samples is selected reaction monitoring capillary electrophoresis mass spectrometry (SRM-CE/MS), which has a detection limit of 7.3 ng/mL in human plasma and a Detection limit of 15 ng/mL.
EDTA has also been measured in non-alcoholic beverages using high performance liquid chromatography (HPLC) at a level of 2.0 μg/mL.
In popular culture
In the movie Blade (1998), EDTA is used as a weapon to kill vampires, exploding when in contact with vampire blood.
Blood on the sock that was used as evidence against O. J. Simpson, in the killing of Nicole Brown Simpson and Ronald Goldman, had high levels of EDTA, according to defense attorneys.
External links
(
Acetic Acids
)
