Editing Chelation

{{short description|Type of chemical bonding with metal ions}}
{{About|sequestering agents in general|chemicals used in food processing|sequestrant|the isopod genus|Chelator (crustacean)}}
{{Use mdy dates|date=March 2025}}

'''Chelation''' ({{IPAc-en|k|iː|ˈ|l|eɪ|ʃ|ən}}) is a type of bonding of [[ions]] and their molecules to metal ions. It involves the formation or presence of two or more separate [[coordinate bond]]s between a [[Denticity|polydentate]] (multiple bonded) [[ligand]] and a single central metal atom.<ref name="IUPAC">[http://goldbook.iupac.org/C01012.html IUPAC definition of chelation.]</ref><ref>Latin ''[[chela (organ)|chela]]'', from Greek, denotes a claw.</ref> These ligands are called chelants, chelators, chelating agents, or sequestering agents. They are usually [[organic compound]]s, but this is not a necessity.

The word ''chelation'' is derived from [[Greek language|Greek]] χηλή, ''chēlē'', meaning "claw"; the ligands lie around the central atom like the claws of a [[crab]]. The term ''chelate''  ({{IPAc-en|ˈ|k|iː|l|eɪ|t}}) was first applied in 1920 by Sir [[Gilbert Thomas Morgan|Gilbert T. Morgan]] and [[Harry Dugald Keith Drew|H. D. K. Drew]], who stated: "The adjective chelate, derived from the great claw or ''chele'' (Greek) of the crab or other crustaceans, is suggested for the caliperlike groups which function as two associating units and fasten to the central atom so as to produce [[heterocyclic]] rings."<ref>{{cite journal |last1=Morgan |first1=Gilbert T. |last2=Drew |first2=Harry Dugald Keith |name-list-style=vanc |title=CLXII.—Researches on residual affinity and co-ordination. Part II. Acetylacetones of selenium and tellurium |journal=Journal of the Chemical Society, Transactions |volume=117 |year=1920 |pages=1456–65 |doi=10.1039/ct9201701456 |url=https://zenodo.org/record/1429747}}</ref>

Chelation is useful in applications such as providing nutritional supplements, in [[chelation therapy]] to remove toxic metals from the body, as [[contrast medium|contrast agents]] in [[MRI|MRI scanning]], in manufacturing using [[homogeneous catalyst]]s, in chemical [[water treatment]] to assist in the removal of metals, and in [[fertilizer]]s.

== Chelate effect ==
[[file:Me-EN.svg|thumb|[[Ethylenediamine]] [[ligand]] chelating to a metal with two bonds]]
[[file:Cu chelate.svg|thumb|Cu<sup>2+</sup> [[Coordination complex|complexes]] with nonchelating [[methylamine]] (left) ''and'' chelating [[ethylenediamine]] (right) ligands]]

The chelate effect is the greater affinity of chelating ligands for a metal ion than that of similar nonchelating (monodentate) ligands for the same metal.

The thermodynamic principles underpinning the chelate effect are illustrated by the contrasting affinities of [[copper]](II) for [[ethylenediamine]] (en) vs. [[methylamine]].
{{NumBlk|:|Cu<sup>2+</sup> + en {{eqm}} [Cu(en)]<sup>2+</sup>|{{EquationRef|1}}}}
{{NumBlk|:|Cu<sup>2+</sup> + 2 MeNH<sub>2</sub> {{eqm}} [Cu(MeNH<sub>2</sub>)<sub>2</sub>]<sup>2+</sup>|{{EquationRef|2}}}}
In ({{EquationNote|1}}) the ethylenediamine forms a chelate complex with the copper ion. Chelation results in the formation of a five-membered CuC<sub>2</sub>N<sub>2</sub> ring. In ({{EquationNote|2}}) the bidentate ligand is replaced by two [[Denticity|monodentate]] methylamine ligands of approximately the same donor power, indicating that the Cu–N bonds are approximately the same in the two reactions.

