Pb-EDTA complex

Figure 22.8 (a) EDTA complex of lead. The complex bears a net charge of -2 because each O donor atom has one negative charge and the lead ion carries two positive charges. Only the lone pairs that participate in bonding are shown. Note the octahedrai geometry around the ion. p) Molecular model of the Pb -EDTA complex. The yellow sphere is the Pb ion. 

EDTA forms 1 1 complexes with a large number of cations, including those of some of the main-group metals. The complex formed by calcium with EDTA is used to treat lead poisoning. When a solution containing the Ca-EDTA complex is given by injection, the calcium is displaced by lead. The more stable Pb-EDTA complex is eliminated in the urine. EDTA has also been used to remove radioactive isotopes of metals, notably plutonium, from body tissues. 

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. 

This colour change can be observed with the ions of Mg, Mn, Zn, Cd, Hg, Pb, Cu, Al, Fe, Ti, Co, Ni, and the Pt metals. To maintain the pH constant (ca 10) a buffer mixture is added, and most of the above metals must be kept in solution with the aid of a weak complexing reagent such as ammonia or tartrate. The cations of Cu, Co, Ni, Al, Fe(III), Ti(IV), and certain of the Pt metals form such stable indicator complexes that the dyestuff can no longer be liberated by adding EDTA direct titration of these ions using solochrome black as indicator is therefore impracticable, and the metallic ions are said to block the indicator. However, with Cu, Co, Ni, and Al a back-titration can be carried out, for the rate of reaction of their EDTA complexes with the indicator is extremely slow and it is possible to titrate the excess of EDTA with standard zinc or magnesium ion solution. 

Another material based on the crown ether extractant 4,4 (5 )-bis(t-butyl-cyclohexano)-18 crown-6, marketed under the name Sr-Spec, is useful for separations involving divalent cations including Pb, Ba, and Ra (Horwitz et al. 1991). For Ra analysis by TIMS, Ra-Ba separations are required because the presence of Ba drastically decreases the ionization efficiency of fg Ra samples from 10% to <1%. This material has been widely used for separations of Ra from Ba (e.g., Chabaux et al. 1994 Lundstrom et al. 1998 Rihs et al. 2000 Joannon and Pin 2001 Pietruszka et al. 2002) and is a complement or alternative to cation exchange separations for EDTA complexes of these elements (Volpe et al. 1991 Cohen and O Nions 1991). Sr-Spec material would also be useful for °Pb analysis, since Pb has a greater distribution coefficient than Sr with this extractant. 

An indirect method for the determination of lead by coupling reactions was developed based on the replacement of Fe(II) by Pb(II) from the Fe(II)-EDTA complex. The subsequent CL reaction was based on the Fe(II)-luminol-02 system. The method was used to determine lead in polluted water samples [75], Such methods may be extended to other ions with proper complex constants as compared to the Fe(II)-EDTA complex, after HPLC separation. Analysis of elements based on indirect reactions is summarized in Table 4. 

Predicted formation times0 of Cd, Pb, Zn complexes with EDTA 10 6 - 1... 

Speciation of Pb(II) in Glatt river. The concentrations given for CO2, Pb(II), Cu(II) and [Ca2+] as well as for the pollutants EDTA and NTA are representative of concentrations encountered in this river, The speciation is calculated from the surface complex formation constants determined with the particles of the river and the stability constants of the hydroxo-, carbonate-, NTA- and EDTA-complexes.The presence of [Ca2+] and [Cu2+] is considered. 

The electrochemical deposition of metal (Cd, Zn, Pb) from Me(II)-ethylenedia-minetetraacetic acid (EDTA) complexes... 

Chiu prepared monodisperse crystalline particles of metal sulfides, such as lead sulfide (PbS cubes 100 A) (I), cupric sulfide (CuS hexagonal bipyramids 200 A) (2), and zinc sulfide (ZnS multifaceted spheres 0.1-0.4 p,m) (3) by introducing hydrogen sulfide gas into dilute acidic solutions of the ethylenediamine tetraacetic acid (EDTA) complexes of the corresponding metal ions (10 4-10-1 mol dm-3) for several minutes at room temperature. 

Auxiliary complexing agents such as NH3, tartrate, citrate, or triethanolamine may be employed to prevent metal ion from precipitating in the absence of EDTA. For example, Pb2+ is titrated in NH, buffer at pH 10 in the presence of tartrate, which complexes Pb2+ and does not allow Pb(OH)2 to precipitate. The lead-tartrate complex must be less stable than the lead-EDTA complex, or the titration would not be feasible. 

FIGURE2.14 The equivalent fractions of lead ion on the surface of montmorillonite and the ratios of different lead-EDTA complexes in the solution at Pb EDTA =1 1. (Reprinted from Nagy and Konya 1998, with permission from Elsevier.)... 

Cadmium (Cd, at. mass 112.40) occurs in its compounds exclusively in the II oxidation state. Unlike Zn(OH)2 and Pb(OH)2, Cd(OH)2 shows no amphoteric properties and is insoluble in excess of NaOH. Cadmium forms ammine, cyanide, halide, and EDTA complexes. 

Lead (Pb, at. mass 207.19) occurs in its compounds in the II and IV oxidation states. Compounds of lead(IV) have acidic properties. Lead hydroxide, Pb(OH)2, is amphoteric, precipitating within the pH range 7-13. Lead(II) forms strong tartrate, acetate, thiosulphate, and EDTA complexes. The halide complexes are relatively weak. 

Silver may be separated from Ce, Zr, Th, Be, and Fe(III), on strongly acidic cation-exchangers, by converting these metals into anionic complexes, or separated from Cu, U, Al, and Zn by selective elution with nitric acid [14]. After retention of Pb, Ag, and Hg on Dowex 50, lead is eluted first with 0.25 M ammonium acetate, then silver with 0.5 M ammonia solution. Silver has been separated on a cation-exchanger from Hg, Co, Ni, and Zn on the basis of the differing stabilities of their EDTA complexes at pH 4.6 [15]. Silver retained in a column with a macroporous cation-exchange resin bed has been eluted with 2 M HNO3 or 0.5 M HBr in aqueous acetone solution [16]. 

Fig. 8 The effect of the addition of the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) in the separation buffer for the separation of ion complexes with the chelating agent EDTA. TTAB, 0.5 mM, was added to the separation buffer for the separation of N03 (1), Cu-EDTA, Pb-EDTA, EDTA (2), Cr-EDTA (3), and Fe-EDTA (4). (For details of experimental conditions, see Ref. 341.)...

In another investigation, the exchange between [Ce(edta)aq] and hydrated Pb, Ni or Co ions again show reaction by dissociation of protonated [Ce(Hedta)aq] as well as by the direct attack of metal ions on [Ce(edta)aq]" or [Ce(Hedta)aq]. The kinetic parameters for the Ni " " or Co " " ions could be related to the relatively dow (k = 2.6 x 10 s for Co and 3.4 X 10 s for NP" ") water exchange reactions of these ions. The direct attack was interpreted in terms of an intermediate in which one of the carboxylate groups was coordinated to the incoming ion rather than to Ce. These reactions were followed by spectrophotometry at 280 nm, where the absorbance of Ce aq is much lower than the edta complex. 

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