Cadmium-EDTA chelate

Similarly, ions such as cadmium and zinc, which form more stable EDTA chelates than does magnesium, can be determined in the presence of the latter ion by buffering the mixture to a pH of 7 before titration. Eriochrome Black T serves as an indicator for the cadmium or zinc end points without interference from magnesium because the indicator chelate with magnesium is not formed at this pH. 

Chelates are often named merely as a complex, eg, cadmium complex with acetylacetone. A common practice ia the Hterature is to give the symbol of the central atom and an abbreviation for the ligand with or without an iadication of ionic charges, oxidation states, stmcture, or counterions, as ia the foUowiag Pb-EDTA, Cacit , Cu(en)2, Co(II)-(phen), [Cu(dipy)2]S04, [Ru(dipy)2(en)], and Na[Co(acac)2]. Ligand abbreviations are given ia Table 1. 

Koehler, F.M., Rossier, M., Waelle, M., Athanassiou, E.K., Limbach, L.K., Grass, R.N., Gunther, D. and Stark, W.J. (2009) Magnetic EDTA coupling heavy metal chelators to metal nanomagnets for rapid removal of cadmium, lead and copper from contaminated water. Chemical Communications, (32), 4862—4864. 

Gardner and Yates [26] developed a method for the determination of total dissolved cadmium and lead in estuarine waters. Factors leading to the choice of a method employing extraction by chelating resin, and analysis by carbon furnace atomic absorption spectrometry, are described. To ensure complete extraction of trace metals, inert complexes with humic-like material are decomposed by ozone [27]. The effect of pH on extraction by and elution from chelating resin is discussed, and details of the method were presented. These workers found that at pH 7 only 1-2 minutes treatment with ozone was needed to completely destroy complexing agents such as EDTA and humic acid in the samples. 

The most efficient system devised by Monsanto uses electrodes fabricated from carbon steel plate, electro-coated on one face with cadmium. These are stacked in parallel so that the electrolyte can be pumped through the gap between successive plates. Overall tire system forms a series of electrochemical cells with a cadmium cathode and a carbon steel anode. Each plate of metal forms the cathode of one cell and the anode of the next in the stack. Electric current is passed across the stack. The electrolyte contains phosphate and borate salts as corrosion inhibitors, EDTA to chelate any cadmium and iron ions generated by corrosion together with hex-amethylenebis(ethyldibutylammonium) phosphate to provide the necessary telraal-kylammonium ions. This electrolyte circulates through the cell from a reservoir and there is provision for the introduction of acjylonitrile and water as feedstock. The overall cell reaction is ... 

The BS I lectin requires bound calcium for activity.131 Two moles of calcium and 1.25 moles of magnesium per mole of protein were found by atomic absorption spectroscopy. Inactive, metal-free lectin, obtained by exhaustive dialysis, could be reconstituted by addition of calcium, cadmium, or strontium (magnesium restored 80% of the activity). Although bound-calcium was not removed by dialysis against EDTA, inclusion of this chelating agent in the precipitin reaction resulted in complete inhibition. 

The complexing ability of ethylenediaminetetra-acetic acid (EDTA, H4Y) has been exploited in the coulometric titration of metal ions. The method depends on the reduction of the mercury(II) or cadmium chelate of EDTA and on the titration of the metal ion (for example, magnesium) to be determined by the anion of EDTA that is released. 

The concentration of (EDTA) ", and thus the ability to complex metal ions, will depend upon the pH. A decrease in pH results in an increase in the deprotonation of EDTA and hence an increase in the concentration of the ED I A ion. The effect of this is that only metal ions with a very high affinity for EDTA will be able to form stable complexes. The stability constants for the EDTA and [diethylenetriaminepentaacetic acid] - (DTPA ) complexes with some important metal ions that are of particular interest for chelation therapy are listed in Table 7.3. It is important to note that the stability of the EDTA and DTPA complexes with toxic metals, such as lead, mercury, cadmium, or plutonium are quite similar to those with essential metals such as zinc, cobalt or copper however, the Ca complex is many orders of magnitude lower. This has important implications for chelation therapy. First, the mobilization and excretion of zinc and other essential metals are likely to be increased, along with that of the toxic metal during EDTA treatment and secondly, the chelation of the ionic calcium in the blood, that can cause tetany and even death, can be avoided by administering the chelator as the calcium salt. 