The [[equilibrium thermodynamics|thermodynamic]] approach to describing the chelate effect considers the [[equilibrium constant]] for the reaction: the larger the equilibrium constant, the higher the concentration of the complex.
{{NumBlk|:|[Cu(en)] {{=}} ''β''<sub>11</sub>[Cu][en]|{{EquationRef|3}}}}
{{NumBlk|:|[Cu(MeNH<sub>2</sub>)<sub>2</sub>] {{=}} ''β''<sub>12</sub>[Cu][MeNH<sub>2</sub>]<sup>2</sup>|{{EquationRef|4}}}}
Electrical charges have been omitted for simplicity of notation. The square brackets indicate concentration, and the subscripts to the [[Stability constants of complexes|stability constant]]s, ''β'', indicate the [[stoichiometry]] of the complex. When the [[analytical concentration]] of methylamine is twice that of ethylenediamine and the concentration of copper is the same in both reactions, the concentration [Cu(en)] is much higher than the concentration [Cu(MeNH<sub>2</sub>)<sub>2</sub>] because {{nowrap|''β''<sub>11</sub> ≫ ''β''<sub>12</sub>}}.

An equilibrium constant, ''K'', is related to the standard [[Gibbs energy|Gibbs free energy]], {{tmath|\Delta G^\ominus}} by
: <math>\Delta G^\ominus = - RT \ln K = \Delta H^\ominus - T \Delta S^\ominus</math>
where ''R'' is the [[gas constant]] and ''T'' is the temperature in [[kelvin]]s. {{tmath|\Delta H^\ominus}} is the standard [[enthalpy]] change of the reaction and {{tmath|\Delta S^\ominus}} is the standard [[Entropy (statistical thermodynamics)|entropy]] change.

Since the enthalpy should be approximately the same for the two reactions, the difference between the two stability constants is due to the effects of entropy. In equation ({{EquationNote|1}}) there are two particles on the left and one on the right, whereas in equation ({{EquationNote|2}}) there are three particles on the left and one on the right. This difference means that less [[Entropy (order and disorder)|entropy of disorder]] is lost when the chelate complex is formed with bidentate ligand than when the complex with monodentate ligands is formed. This is one of the factors contributing to the entropy difference. Other factors include solvation changes and ring formation. Some experimental data to illustrate the effect are shown in the following table.<ref name="GE">{{Greenwood&Earnshaw2nd|page=910| name-list-style=vanc}}</ref>

{| class="wikitable"
! Equilibrium !! log ''β'' !! {{tmath|\Delta G^\ominus}} !! <math>\Delta H^\ominus \mathrm{/kJ\ mol^{-1}}</math> !! <math>-T\Delta S^\ominus \mathrm{/kJ\ mol^{-1}}</math>
|-
| Cu<sup>2+</sup> + 2 MeNH<sub>2</sub> {{eqm}} Cu(MeNH<sub>2</sub>)<sub>2</sub><sup>2+</sup>
||6.55|| −37.4 || −57.3||19.9
|-
| Cu<sup>2+</sup> + en {{eqm}} Cu(en)<sup>2+</sup>
||10.62|| −60.67 || −56.48||−4.19
|}

These data confirm that the enthalpy changes are approximately equal for the two reactions and that the main reason for the greater stability of the chelate complex is the entropy term, which is much less unfavorable. In general it is difficult to account precisely for thermodynamic values in terms of changes in solution at the molecular level, but it is clear that the chelate effect is predominantly an effect of entropy.

Other explanations, including that of [[Gerold Schwarzenbach|Schwarzenbach]],<ref>{{cite journal |vauthors=Schwarzenbach G |title=Der Chelateffekt |trans-title=The Chelation Effect |language=de |journal=Helvetica Chimica Acta |volume=35 |issue=7 |year=1952 |pages=2344–59 |doi=10.1002/hlca.19520350721}}</ref> are discussed in Greenwood and Earnshaw (''loc.cit'').

