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EDTA
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EDTA is a widely used acronym for the chemical compound ethylenediaminetetraacetic acid (which has many other names, see Table). EDTA is a polyamino carboxylic acid with the formula [CH2N(CH2CO2H)2]2. This colourless, water-soluble solid produced on a large scale for many applications. Its prominence as a chelating agent arises from its ability to "sequester" di- and tricationic metal ions such as Ca2+ and Fe3+.
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EDTA is a widely used acronym for the chemical compound ethylenediaminetetraacetic acid (which has many other names, see Table). EDTA is a polyamino carboxylic acid with the formula [CH2N(CH2CO2H)2]2. This colourless, water-soluble solid produced on a large scale for many applications. Its prominence as a chelating agent arises from its ability to "sequester" di- and tricationic metal ions such as Ca2+ and Fe3+. After being bound by EDTA, metal ions remain in solution but exhibit diminished reactivity.
Synthesis
The compound was first described in 1935 by Ferdinand Munz, who prepared the compound from ethylenediamine and chloroacetic acid. Today, EDTA is mainly synthesised from ethylenediamine (1,2-diaminoethane), formaldehyde (methanal), and sodium cyanide. This route yields the sodium salt, which can be converted in a subsequent step into the acid forms:
- H2NCH2CH2NH2 + 4 CH2O + 4 NaCN + 4 H2O ? (NaO2CCH2)2NCH2CH2N(CH2CO2Na)2 + 4 NH3
2NCH2CH2N(CH2CO2Na)2 + 4 HCl ? (HO2CCH2)2NCH2CH2N(CH2CO2H)2 + 4 NaCl
In this way, about 80M kilograms are produced 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 protonated forms, chemists distinguish between EDTA4-, the conjugate base that is the ligand, and H4EDTA, the precursor to that ligand. At very low pH (very acidic conditions) the fully protonated H6Y2+ forms predominates (where Y denotes EDTA), whereas at very high pH or very basic condition, the fully deprotonated Y4− form is prevalent. In this article, the term EDTA is used to mean H4-xEDTAx-, whereas in its complexes edta4- stands for the tetra-deprotonated ligand.
Coordination chemistry principles
In coordination chemistry, EDTA4- is a member of the polyamino carboxylic acid family of ligands. EDTA4- usually binds to a metal cation through its two amines and four carboxylates. Many of the resulting coordination compounds adopt octahedral geometry. Although of little consequence for its applications, these octahedral complexes are chiral. The anion [Co(edta)]- has been resolved into enantiomers. Many complexes of edta4- adopt more complex structures due to (i) the formation of an additional bond to water, i.e. seven-coordinate complexes, or (ii) the displacement of one carboxylate arm by water. Early work on the development of EDTA was undertaken by Gerold Schwarzenbach in the 1940s. EDTA forms especially strong complexes with Mn(II), Cu(II), Fe(III), Pb (II) 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:
- [Fe(H2O)6]3+ + H4EDTA [Fe(edta)]- + 6 H2O + 4 H+ 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 catalytic properties are often suppressed. Finally, since complexes of EDTA4- are anionic, they tend to be highly soluble in water. For this reason, EDTA is able to dissolve deposits of metal oxides and carbonates.
Uses
The most important industrial uses involve the sequestration of metal ions in aqueous solution. In the textile industry, 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 Mn2+, from catalyzing the disproportionation of hydrogen peroxide, which is used in "chlorine-free bleaching." Similarly, EDTA is added to some food as a preservative or stabilizer to prevent catalytic oxidative decoloration which is catalyzed by metal ions. In personal care products, it is added to cosmetics to improve their stability toward air. In Soft drinks containing ascorbic acid and sodium benzoate, EDTA mitigates formation of benzene (a carcinogen).
The reduction of water hardness in laundry applications and the dissolution of scale in boilers both rely on EDTA and related complexants to bind Ca2+ and Mg2+ ions as well as other metal ions. Once bound to EDTA, these metal centers tend not to form precipitates or to interfere with the action of the detergents. For similar reasons, cleaning solutions often contain EDTA.
The solubilization of ferric ions near neutral pH is accomplished using EDTA. This property is useful in agriculture including hydroponics, especially in calcareous soils. Otherwise, at near neutral pH, iron(III) salts form insoluble hydroxides, which are less bioavailable. Aqueous [Fe(edta)]- is used as a recyclable oxidizing agent for removing ("scrubbing") hydrogen sulfide from gas streams. This conversion is achieved by oxidizing the hydrogen sulfur to elemental sulfur, which is non-volatile:
- 2 [Fe(edta)]- + H2S ? 2 [Fe(edta)]2- + S + 2 H+
In this application the ferric center is reduced to its ferrous derivative, which can then be reoxidized by air. Similarly, nitrogen oxides are removed from gas streams using [Fe(edta)]2-. The oxidizing properties of [Fe(edta)]- are also exploited in photography where it is used to solubilize silver particles.
Medicine
EDTA is used to detoxify metal ions in chelation therapy, e.g. for mercury and lead poisoning. Similarly it is used to remove excess iron from the body. This therapy is used to treat the disease thalassemia, which arises from repeated blood transfusions.
EDTA finds many specialized uses in the biomedical laboratories, such as in veterinary ophthalmology as an anticollagenase to prevent the worsening of corneal ulcers in animals. Dentists used EDTA solutions to remove inorganic debris (smear layer) and prepare root canals for obturation. It serves as a preservative (usually to enhance the action of another preservative such as benzalkonium chloride or thiomersal) in ocular preparations and eyedrops. In cell cultures EDTA is used as a chelating agent which binds to calcium and prevents joining of cadherins between cells, preventing cell clumping. In evaluating kidney function, the complex [Cr(edta)]- is administered intravenously and its filtration into the urine is monitored. This method is useful for evaluating glomerular filtration rate.
EDTA is used extensively in the analysis of blood. It is an anticoagulant for blood samples for CBC/FBE's (complete blood count also know as full blood examination). Laboratory studies also suggest that EDTA chelation may prevent collection of platelets ([or plaque] which can otherwise lead to formation of blood clots and prevent blood flow) on the walls of blood vessels [such as arteries]. These ideas are theoretical, however a major clinical study of the effects of EDTA on coronary arteries is currently (2008) proceeding [|http://www.clinicaltrials.gov/ct/show/NCT00044213?order=2] EDTA played a role in the O.J. Simpson trial when one of the blood samples collected from Simpson's estate was found to contain traces of the compound..
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 metal-dependent enzymes, either as an assay for their reactivity or to suppress damage to DNA or proteins. 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 is added to some buffer solutions.
Toxicity and environmental considerations
EDTA is in such widespread use that it has emerged as a persistent organic pollutant. Its degradation entails conversion to ethylenediaminetriacetic acid, which then cyclizes.
EDTA exhibits low toxicity with LD50 (rat) of 2.0 – 2.2 g/kg. It has been found to be both cytotoxic and weakly genotoxic in laboratory animals. Oral exposures have been noted to cause reproductive and developmental effects. The same study by Lanigan also found that both dermal exposure to EDTA in most cosmetic formulations and inhalation exposure to EDTA in aerosolized cosmetic formulations would produce systemic effects below those seen to be toxic in oral dosing studies.
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 (abbreviation SRM-CE/MS) which has a detection limit of 7.3 ng/mL in human plasma and a quantitation limit of 15 ng/mL. This method works with sample volumes as small as ~7-8 nL.
EDTA has also been measured in non-alcoholic beverages using high performance liquid chromatography (HPLC) of 2.0 µg/mL.
Endnotes
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