Indicators
To carry out metal cation titrations using EDTA, it is almost always necessary to use a complexometric indicator to determine when the end point has been reached. Common indicators are organic dyes such as Fast Sulphon Black, Eriochrome Black T, Eriochrome Red B or Murexide. Color change shows that the indicator has been displaced (usually by EDTA) from the metal cations in solution when the endpoint has been reached. Thus, the free indicator (rather than the metal complex) serves as the endpoint indicator.
Read more about this topic: Complexometric Titration