Calcium-EDTA chelate

Fujishiro, Y., Sato, T., and Okuwaki, A., Coating of hydroxyapatite on metal plates using thermal dissociation of calcium-EDTA chelate in phosphate solutions under hydrothermal conditions, 7. Mater. Sci. Mater. Med., 6, 172, 1995. 

Janackovic, D. et al.. Surface properties of HAP particles obtained by hydrothermal decomposition of urea and calcium-EDTA chelates. Key Eng. Mater., 240-242,437, 2003. 

The conditional formation constant for the calcium-EDTA chelate was calculated for pH 10 in Example 9.4 to be 1.8 X 10 °. Calculate the conditional formation constant at pH 3. Compare this with that calculated for lead at pH 3 in Problem 8. Could lead be titrated with EDTA at pH 3 in the presence of calcium ... 

We studied the calcium—EDTA chelation using Calcein [4] as the fluorescent indicator. This indicator forms a fluorescent complex with free calcium ions. In order to determine calcium in a sample, the excess EDTA remaining after stoichiometric chelation of calcium in the sample is "back-titrated" with a known concentration of calcium solution. The equivalence point is reached when all of the excess EDTA is chelated and is detected by fluorescence of the Calcein-calcium complex formed by a slight excess of free calcium ions. The calcium content of the sample (in micrograms) is equal to the volume (in milliliters) of titrant added when no sample was introduced, minus the volume (in milliliters) of titrant added when the sample was introduced, times the calcium content of the titrant (in micrograms per milliliter). 

In patients on EDTA therapy, calcium cannot be determined by the Indirect colorimetric or fluorometrlc methods based on the chelation of a calcium - EDTA complex. However, In calcium determinations by atomic-absorption spectroscopy, the complexlng agent Is destroyed In the flame and the direct concentration of calcium can be determined. 

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. 

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. 

The longer the half-life of a metal in a particular organ, the less effectively it will be removed by chelation. For example, in the case of lead chelation with calcium EDTA or succimer, or of plutonium chelation with DTPA, the metal is more effectively removed from soft tissues than from bone, where incorporation into bone matrix results in prolonged retention. 

EDTA is marketed in its salt forms such as sodium EDTA or calcium EDTA. EDTA has industrial and medical uses as a chelating agent. Much of its utility is related to the fact that... 

In addition to removing the target metal that is exerting toxic effects on the body, some chelating agents (such as calcium EDTA used for lead intoxication) may enhance the excretion of essential cations such as zinc or copper. However, this side effect is seldom of clinical significance during the limited time frame that characterizes most courses of therapeutic chelation. 

Foreman and Trujillo (F6) have shown that disodium ethylenedia-minetetraacetate forms chelate compounds with calcium which are soluble and nonionized and therefore biologically inactive. These are virtually unabsorbed from the alimentary tract. It has been shown that 95 % of intravenously administered calcium EDTA is excreted in the urine in 24 hours. 

Ca2 -chelators (B) prevent the enzymatic activity of Ca2+-dependent factors they contain COO groups that bind Ca2+ ions (C) citrate and EDTA (ethylenediaminetetraace-tic acid) form soluble complexes with Ca2+ oxalate precipitates Ca2+ as insoluble calcium oxalate. Chelation of Ca2+ cannot be used in vivo for therapeutic purposes because Ca2 concentrations would have to be lowered to a level incompatible with life (hypo-calcemic tetany). These compounds (sodium salts) are, therefore, used only for rendering blood incoagulable outside the body. This effect can be reversed at any time by addition of Ca2 ions. 

Tetra sodium salt (Na4) of EDTA. Chelating agent for calcium, magnesium and other divalent and trivalent metal ions in acid, neutral and alkaline conditions. 

Sodium calciumedetate is the calcium chelate of the disodium salt of ethylenediaminetetra-acetic acid (calcium EDTA). It is effective in acute lead poisoning because of its capacity to exchange calcium for lead the lead chelate is excreted in the urine, leaving behind a harmless amount of calcium. Dimercaprol may usefully be combined with sodium calciumedetate when lead poisoning is severe, e.g. with encephalopathy. 

Synonyms calcium disodium edetate calcium disodium ethylenediaminetetraacetate calcium disodium (ethylene-dinitrilo)tetraacetate E385 edathamil calcium disodium edetic acid calcium disodium salt EDTA calcium ethylene-diaminetetraacetic acid calcium disodium chelate [(ethyl-enedinitrilo)tetraacetato]calciate(2-) disodium sodium calciumedetate Nersene CA. 

Clinical management is supportive. Gastric decontamination should be considered only in the case of massive ingestions. Normal zinc levels in the blood are between 68 and 136pgdl . Chelating agents such as BAL (British Antilewisite 2,3-dimercapto-propanol) or calcium EDTA will enhance removal of zinc, but are not likely indicated unless the unusual case of massive chronic exposure. Hemodialysis and other methods of extracorporeal elimination are not necessary. 

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... 

Masking can be achieved by precipitation, complex formation, oxidation-reduction, and kinetically. A combination of these techniques may be employed. For example, Cu " can be masked by reduction to Cu(I) with ascorbic acid and by complexation with I . Lead can be precipitated with sulfate when bismuth is to be titrated. Most masking is accomplished by selectively forming a stable, soluble complex. Hydroxide ion complexes aluminum ion [Al(OH)4 or AlOa"] so calcium can be titrated. Fluoride masks Sn(IV) in the titration of Sn(II). Ammonia complexes copper so it cannot be titrated with EDTA using murexide indicator. Metals can be titrated in the presence of Cr(III) because its EDTA chelate, although very stable, forms only slowly. 

Complexometric titrations in the clinical laboratory are limited to those substances that occur in fairly high concentrations since volumetric methods are generally not too sensitive. The most important complexometric titration is the determination of calcium in blood (see Ref. 8). Chelating agents such as. EDTA are used in the treatment of heavy-metal poisoning, for example, when children ingest chipped paint that contains lead. The calcium chelate (as Na2CaY) is administered to prevent complexation and removal of calcium in the bones. Heavy metals such as lead form more stable EDTA chelates than calcium does and will displace the calcium from the EDTA. The chelated lead is then excreted via the kidneys. 

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