Chemistry and Properties of EDTA

Ethylenediaminetetraacetic acid, or EDTA, is an aminocar-boxylic acid. The structure of EDTA is shown in Eigure 9.25a. 

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

EDTA Is a Weak Acid Besides its properties as a ligand, EDTA is also a weak acid. The fully protonated form of EDTA, HeY, is a hexaprotic weak acid with successive pKa values of 

The first four values are for the carboxyl protons, and the remaining two values are for the ammonium protons. A ladder diagram for EDTA is shown in figure 9.26. The species Y becomes the predominate form of EDTA at pH levels greater than 10.17. It is only for pH levels greater than 12 that Y becomes the only significant form of EDTA. 

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. 

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