== In nature ==

Numerous [[biomolecules]] exhibit the ability to dissolve certain metal [[cation]]s. Thus, [[protein]]s, [[polysaccharide]]s, and polynucleic acids are excellent polydentate ligands for many metal ions.  Organic compounds such as the amino acids [[glutamic acid]] and [[histidine]], organic diacids such as [[malate]], and polypeptides such as [[phytochelatin]] are also typical chelators. In addition to these adventitious chelators, several biomolecules are specifically produced to bind certain metals (see next section).<ref>{{cite journal |last1=Krämer |first1=Ute |last2=Cotter-Howells |first2=Janet D. |last3=Charnock |first3=John M. |last4=Baker |first4=Alan J. M. |last5=Smith |first5=J. Andrew C. |name-list-style=vanc |title=Free histidine as a metal chelator in plants that accumulate nickel |journal=Nature |volume=379 |issue=6566 |year=1996 |pages=635–8 |bibcode=1996Natur.379..635K |doi=10.1038/379635a0 |s2cid=4318712}}</ref><ref>{{cite journal |vauthors=Magalhaes JV |title=Aluminum tolerance genes are conserved between monocots and dicots |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=103 |issue=26 |pages=9749–50 |date=June 2006 |pmid=16785425 |pmc=1502523 |doi=10.1073/pnas.0603957103 |bibcode=2006PNAS..103.9749M |doi-access=free}}</ref><ref>{{cite journal |vauthors=Ha SB, Smith AP, Howden R, Dietrich WM, Bugg S, O'Connell MJ, Goldsbrough PB, Cobbett CS |title=Phytochelatin synthase genes from Arabidopsis and the yeast Schizosaccharomyces pombe |journal=The Plant Cell |volume=11 |issue=6 |pages=1153–64 |date=June 1999 |pmid=10368185 |pmc=144235 |doi=10.1105/tpc.11.6.1153}}</ref><ref name="Lippard" />

Virtually all metalloenzymes feature metals that are chelated, usually to peptides or cofactors and prosthetic groups.<ref name="Lippard">{{cite book |vauthors=Lippard SJ, Berg JM |title=Principles of Bioinorganic Chemistry |publisher=University Science Books |location=Mill Valley, Calif. |year=1994 |isbn=978-0-935702-73-6}}.{{page needed|date=December 2015}}</ref>  Such chelating agents include the [[porphyrin]] rings in [[hemoglobin]] and [[chlorophyll]].  Many microbial species produce water-soluble pigments that serve as chelating agents, termed [[siderophores]]. For example,  species of ''[[Pseudomonas]]'' are known to secrete [[pyochelin]] and [[pyoverdine]] that bind iron. [[Enterobactin]], produced by ''[[Escherichia coli|E. coli]]'', is the strongest chelating agent known. The marine [[mussel]]s use metal chelation, especially Fe<sup>3+</sup> chelation with the [[L-DOPA|Dopa]] residues in mussel foot protein-1 to improve the strength of the threads that they use to secure themselves to surfaces.<ref>{{cite journal |vauthors=Das S, Miller DR, Kaufman Y, Martinez Rodriguez NR, Pallaoro A, Harrington MJ, Gylys M, Israelachvili JN, Waite JH |title=Tough coating proteins: subtle sequence variation modulates cohesion |journal=Biomacromolecules |volume=16 |issue=3 |pages=1002–8 |date=March 2015 |pmid=25692318 |pmc=4514026 |doi=10.1021/bm501893y}}</ref><ref>{{cite journal |vauthors=Harrington MJ, Masic A, Holten-Andersen N, Waite JH, Fratzl P |title=Iron-clad fibers: a metal-based biological strategy for hard flexible coatings |journal=Science |volume=328 |issue=5975 |pages=216–20 |date=April 2010 |pmid=20203014 |pmc=3087814 |doi=10.1126/science.1181044 |bibcode=2010Sci...328..216H}}</ref><ref>{{cite journal |vauthors=Das S, Martinez Rodriguez NR, Wei W, Waite JH, Israelachvili JN |title=Peptide Length and Dopa Determine Iron-Mediated Cohesion of Mussel Foot Proteins |journal=Advanced Functional Materials |volume=25 |issue=36 |pages=5840–5847 |date=September 2015 |pmid=28670243 |pmc=5488267 |doi=10.1002/adfm.201502256}}</ref>

