Formation constant, EDTA

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 ...

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 values of some representative EDTA formation constants are given in Appendix C. 

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

Tabie 11.34 Formation Constants of EDTA Compiexes at 25°C, ionic Strength... 

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 ... 

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. 

To correct the formation constant for EDTA s acid-base properties, we must account for the fraction, ayi-, of EDTA present as Y . 

Since the titration is carried out at a pH of 10, some of the EDTA is present in forms other than Y . In addition, the presence of NH3 means that the EDTA must compete for the Cd +. To evaluate the titration curve, therefore, we must use the appropriate conditional formation constant. Erom Tables 9.12 and 9.14 we find that ay4- is 0.35 at a pH of 10, and that acd + is 0.0881 when the... 

After the equivalence point, EDTA is in excess, and the concentration of Cd + is determined by the dissociation of the CdY complex. Examining the equation for the complex s conditional formation constant (equation 9.15), we see that to calculate Ccd we must first calculate [CdY ] and Cedxa- After adding 30.0 mb of EDTA, these concentrations are... 

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. 

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