metal-ion indicator
/ MET-ul EYE-on IN-dih-kay-tor /
Imagine a dye that wears one colour when it is holding hands with a metal and a different colour when it lets go. A metal-ion indicator is exactly that kind of colour-changing partner, added in a tiny amount so you can watch with your eyes when the metal has all been taken away.
Technically, it is a dye that itself forms a coloured complex with the metal ion. During a complexometric titration, the indicator weakly holds a little of the metal and shows one colour; when the titrant (such as EDTA) has consumed all the free metal, it finally strips the metal from the indicator too, and the dye flips to its free colour — that flip marks the end point.
It matters because it lets you find the end point with no instrument, just a sharp colour change. The catch is that the indicator must bind the metal more weakly than the titrant does, and only over the right pH window, or the colour change will arrive too early, too late, or be muddy.
Eriochrome Black T is wine-red while bound to magnesium but pure blue when free; at the end of an EDTA titration the colour jumps from red to blue, telling you every metal ion has been complexed.
The dye changes colour the instant the titrant takes the last metal away.
Most metal-ion indicators are also acid-base indicators, so their free colour depends on pH; this is why a buffer is essential — it fixes both the titration conditions and the indicator colour at once.