In earth science, chemical [[weathering]] is attributed to organic chelating agents (e.g., [[peptide]]s and [[sugar]]s) that extract [[metal ions]] from minerals and rocks.<ref>{{cite web |title=Introduction to the Lithosphere: Weathering |first=Michael |last=Pidwirny |name-list-style=vanc |location=University of British Columbia Okanagan |url=http://www.physicalgeography.net/fundamentals/10r.html}}</ref>  Most metal complexes in the environment and in nature are bound in some form of chelate ring (e.g., with a [[humic acid]] or a protein). Thus, metal chelates are relevant to the mobilization of [[metals]] in the [[soil]], the uptake and the accumulation of [[metals]] into [[plants]] and [[microorganism]]s. Selective chelation of [[heavy metals]] is relevant to [[bioremediation]] (e.g., removal of [[Caesium-137|<sup>137</sup>Cs]] from [[radioactive waste]]).<ref>{{cite book |last1=Prasad  |first1=MNV |name-list-style=vanc |title=Metals in the Environment: Analysis by Biodiversity |date=2001 |publisher=Marcel Dekker |location=New York |isbn=978-0-8247-0523-7}}{{page needed|date=December 2015}}</ref>

== Applications ==
=== Animal feed additives ===
Synthetic chelates such as [[ethylenediaminetetraacetic acid]] (EDTA) proved too stable and not nutritionally viable. If the mineral was taken from the EDTA ligand, the ligand could not be used by the body and would be expelled. During the expulsion process, the EDTA ligand randomly chelated and stripped other minerals from the body.<ref>{{cite book |last=Ashmead |first=H. DeWayne |name-list-style=vanc |title=The Roles of Amino Acid Chelates in Animal Nutrition |year=1993 |publisher=Noyes Publications |location=Westwood|isbn=0815513127}}{{page needed|date=December 2015}}</ref> According to the Association of American Feed Control Officials (AAFCO), a metal–amino acid chelate is defined as the product resulting from the reaction of metal ions from a soluble metal salt with amino acids, with a [[mole ratio]] in the range of 1–3 (preferably 2) moles of amino acids for one mole of metal.{{Citation needed|date=October 2018}} The average weight of the hydrolyzed amino acids must be approximately 150 and the resulting molecular weight of the chelate must not exceed 800&nbsp;[[Dalton (unit)|Da]].{{citation needed|date=December 2015}} Since the early development of these compounds, much more research has been conducted, and has been applied to human nutrition products in a similar manner to the animal nutrition experiments that pioneered the technology. [[Iron supplement|Ferrous bis-glycinate]] is an example of one of these compounds that has been developed for human nutrition.<ref>{{cite web |publisher=Albion Laboratories, Inc. |title=Albion Ferrochel Website |url=http://www.albionferrochel.com/ |access-date=July 12, 2011 |archive-date=September 3, 2011 |archive-url=https://web.archive.org/web/20110903054502/http://www.albionferrochel.com/ |url-status=dead}}</ref>

=== Dental use ===
[[Dentin]] adhesives were first designed and produced in the 1950s based on a co-monomer chelate with calcium on the surface of the tooth and generated very weak water-resistant chemical bonding (2–3 MPa).<ref>{{cite book |last1=Anusavice |first1=Kenneth J. |name-list-style=vanc |title=Phillips' Science of Dental Materials |publisher=Elsevier Health |isbn=978-1-4377-2418-9 |chapter=Chapter 12: Bonding and Bonding Agents |pages=257–268 |edition=12th |oclc=785080357 |date=September 27, 2012}}</ref>

=== Chelation therapy ===
[[Chelation therapy]] is an antidote for poisoning by [[mercury poisoning|mercury]], [[arsenic]], and [[lead]].  Chelating agents convert these metal ions into a chemically and biochemically inert form that can be excreted. Chelation using [[sodium calcium edetate]] has been approved by the [[U.S. Food and Drug Administration]] (FDA) for serious cases of [[lead poisoning]]. It is not approved for treating "[[heavy metal toxicity]]".<ref name="warning">{{cite web |url=http://www.chelationwatch.org/reg/fda_warning.shtml |title=FDA Issues Chelation Therapy Warning |date=September 26, 2008 |access-date=May 14, 2016}}</ref> Although beneficial in cases of serious lead poisoning, use of disodium EDTA (edetate disodium) instead of calcium disodium EDTA has resulted in fatalities due to [[hypocalcemia]].<ref>{{cite journal |author=Centers for Disease Control Prevention (CDC) |title=Deaths associated with hypocalcemia from chelation therapy--Texas, Pennsylvania, and Oregon, 2003–2005 |journal=MMWR. Morbidity and Mortality Weekly Report |volume=55 |issue=8 |pages=204–7 |date=March 2006 |pmid=16511441 |url=https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5508a3.htm}}</ref> Disodium EDTA is not approved by the FDA for any use,<ref name="warning" /> and all FDA-approved chelation therapy products require a prescription.<ref>{{cite web |url=https://www.fda.gov/Drugs/ResourcesForYou/Consumers/BuyingUsingMedicineSafely/MedicationHealthFraud/ucm229313.htm |archive-url=https://web.archive.org/web/20101017040749/http://www.fda.gov/Drugs/ResourcesForYou/Consumers/BuyingUsingMedicineSafely/MedicationHealthFraud/ucm229313.htm |url-status=dead |archive-date=October 17, 2010 |title=Questions and Answers on Unapproved Chelation Products |publisher=[[U.S. Food and Drug Administration|FDA]] |date=February 2, 2016 |access-date=May 14, 2016}}</ref>

