Metal ion indicators (complexometric indicators) are organic dyes that form weak complexes with metal ions and change color during the titration endpoint. They ensure accurate detection of the endpoint in complexometric titrations.
Properties of a Good Indicator:
- Forms a distinctively colored complex with the metal ion.
- The color change should be sharp at the endpoint.
- Should not form too strong a complex with the metal ion (to avoid interference with the titration).
Examples of Metal Ion Indicators:
- Eriochrome Black T (EBT):
- Commonly used for calcium and magnesium.
- Wine-red (metal-indicator complex) → Blue (free indicator) at the endpoint.
- Murexide:
- Used for calcium estimation.
- Pink → Purple at the endpoint.
- Xylenol Orange:
- Used for metals like aluminum and iron.
- Yellow → Red at the endpoint.
- Calcon:
- Suitable for calcium titrations.
- Blue → Purple at the endpoint.