Metal-EDTA formation constants

Metal—EDTA Formation Constants To illustrate the formation of a metal-EDTA complex consider the reaction between Cd + and EDTA... 

Table 5.8 Some selected metal-EDTA formation constants ...

C6, the most widely used complexing agent is ethylenediaminetetracetic acid or EDTA, and Table 1 gives a selection of metal EDTA formation constants. 

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 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. 

Any metal ion that has an EDTA formation constant higher than calcium or magnesium will interfere. Cyanide complexes strongly with copper, cobalt, nickel, zinc, and ferrous iron. Hydroxylamine or ascorbic acid is added to reduce iron to the ferrous state. If the solution is buffered to pH 10 before the indicator is added, then iron will not interfere because it precipitates as the hydroxide before it can react with the indicator or the EDTA. 

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 ... 

Finding the End Point with a Visual Indicator Most indicators for complexation titrations are organic dyes that form stable complexes with metal ions. These dyes are known as metallochromic indicators. To function as an indicator for an EDTA titration, the metal-indicator complex must possess a color different from that of the uncomplexed indicator. Furthermore, the formation constant for the metal-indicator complex must be less favorable than that for the metal-EDTA complex. 

This reaction will proceed if the metal indicator complex M-In is less stable than the metal-EDTA complex M EDTA. The former dissociates to a limited extent, and during the titration the free metal ions are progressively complexed by the EDTA until ultimately the metal is displaced from the complex M-In to leave the free indicator (In). The stability of the metal-indicator complex may be expressed in terms of the formation constant (or indicator constant) Ku ... 

The basis for the toxicological activity of this substance is the reaction of cobalt ion with cyanide ion to form a relatively nontoxic and stable ion complex. The hexacyanocobaltate ion contains a Co2+ central metal ion with six cyanide ions as ligands. This coordination complex involves six coordinate covalent bonds whereby each cyanide ion supplies a pair of electrons to form each covalent bond with the central cobalt ion. The formation constant for the hexacyanocobaltate ion is even larger than for dicobalt EDTA,3 and thus the cobalt ion preferentially exchanges an EDTA ligand for six cyano ligands ... 

TABLE 9.1 Formation Constants for Complexes of EDTA with Metal Cations at 25°C... 

The larger the formation constant, K, the more likely the M - EDTA complex is to form and the fewer free metal ions remain. 

EDTA salts are used for the treatment of heavy metal poisoning. Roosels and Vanderkeel142) were able to extract lead from urine in the presence of EDTA with dithizone by adding calcium to presumably release the lead from EDTA. In view of the fact that the formation constant of the lead-EDTA chelate is 20,000,000 times larger than that of the corresponding calcium chelate, it is doubtful that the calcium actually releases the EDTA from the lead. 

EDTA complexes of trivalent metals can be extracted successively with liquid anion exchangers such as Aliquat 336-S by careful pH control. Mixtures of lanthanides can be separated by exploiting differences in their EDTA complex formation constants. 

Metal ion complex formation is typically measured via stability, or formation, constants. Thus, for -i- EDTA ... 

Formation Constants for Selected Metal-EDTA Complexes... 

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. 

Table 12-2 Formation constants for metal-EDTA complexes...

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. 

Box 12-2 Metal Ion Hydrolysis Decreases the Effective Formation Constant for EDTA Complexes... 

Equation 12-18 states that the effective (conditional) formation constant for an EDTA complex is the product of the formation constant, Kf. times the fraction of metal in the form M" + times the fraction of EDTA in the form Y4- K" = aM, + aY4 h t. Table 12-1 told us that aY4- increases with pH until it levels off at 1 near pH 11. 

In a direct titration, analyte is titrated with standard EDTA. The analyte is buffered to a pH at which the conditional formation constant for the metal-EDTA complex is large and the color of the free indicator is distinctly different from that of the metal-indicator complex. 

Formation constants for EDTA are expressed in terms of [Y4-], even though there are six protonated forms of EDTA. Because the fraction (aY4 1 of free EDTA in the form Y4 depends on pH, we define a conditional (or effective) formation constant as K = aYj Kf = MY" 4 /[M"+ [EDTA], This constant describes the hypothetical reaction Mn+ + EDTA MY 1-4, where EDTA refers to all forms of EDTA not bound to metal ion. Titration calculations fall into three categories. When excess unreacted M"+ is present, pM is calculated directly from pM = — log M l+]. When excess EDTA is present, we know both [MY"-4] and [EDTA], so IM"+] can be calculated from the conditional formation constant. At the equivalence point, the... 

The greater the effective formation constant, the sharper is the EDTA titration curve. Addition of auxiliary complexing agents, which compete with EDTA for the metal ion and thereby limit the sharpness of the titration curve, is often necessary to keep the metal in solution. Calculations for a solution containing EDTA and an auxiliary complexing agent utilize the conditional formation constant K" = aM aY4- Kt, where aM is the fraction of free metal ion not complexed by the auxiliary ligand. 

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