=== Contrast agents ===
Chelate complexes of [[gadolinium]] are often used as [[contrast medium|contrast agent]]s in [[MRI|MRI scan]]s, although [[iron]] particle and [[manganese]] chelate complexes have also been explored.<ref name=":0">{{cite journal |vauthors=Caravan P, Ellison JJ, McMurry TJ, Lauffer RB |title=Gadolinium(III) Chelates as MRI Contrast Agents: Structure, Dynamics, and Applications |journal=Chemical Reviews |volume=99 |issue=9 |pages=2293–352 |date=September 1999 |pmid=11749483 |doi=10.1021/cr980440x}}</ref><ref>{{cite journal |vauthors=Pan D, Schmieder AH, Wickline SA, Lanza GM |title=Manganese-based MRI contrast agents: past, present and future |journal=Tetrahedron |volume=67 |issue=44 |pages=8431–8444 |date=November 2011 |pmid=22043109 |pmc=3203535 |doi=10.1016/j.tet.2011.07.076}}</ref> Bifunctional chelate complexes of [[zirconium]], [[gallium]], [[fluorine]], [[copper]], [[yttrium]], [[bromine]], or [[iodine]] are often used for conjugation to [[monoclonal antibodies]] for use in antibody-based [[PET imaging]].<ref>{{cite journal |vauthors=Vosjan MJ, Perk LR, Visser GW, Budde M, Jurek P, Kiefer GE, van Dongen GA |title=Conjugation and radiolabeling of monoclonal antibodies with zirconium-89 for PET imaging using the bifunctional chelate p-isothiocyanatobenzyl-desferrioxamine |journal=Nature Protocols |volume=5 |issue=4 |pages=739–43 |date=April 2010 |pmid=20360768 |doi=10.1038/nprot.2010.13 |s2cid=5087493}}</ref> These chelate complexes often employ the usage of [[hexadentate ligand]]s such as [[desferrioxamine B]] (DFO), according to Meijs ''et al.'',<ref>{{Cite journal|last1=Price|first1=Eric W.|last2=Orvig|first2=Chris |date=January 7, 2014|title=Matching chelators to radiometals for radiopharmaceuticals|journal=Chemical Society Reviews|volume=43|issue=1|pages=260–290|doi=10.1039/c3cs60304k|issn=1460-4744|pmid=24173525}}</ref> and the gadolinium complexes often employ the usage of octadentate ligands such as DTPA, according to Desreux ''et al''.<ref>{{Cite journal|last1=Parac-Vogt|first1=Tatjana N.|last2=Kimpe|first2=Kristof|last3=Laurent |first3=Sophie|last4=Vander Elst|first4=Luce|last5=Burtea|first5=Carmen|last6=Chen|first6=Feng|last7=Muller |first7=Robert N.|last8=Ni |first8=Yicheng|last9=Verbruggen|first9=Alfons|date=May 6, 2005|title=Synthesis, characterization, and pharmacokinetic evaluation of a potential MRI contrast agent containing two paramagnetic centers with albumin binding affinity |journal=Chemistry: A European Journal|volume=11 |issue=10|pages=3077–3086|doi=10.1002/chem.200401207|issn=0947-6539|pmid=15776492 |url=https://lirias.kuleuven.be/handle/123456789/20303|url-access=subscription}}</ref> [[Auranofin]], a chelate complex of [[gold]], is used in the treatment of rheumatoid arthritis, and [[penicillamine]], which forms chelate complexes of [[copper]], is used in the treatment of [[Wilson's disease]] and [[cystinuria]], as well as refractory rheumatoid arthritis.<ref>{{cite journal |vauthors=Kean WF, Hart L, Buchanan WW |title=Auranofin |journal=British Journal of Rheumatology |volume=36 |issue=5 |pages=560–72 |date=May 1997 |pmid=9189058 |doi=10.1093/rheumatology/36.5.560 |doi-access=free}}</ref><ref>{{cite journal |vauthors=Wax PM |title=Current use of chelation in American health care |journal=Journal of Medical Toxicology |volume=9 |issue=4 |pages=303–307 |date=December 2013 |pmid=24113860 |pmc=3846961 |doi=10.1007/s13181-013-0347-2}}</ref>

