Equilibrium EDTA complex formation

B. (a) Write the reaction whose equilibrium constant is the formation constant for EDTA complex formation and write the algebraic form of Kf. 

EDTA forms 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... 

Note that Kf for EDTA is defined in terms of the species Y4 reacting with the metal ion. The equilibrium constant could have been defined for any of the other six forms of EDTA in the solution. Equation 12-5 should not be interpreted to mean that only Y4 reacts with metal ions. Table 12-2 shows that formation constants for most EDTA complexes are quite large and tend to be larger for more positively charged cations. 

You can see from the example that a metal-EDTA complex becomes less stable at lower pH. For a titration reaction to be effective, it must go to completion (say, 99.9%), which means that the equilibrium constant is large—the analyte and titrant are essentially completely reacted at the equivalence point. Figure 12-9 shows how pH affects the titration of Ca2+ with EDTA. Below pH 8, the end point is not sharp enough to allow accurate determination. The conditional formation constant for CaY2" is just too small for complete reaction at low pH. 

EDTA (ethylenediaminetetraacetic acid) (H02CCH2)2NCH2CH2N-(CH2C02H)2, the most widely used reagent for complexometric titrations. It forms 1 1 complexes with virtually all cations with a charge of 2 or more, effective formation constant Equilibrium constant for formation of a complex under a particular stated set of conditions, such as pH, ionic strength, and concentration of auxiliary complexing species. Also called conditional formation constant. 

Aminolevulinic acid, an element in the biosynthesis of heme proteins, may be overexpressed under certain pathological conditions and its accumulation has been correlated with some hepatitic cancers. It is in equilibrium with its enol form and can complex transition-metal ions. In the presence of 02, this may lead to the formation of OH and hence in the presence of DNA to DNA damage which is enhanced in the presence of ferritin (Douki et al. 1998 di Mascio et al. 2000). Superoxide radicals have been assumed to be intermediates in these reactions. Mechanistically, the formation of 02 is certainly very complex, because even in the case of the Fe(II)-EDTA-complex the reduction potential is not low enough to reduce O2 by simple one-electron donation. 

EDTA Complexes. Ethylenediaminetetraacetic acid (EDTA) and its homologues form the most stable known complexes of plutonium. This discussion will be limited to EDTA, which is most likely to be found in the environment as a result of its use as a medium for the addition of soluble iron to soils. The equilibrium constant for formation of the 1 1 chelate of plutonium(III), as given by the expression... 

Table 17-3 lists formation constants for common EDTA complexes. Note that the constant refers to the equilibrium involving the fully unprotonated species Y" with the metal ion ... 

Bryce et al. (1994) demonstrated that in the ternary Ni-EDTA-ferrihydrite system (initial Ni/EDTA molar ratio of 1) the fraction of nickel sorbed was dependent on the component addition sequence (Figure 5.9a), but the fraction of sorbed EDTA was not. EDTA sorption could always be described in terms of the binary EDTA-ferrihydrite system, with the fraction sorbed decreasing sharply above pH 6 (Figure 5.9Zj). When Ni and EDTA were preequilibrated at pH 7, which favored the formation of 1 1 Ni-EDTA complex, the fraction of Ni and EDTA sorbed onto ferrihydrite after 2, 24, and 48 hours was near 100% at pH < 6 but decreased sharply to approximately 0% at pH > 7 (Figure 5.9c). In this system, equilibrium was achieved within 2 hours and the fraction of Ni sorbed at a given pH was equivalent to the fraction of EDTA sorbed, suggesting... 

Erio-T is a tribasic acid and forms a colored complex with magnesium ions. This is a beautiful example of how a knowledge of equilibrium constants can be used to solve a problem. The formation constants of Ca and Mg with EDTA are 3.0 x 10 and 5.0 x 10. The magnesium complex with the indicator is more stable (lx 10 ) than the complex with the calcium (2.5 x 10 ) but less stable than the Mg EDTA complex. Thus, during a titration, the EDTA reacts first with the free Ca ions, then with the free Mg" ions, and finally with the Mg in the indicator complex... 

Osthols et al. have also reanalysed the experimental data in [1987JOA/BIG2] they point out that the Th(IV) EDTA complex used to evaluate the equilibrium constant for the formation of Th(C03)5 is Th(EDTA)(OH) , not Th(EDTA)(aq), but it is not clear from [19940ST/BRU] how they arrived at the recalculated value of logj this review has therefore reanalysed the data in [1987JOA/BIG2] as described in the Appendix A entry for that paper. 

Hydration of lanthanide complexes. X-ray diffraction studies of the solid complexes of KLn(EDTA)(H20),c showed the number of water molecules in the coordination sphere to be three for the lighter lanthanides and two for the heavier ones for total coordination numbers of nine and eight, respectively, since EDTA is hexadentate (Hoard et al. 1967). Ots (1973) measured a maximum at europium for the heat capacity change AC° for the formation of lanthanide-EDTA complexes. This maximum was taken as a strong evidence for hydration equilibrium between the complexed species,... 

In the now familiar pattern discussed above, the titration involves the buret addition of EDTA solution to the metal ion solution, which generates a titration curve with an abrupt change in — log[M " ] (pM). This is governed by the equilibrium constant for the formation of the metal-EDTA complex ... 

When the potential of an electrode of the first kind responds to the potential of another ion that is in equilibrium with M"+, it is called an electrode of the second kind. Two common electrodes of the second kind are the calomel and silver/silver chloride reference electrodes. Electrodes of the second kind also can be based on complexation reactions. Eor example, an electrode for EDTA is constructed by coupling a Hg +/Hg electrode of the first kind to EDTA by taking advantage of its formation of a stable complex with Hg +. 

The equilibrium constant (log K) for the formation of the quinquedentate complex is 23.40.1219 Perhaps the most remarkable feature of chromium(III)-edta chemistry is the kinetic... 

The method of continuous variation can be carried out with many separate solutions, as in Table 19-1. However, a titration is more sensible. Figure 19-9a shows a titration of EDTA with Cu2+. In Figure 19-9b, the abscissa has been transformed into mole fraction of Cu2+(= [moles of Cu2+]/[moles of Cu2+ + moles of EDTA]) instead of volume of Cu2+. The sharp maximum at a mole fraction of 0.5 indicates formation of a 1 1 complex. If the equilibrium constant is not large, the maximum is more curved than in Figure 19-9b. The curvature can be used to estimate the equilibrium constant.7... 

The result of Fe +Cl O) 5(NO) is in good agreement with that determined by Kastin et al. (15) using the same experimental technique. For both Fe2+(EDTA)(NO) and Fe2+(NTA)(NO), the relaxation times due to the temperature jump were too fast to be measured. However, an upper limit of 10 /is was established for the relaxation times for both complexes. By use of this value with the equilibrium constants determined for Fe2+(EDTA)(NO) (16) and Fe2+(NTA)(NO) (10), the lower limits of formation rate constants were calculated to be 7 x 10 and 6 x 107 Z/nol -sec at 35 °C, which is in good agreement with that determined by the temperature-jump technique. From the results listed in Table I, we can conclude that the formation rate of Fe2+(EDTA)(NO) is at least 85 times faster than that of Fe2+(H20)5(N0), whereas, the dissociation rate of Fe2+(EDTA)(NO) is about 250 times slower than that of Fe2+(H20)5(N0) at 25 °C. 

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