Case of Metal Indicators

Metal indicators are derivatives, more precisely dyestuffs, that exhibit a color change when the concentration of the metal cation they indicate changes within a certain concentration range. They form complexes with specific metal cations. In principle, they exhibit two colors depending on whether they are free or engaged in the complex with the metallic ion. They are used to detect endpoints in complexometric titrations. 

Like numerous ligands, metal indicators can take up protons. The protons taken up also produce a color change therefore, these derivatives are equally pH indicators. Of course, free metal indicators, once protonated, have lost a fraction of their complexing properties compared to their basic forms. 

For analytical purposes, metal indicators can be smdied from two standpoints, which, additionally, are interrelated. The first concerns their ability to complex metallic ions as a function of pH. The second standpoint concerns the stability of metal-indicator complexes compared to that of metal-titrant complexes. The latter standpoint will be studied in Chap. 29. 

Using conditional stability constants is a good strategy to orient and control the use of metal indicators as a function of pH. As an example, we now consider the case of eriochrome black T in details. The structure of eriochrome black T in acidic 

It can be symbolized by H2ln . The sulfonic acid group remains, at least partly, ionized in acidic medium. The presence of two phenol functions in its stmcture makes eriochrome black T diacidic. Its predominance areas as a function of pH are given in Fig. 26.6. They are delimited by thepK values pK = 6.3 andpK 2 = 11.6. 

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