=== Nutritional advantages and issues ===
Chelation in the intestinal tract is a cause of numerous interactions between drugs and metal ions (also known as "[[Dietary mineral|minerals]]" in nutrition). As examples, [[antibiotic]] [[medication|drug]]s of the [[tetracycline]] and [[Quinolone antibiotic|quinolone]] families are chelators of [[iron|Fe]]<sup>2+</sup>, [[Calcium|Ca]]<sup>2+</sup>, and [[Magnesium|Mg]]<sup>2+</sup> ions.<ref>{{cite journal |vauthors=Campbell NR, Hasinoff BB |title=Iron supplements: a common cause of drug interactions |journal=British Journal of Clinical Pharmacology |volume=31 |issue=3 |pages=251–5 |date=March 1991 |pmid=2054263 |pmc=1368348 |doi=10.1111/j.1365-2125.1991.tb05525.x}}</ref><ref>{{cite journal |vauthors=Lomaestro BM, Bailie GR |title=Absorption interactions with fluoroquinolones. 1995 update |journal=Drug Safety |volume=12 |issue=5 |pages=314–33 |date=May 1995 |pmid=7669261 |doi=10.2165/00002018-199512050-00004 |s2cid=2006138}}</ref>

EDTA, which binds to calcium, is used to alleviate the [[hypercalcemia]] that often results from [[band keratopathy]].  The calcium may then be removed from the [[cornea]], allowing for some increase in clarity of vision for the patient.<ref>{{Cite journal |last=Najjar |first=Dany M. |last2=Cohen |first2=Elisabeth J. |last3=Rapuano |first3=Christopher J. |last4=Laibson |first4=Peter R. |date=June 2004 |title=EDTA chelation for calcific band keratopathy: results and long-term follow-up |url=https://pubmed.ncbi.nlm.nih.gov/15183790/ |journal=American Journal of Ophthalmology |volume=137 |issue=6 |pages=1056–1064 |doi=10.1016/j.ajo.2004.01.036 |issn=0002-9394 |pmid=15183790}}</ref><ref>{{Cite journal |last=Al-Hity |first=A |last2=Ramaesh |first2=K |last3=Lockington |first3=D |date=December 1, 2017 |title=EDTA chelation for symptomatic band keratopathy: results and recurrence |url=https://pmc.ncbi.nlm.nih.gov/articles/PMC5770724/ |journal=Eye |language=en |volume=32 |issue=1 |pages=26–31 |doi=10.1038/eye.2017.264 |issn=0950-222X |archive-url=http://web.archive.org/web/20250202182230/https://pmc.ncbi.nlm.nih.gov/articles/PMC5770724/ |archive-date=February 2, 2025|pmc=5770724}}</ref>

[[Homogeneous catalyst]]s are often chelated complexes. A representative example is the use of [[BINAP]] (a bidentate [[phosphine]]) in [[Noyori asymmetric hydrogenation]] and asymmetric isomerization. The latter has the practical use of manufacture of synthetic [[Menthol|(–)-menthol]].

=== Cleaning and water softening ===
A chelating agent is the main component of some rust removal formulations.
[[Citric acid#Cleaning and chelating agent|Citric acid]] is used to [[water softening|soften water]] in [[soap]]s and laundry [[detergent]]s. A common synthetic chelator is [[EDTA]]. [[Phosphonate]]s are also well-known chelating agents. Chelators are used in water treatment programs and specifically in [[steam engineering]].{{cn|date=September 2023}}  Although the treatment is often referred to as "softening", chelation has little effect on the water's mineral content, other than to make it soluble and lower the water's [[pH]] level.

