Metal-EDTA chelates

A proposed structure for a metal-EDTA chelate showing its octahedral geometry. 

The Formation and Stability of Metal-EDTA Chelates EDTA fonns stable complexes with a wide range of metal ions. The exceptional stability is conferred by the large number of donor groups and the subsequent isolation of the metal ion in the cage-like structure. It is clear from the foregoing section that complex formation will be pH dependent and may be represented by different equations depending on the pH and the ionic form of the EDTA. However, for purposes of comparison it is best to use the equilibrium... 

Table 40-1. Formation constants of metal-EDTA chelates...

The pH can affect stability of the complex (i.e., K ) by affecting not only the form of the EDTA but also that of the metal ion. For example, hydroxy species may form (M2+ -I- OH- MOH" "). That is, OH- competes for the metal ion just as H" " competes for the Y ". Figure 9.2 (prepared from a spreadsheet—see Problem 20) shows how Kf changes with pH for three metal-EDTA chelates with moderate (Ca) to strong (Hg) formation constants. The calcium chelate is obviously too weak to be titrated in acid solution K < 1), while the mercury chelate is strong enough... 

Fig. 28.1 Octahedral structure of a metal-EDTA chelate (example of a bivalent metal)...

According to the medium pH value, one of these reactions or even several ones occur to form the complex. This explains why the apparent stability constants of the metal-EDTA chelates may vary considerably with the pH value. 

B. R. Saunders, K. S. Murray, R. J. Fleming, D. G. McCulloch, L. J. Brown, and J. D. Cushion, Physical and spectroscopic studies of polypyrrole films containing transition metal EDTA chelates, Chem. Mater. 6(5) 697 (1994). 

Fig. 4. pM vs pH for A Cu(II), and B Mn (IT) EDTA chelates. For each family of curves, the lowest curve represents 1% the second, 10% and the top curve, 100% of free ligand species ia excess of the amount needed to form the metal chelate. Broken lines represent soHd—solution equiUbria for... 

Wetai Ion Analysis. We have reported a sensitive trace-metal analysis based upon HPLC separation of p-aminophenyl EDTA chelates and fluorescence detection by postcolumn reaction with fluorescamine (23). An application of the pyridone chemistry already discussed leads to a fluorescent-labeled EDTA (VIII). 

A common form of EDTA used as a preservative is calcium disodium EDTA (CaNa2EDTA). What metals will this form of the sequestrant scavenge effectively The dissolution of the solid will yield calcium ions, sodium ions, and the EDTA anion. Any metal more effectively complexed than calcium will be readily scavenged, including all ions listed in Table 9.1 except silver (Ag+) and magnesium (Mg2+). (In the absence of the calcium counterion, as in the case of the acid form of EDTA, chelation of calcium in the body can occur. In fact, EDTA administered orally is an FDA-approved treatment for calcium deposits in the bloodstream that lead to cardiovascular disease.) Citric acid (Fig. 9.3.3) is another sequestrant of metal ions in foodstuffs. 

In this laboratory, we also include the metal ion chelators EDTA (ethylene diamine tetraacetic acid binds, e.g., Mg2 1 -ions) and EGTA (ethylene glycol-bis(2-aminoethyl)-Al,iV,iV/,iV/,-tetraacetic acid binds, e.g., Ca2+-ions) in our lysis buffers. These agents help prevent phosphatase action (by the metal ion-dependent phosphatase PP2C, which is not inhibited by microcystin-LR), metal (Ca2+) dependent proteinases, and protein kinases, which require divalent cations such as Mg2 1 (and, in some cases, also Ca2+). We also use a mix of proteinase inhibitors that inhibit a broad range of proteolytic enzymes, including serine and cysteine proteinases. 

Figure 13.8 TMS-EDTA can be used to modify an inorganic substrate to containing EDTA chelating groups for complexation with metal ions.

EDTA salts are used for the treatment of heavy metal poisoning. Roosels and Vanderkeel142) were able to extract lead from urine in the presence of EDTA with dithizone by adding calcium to presumably release the lead from EDTA. In view of the fact that the formation constant of the lead-EDTA chelate is 20,000,000 times larger than that of the corresponding calcium chelate, it is doubtful that the calcium actually releases the EDTA from the lead. 

Chelating agents—to bind trace metals, EDTA 0.01-0.05%... 

Experimentally based intuitive arguments have been presented to arrive at a regional rule for optical activity of d-d transitions of conformational isomers of octahedral metal complexes. Conformational preferences for chelate rings formed by 1,3-pn in its octahedral mono, bis, and tris metal complexes have been studied by calculation of the conformational energies. In all cases, the chair conformation was found to be the most stable. The lowest energy pathway for converting from one chair configuration into another has a barrier to activation of about 7 kcal mol Conformational types of metal-edta complexes have been studied. ... 

Laboratory research has demonstrated that the liquid corona technology can treat a variety of wastewater contaminants such as carbon tetrachloride, metal ion chelators, and industrial dyes. The technology successfully reduced initial organic contaminant concentrations (by more than 99%) for the following contaminants after exposure to corona discharge trichloroethylene (TCE), ethylene-diamine-tetraacetic acid (EDTA), and benzoic acid. Additionally, liquid corona has demonstrated removal success with carbon tetrachloride, pentachlorophenol, and perchloroethylene. 

EDTA Chelators of cations, particularly heavy metals... 

In cupric sulfide systems, a high stability constant of a chelate does not necessarily mean a slow release of metal ions, due to the extremely high rates in both association and dissociation of the chelate. For instance, the reactions of chelates, Cu(TMD)22+ and Cu(DETA)22+, with TAA under standard conditions A finish within 2 min at 25°C to yield rather small CuS particles of 40 to 50 nm, despite the respective high stability constants, 10169 and 102l comparable to or even much higher than the stability constants of EDTA chelates of the other kinds of metal ions, such as Cd2+, Zn2+, and Pb2+, which are much slower in releasing these metal... 

EDTA Chelates alkaline earth and heavy metals 0.005-0.01% w/v antimicrobial activity synergy Often used in combination with other antimicrobial preservatives, and other antioxidants based on their synergy... 

In many complexes, EDTA engulfs the metal ion, forming the six-coordinate species in Figure 12-1. If you try to build a space-filling model of a six-coordinate metal-EDTA complex, you will find that there is considerable strain in the chelate rings. This strain is relieved when the O ligands are drawn back toward the N atoms. Such distortion opens up a seventh... 

EDTA chelation The EDTA chelation test evaluates the extent to which cationic metals (e.g., Al, Ba, Cd, Co, Cu) can be made less toxic or non-toxic by the addition of EDTA (Ethylenediaminetetraacetate). A cationic metal may be suspected as the cause of toxicity if both EDTA and sodium thiosulfate reduce toxicity. EDTA is typically added as a gradient of concentrations (based on its toxicity to the species of interest) to a single effluent concentration. 

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