Formation constants of EDTA complexes

The formation constants of EDTA complexes are gathered in Table 11.34. Based on their stability, the EDTA complexes of the most common metal ions may be roughly divided into three groups ... 

TABLE 11.34 Formation Constants of EDTA Complexes at 25°C, Ionic Strength Approaching Zero ... 

Fonnation of metal-EDTA complexes The formation constants of EDTA complexes of various metals are listed in Table 11-2. Written as concentration constants, they involve reactions of the type... 

Table 11-2 FORMATION CONSTANTS OF EDTA COMPLEXES AT 20°C, IONIC STRENGTH 0.1...

Separations based on complex formation The relative retentions for closely similar metal ions often may be enhanced by use of a suitable reagent (such as EDTA) to take advantage of differences in formation constants of the complexes. A classic example is the separation of the lanthanides with a buffered dtrate solution as the eluting agent. Especially effective separations are possible when one metal is converted to an anionic complex while another is present as a cation. Teicher and... 

The conditional formation constant for the calciumyEDTA complex at pH 10 is obtained from the formation constant of the complex (see Table 17-3) and the value for EDTA at pH 10 (see Figure 17-4). Thus, if we substitute into Equation 17-25, we get... 

Table 8 Formation Constants of Metal Complexes of EDTA ...

Formation constants for EDTA complexes in Table 13-1 are large and tend to be larger for more positively charged metal ions. Note that K( is defined for reaction of the species Y" " with the metal ion. At low pH, most EDTA is in one of its protonated forms, not Y . 

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

Conditional Metal—Ligand Formation Constants Recognizing EDTA s acid-base properties is important. The formation constant for CdY in equation 9.11 assumes that EDTA is present as Y . If we restrict the pH to levels greater than 12, then equation 9.11 provides an adequate description of the formation of CdY . for pH levels less than 12, however, K overestimates the stability of the CdY complex. 

Figure 3.29 The formation constants of RE(III) complexes with IDA, NTA, TEDTA, EDTA, and DTPA.

The papers cited above include studies of La and Lu " EDTA-type complexes, (308, 309, 311, 312) Cu complexes, (314, 315) Fe " complexes, (310) and Sm " complexes. (316, 317) Kinetic parameters and stability constants have been calculated in many cases. In one study (315) the variation in the relaxation time of [Cu(EDTA)(H20)] was followed on gradual addition of ammonia at pH 9. Values were found to shorten and then remain constant after addition of a ten-fold excess of ammonia. This is due to the formation of [Cu(EDTA)(NH3)2] " with a formation constant of 18-8. 

Figure 11-7 depicts the relation between the titration error and the product Cm STm-l -Figure 11-3 indicates that conditional formation constants of at least 10 ° are possible for most metal ions in a selected pH region. According to Figure 11-7 a titration error of 0.1% should be easily attainable for 0.01 M solutions of such metals. For a 0.1% relative excess of reagent in the titration of 0.01 M metal ion, [Y ] of the reagent then equals 10" M. A value of = 10 then corresponds to a 99.9% conversion of metal ion to EDTA complex since [MY]/Cm = 1000. 

Assume that log P4, for chloride, bromide, and iodide with Zn is — 1, — 0.74, and — 1.25 and with Cd" " is 0.9, 2.53, and 6.1. (a) Calculate the masking index for Zn" " and Cd" " in 1.0 Af chloride, 1.0 Af bromide, and 0.1 Af and 1.0 Af iodide. Assume activity coefficients of unity and that the metal ions are present predominately as their highest complexes, (b) Assuming that the conditional formation constants of Zn" " and Cd with EDTA are the same (Figure 11-3), which condition is most favorable for the titration of zinc in the presence of cadmium ... 

Molecular model of NiY- . This complex is typical of the strong complexes that EDTA forms with metal ions. The formation constant of the Ni complex is 4.2 X 10 . 

Eriochrome Black T forms red complexes with more than two dozen metal ions, but the formation constants of only a few are appropriate for end point detection. As shown in Example 17-5, the applicability of a given indicator for an EDTA titration can be determined from the change in pM in the equivalence-point region, provided that the formation constant for the metal indicator complex is known. ... 

Figure 7.12 shows a chromatogram of the same sample in which EDTA is added to complex the iron(lll). The additional peak is from an iron(ll) impurity in the iron(II) solution used. Work thus far indicated that any metal ion that has an EDTA formation constant of about 10 - or higher should be masked effectively by adding EDTA to the sample. 

Formation and Dissociation Rate Constants of Nitrosyl Metal Chelates. The absorption rates of NO in an aqueous solution of Fe(II)EDTA were measured by Teramoto et al. (31) and Sado et al. (32) using a stirred vessel with a free flat gas-liquid interface. The forward rate constants of the complexing reaction were derived on the basis of the theory of gas absorption. The results are given in Table II. 

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... 

The formation constants of the EDTA complexes of calcium and magnesium are too close to differentiate between them in an EDTA titration, even by adjusting pH (see Figure 9.4). So they will titrate together, and the Eriochrome Black T end point can be used as above. This titration is used to determine total hardness of water, (Ca " plus Mg " —see Experiment 9). Eriochrome Black T cannot be... 

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