=== Fertilizers ===
Metal chelate compounds are common components of fertilizers to provide micronutrients.  These micronutrients (manganese, iron, zinc, copper) are required for the health of the plants.  Most fertilizers contain phosphate salts that, in the absence of chelating agents, typically convert these metal ions into insoluble solids that are of no nutritional value to the plants.  [[EDTA]] is the typical chelating agent that keeps these metal ions in a soluble form.<ref name="Ullmann">{{cite book |last1=Hart |first1=J. Roger |name-list-style=vanc |chapter=Ethylenediaminetetraacetic Acid and Related Chelating Agents |title=Ullmann's Encyclopedia of Industrial Chemistry |year=2011 |doi=10.1002/14356007.a10_095.pub2 |isbn=978-3527306732}}</ref>

=== Economic situation ===
Because of their wide needs, the overall chelating agents growth was 4% annually during 2009–2014<ref name=":1">(2013) IHS Chemical, Chemical Insight and Forecasting: Chelating Agents.</ref> and the trend is likely to increase. [[Aminopolycarboxylic acid|Aminopolycarboxylic acids]] chelators are the most widely consumed chelating agents; however, the percentage of the greener alternative chelators in this category continues to grow.<ref name=":2">{{cite book |author=Dixon NJ |year=2012 |chapter=Greener chelating agents |title=Handbook of green chemistry: Designing safer chemicals. |publisher=Wiley |pages=281–307}}</ref> The consumption of traditional aminopolycarboxylates chelators, in particular the EDTA ([[ethylenediaminetetraacetic acid]]) and NTA ([[nitrilotriacetic acid]]), is declining (−6% annually), because of the persisting concerns over their toxicity and negative environmental impact.<ref name=":1" /> In 2013, these greener alternative chelants represented approximately 15% of the total aminopolycarboxylic acids demand. This is expected to rise to around 21% by 2018, replacing and aminophosphonic acids used in cleaning applications.<ref>{{cite book |author=Kołodyńska D |year=2011 |chapter=Chelating agents of a new generation as an alternative to conventional chelators for heavy metal ions removal from different waste waters |title=Expanding Issues in Desalination |pages=339–370.}}</ref><ref name=":2" /><ref name=":1" /> Examples of some Greener alternative chelating agents include [[Ethylenediaminedisuccinic acid|ethylenediamine disuccinic acid]] (EDDS), [[polyaspartic acid]] (PASA), [[methylglycinediacetic acid]] (MGDA), [[glutamic diacetic acid]] (L-GLDA), [[Citric acid|citrate]], [[gluconic acid]], amino acids, plant extracts etc.<ref name=":2" /><ref>{{cite journal |author=Kolodynska D |title=Application of a new generation of complexing agents in removal of heavy metal ions from different wastes  |date=March 6, 2013 |journal=Environmental Science and Pollution Research |volume=20 |issue=9 |pages=5939-5949 |pmid=23463276 |doi=10.1007/s11356-013-1576-2|pmc=3720993 }}</ref>

== Reversal ==
{{See also|Transmetalation}}

Dechelation (or de-chelation) is a reverse process of the chelation in which the chelating agent is recovered by acidifying solution with a mineral acid to form a precipitate.<ref>{{Cite journal |last=Ryczkowski |first=Janusz |year=2019 |title=EDTA – synthesis and selected applications |url=https://journals.umcs.pl/aa/article/view/9864/0 |journal=Annales Universitatis Mariae Curie-Sklodowska |volume=74 |issn=2083-358X}}</ref>{{Rp|page=7}}

== See also ==

* {{Annotated link|EDDS}}

== References ==
{{CC-notice|cc=by4|url=https://juniperpublishers.com/omcij/pdf/OMCIJ.MS.ID.555694.pdf|author=Kaana Asemave}}
{{refs}}

== External links ==

* {{wiktionary-inline|chelate}}

{{Chemical equilibria}}
{{Chelating agents}}

[[Category:Chelating agents| ]]
[[Category:Coordination chemistry]]
[[Category:Equilibrium chemistry]]