What is the principle of action of metallochromic indicators based on?

The principle of operation of metallochromic indicators is based on the formation of colored complex compounds with metal ions by them. The color of the resulting complex determines the color of the titrated solution up to the equivalence point. The complex of metal ions with the indicator is less stable than the complexonate; therefore, at the point of equivalence, it is destroyed and the solution acquires a color characteristic of free indicator ions.

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