Cobalt-EDTA chelate

EDTA (ethylenediaminetetraacetic acid) forms stable metal chelates with a number of metal ions. Using this reagent as a complexing- agent, arsenic, bismuth, and selenium can be determined without any interference in the presence of nickel and cobalt. The cobalt-EDTA chelate is stable in 5 M HCl solution, whereas the corresponding bismuth complex is not. The influence of copper on the determination of arsenic can also be eliminated with EDTA, but not in the determination of selenium. Thiourea has been used to eliminate the influence of copper in the determination of antimony and sodium oxalate to eliminate the influence of copper and nickel in the determination of tin. An addition of thiosemicarbazide and 1,10-phenanthro-line reduces the interference of copper, nickel, platinum, and palladium in the determination of arsenic. 

The rate of peroxide decomposition and the resultant rate of oxidation are markedly increased by the presence of ions of metals such as iron, copper, manganese, and cobalt [13]. This catalytic decomposition is based on a redox mechanism, as in Figure 15.2. Consequently, it is important to control and limit the amounts of metal impurities in raw rubber. The influence of antioxidants against these rubber poisons depends at least partially on a complex formation (chelation) of the damaging ion. In favor of this theory is the fact that simple chelating agents that have no aging-protective activity, like ethylene diamine tetracetic acid (EDTA), act as copper protectors. 

The heavy metals copper, manganese, cobalt and zinc were omitted individually and in combination from MS and B5 media to determine the effect on antibody stability in solution [63]. When IgG, antibody was added to these modified media in experiments similar to the one represented in Figure 2.2, only the B5 medium without Mn showed a significant improvement in antibody retention relative to normal culture media. Nevertheless, protein losses were considerable as only about 30% of the added antibody could be detected in the Mn-free medium after about 5 h. The beneficial effect of removing Mn was lost when all four heavy metals, Cu, Mn, Co and Zn, were omitted simultaneously. The reason for these results is unclear. Addition of the metal chelating agent ethylenediaminetetraacetate (EDTA) had a negligible effect on antibody retention in both MS and B5 media [63]. 

FIGURE 8.21 The chelation complex of EDTA with cobalt(lll). Each EDTA ion has six donor sites at which it can bind (by donating lone-pair electrons) to the central metal ion. 

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. 

Quantitative complex formation in the pH range 3-8 requires about 10 min the absorbance is then stable for at least 2 h. In strongly acidic solutions and in the presence of EDTA, the cobalt complex is not formed. However, the cobalt complex formed is resistant to H2SO4 or HNO3 (up to 2 A/) and EDTA (up to 0.1 M). Chelates of many other metals with 5-Br-PADAP would be decomposed under these conditions. This fact, which results from the inertness of the cobalt complex, increases the selectivity of 5-Br-PADAP for cobalt. Palladium, Cu, V, Ni, and Hg still interfere. 

Another advantage of potentiometric titrations is that substances to which the electrode does not respond can be determined, if the electrode responds to the titrant or to some low level of an indicator substance that has been added to the solution. For example, low levels of Al can be determined by titration with standard fluoride solution, using a fluoride electrode [22]. EDTA and other chelates can be determined by titration with standard calcium or copper solution. Manganese(II), vanadium(II), or cobalt(II) can be determined via EDTA titration if a small amount of CuEDTA indicator is added to the solution and a copper electrode is used. The electrode responds directly to the Cu activity which, however, is dependent on the activities of the EDTA and the other metal ion in solution. 

Metal chelation may enhance or inhibit the Fenton reaction, depending on the metal and the chelator in question. Chelation of iron (II) by EDTA enhances the formation of hydroxyl radical, while deferoxamine, another chelator, reduces its formation. This is significant because peptides or proteins can chelate metals in the body, thus influencing the resulting degree of damage. The formation of hydroxyl radicals by nickel (II) and cobalt (II) is enhanced by this type of chelation. In addition to the Fenton and Haber-Weiss reactions, metals can also catalyze the formation of the hydroxyl radical via reaction with hypochlorite (HOCl), which is prodnced by neutrophils. ... 

Sulfur and several sulfides, highly insoluble precipitates with solubility products as low as 1.6 X 10 for mercuric sulfide, have been used to concentrate trace metals from water. Sulfur, produced from (NH4)aS and HNO3 ( 0), coprecipitated several metals including mercury. Iron(III) sulfide (also used in a commercial process SULFEX) removes several metals (61) and is better than hydroxide in the presence of EDTA and other chelating agents (62). Lead sulfide has been used to collect silver for aqueous solution (63), molybdenum sulfide to collect arsenic from 2 M hydrochloric acid solution (64), and copper sulfide to concentrate cobalt and zinc from seawater (65). 

Recently the chelating agent ethylenediaminetetraacetic acid (EDTA) has been used in connection with this problem,and although the relationships are complex, it is possible to show biological activity for Ca++ and Co++. Slater has used 8-hydroxyquinaline to remove trace elements from the basal medium and has demonstrated a cobalt requirement in glucose-free medium. No such requirement was found, however, when glucose was present. 

The extent of precursor complex formation is an important factor in determining the relative rates of chromous reduction of complexes [(NH3)5CoYH ], where Y is 0-bound ida " or edta . With [(NH3)5Co(ida)] the precursor involves chelation at Cr " and leads to the tridentate-bound ligand, whereas protonated cobalt(III) reagents yield only monodentate chromium(III) products. 

It is also possible that the formed chelates are mixed metal-EDTA—(or other ligand) chelates, such as, for example, in the cases of the following compounds [M(OH)Y], [M(H)Y], and so forth. Another example of a mixed complex is provided by the aquo-cobalt(lll)-EDTA complex, whose structure is seen in Fig. 28.2. 

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