Azo Derivatives Possessing a Phenol Function

A first family is constituted by azo derivatives possessing one or several phenol functions in their structure. Eriochrome black T (solochrome black) is one of them (see Chaps. 26 and 28). It is used in the pH range 7-11. With a bivalent ion, for example, the reaction that is accompanied by a color change is 

This is not a very selective indicator since it gives complexes with the following metallic ions  

29 Complexometry III Metal Cation Indicators and Types of EDTA Titrations 

These metal cations are indeed masked by the indicator. The metallic ions are said to block the indicator. However, back titrations of Cu +, Co +, Ni +, and AP+ can be carried out with this indicator. In such titrations, of course, it is the excess of EDTA that is determined. This is carried out by the addition of a standard solution of a metal cation, usually Zn + or Mg +. During the addition of the latter, the indicator is free as long as an excess of EDTA remains. The color change appears just after the equivalence point since then the indicator gives the complex Zn-black eriochrome T or Mg-black eriochrome T with Zn + or Mg +, which are then in excess. 

There are other dyestuffs, named eriochrome black A, B, and R, whose structures are very close to that of black T. Another indicator of the same chemical family is Patton and Reeder s indicator, whose structure is as follows  

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