Stability constants for the complexation of Cd by edta (H L) to yield [Cd(HL)] and [CdL] have been determined, as well as data on the complexation of zinc with propylenediaminetetra-acetic acid, zinc and cadmium with trans-1,2-cyclohexanediaminetetra-acetic acid, ° and mercury with diamino derivatives of succinic and malonic acids. Substituted hydroxamic acids have also been used as chelates in the complexation of zinc and cadmium. ... 

Effective therapy for cadmium poisoning is difficult to achieve. Although there is no proven benefit, some clinicians recommend chelation therapy with CaNaffsDTA. The dose of CaNa DTA is 75 mg/kg/day in 3-6 divided doses for 5 days. After a rrunirrmm of 2 days without treatment, a second 5-day course is given. The total dose of CaNa EDTA per 5-day course should not exceed 500 mg/kg. Animal studies suggest that chelation therapy should be instituted as soon as possible after cadmium exposure because a rapid decrease in effectiveness of chelation therapy occurs in parallel with distribution to sites inaccessible to the chelators. The use of dimercaprol and substituted dithiocarbamates appears promising for individuals chronically exposed to cadmium. 

Cadmium has been used in several experimental studies of metal chelation in sea waters constituted with synthetic ligands such as EDTA (Maljkovic and Branica, 1971) and NTA (Raspor et al., 1977). The conditional stability constants of zinc and cadmium complexes with soil humics and natural water organics are not high (see Table IV). Nevertheless, such interactions have been invoked to explain the enrichment of zinc in marine and estuarine sedimentary pore waters (Elderfield and Hepworth, 1975 Nissenbaum and Swaine, 1976). 

It was shown [23] that addition of ligands that form complexes with Cd influences essentially the size of the CdS nanocolloids. Namely, an increase in the stability of a cadmium complex precursor reduces the equilibrium size of the colloidal particles. The EDTA anions appear to be an exception to this rule because they form strongly chelated Cd complexes with a stability constant of 5 x lO [24] the presence of this ligand dissolves CdS particles of a size less than a certain diameter of the CdS particles in a homogeneous colloidal solution. Data on the CdS synthesis in the inner cavities of the lipid vesicles are in a good agreement with the results of the cited work. 

Some of the most successful and widely used chelating reagents include dimethylglyoxime for the gravimetric determination of nickel 1,10-phe-nanthroline and its derivatives for the colorimetric determination of iron and copper dithizone for the separation and colorimetric determination of a number of metals but particularly lead, silver, zinc, cadmium, and mercury the dithiocarbamates such as diethylammonium diethyldithiocarbamate and ammonium pyrrolidinedithiocarbamate, used for colorimetry but more widely applied now as selective extractants and the most successful titrant, EDTA. 

Jokstad A, Thomassen Y, Bye E, Clench-Aas J, Aaseth J (1992) Dental amalgam and mercury. Pharmacol Toxicol 70 308-313 Jones MM (1991) New developments in therapeutic chelating agents as antidotes for metal poisoning. CRC Crit Rev Toxicol 21 209-233 Jones SG, Basinger MA, Jones MM, Gibbs SG (1982) A comparison of diethyldi-thiocarbamate and EDTA as antidotes for acute cadmium intoxication. Res Commun Chem Pathol Pharmacol 38 271-278 Joyce DA (1989) o-Penicillamine pharmacokinetics and pharmacodynamics in man. Pharmacol Ther 42 405-427... 

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