Eriochrome Black T
/ EH-ree-oh-krohm BLAK TEE /
Think of a mood ring that turns red when it grips a metal and blue when its hand is empty. Eriochrome Black T is exactly that kind of dye: it shows a clear wine-red while holding a metal ion and snaps to blue the moment the metal is taken away.
Technically, it is a metal-ion indicator widely used in EDTA titrations, especially for measuring magnesium and the combined hardness of water. It forms a wine-red complex with metal ions; when EDTA strips the last metal from the dye at the end point, the free indicator returns to its blue colour.
It matters because that crisp red-to-blue change is one of the most reliable visual end points in complexometry. Its limits: it only works well over a narrow pH window around 10, needs a buffer to hold that pH, and is slowly destroyed in solution, so it is often used as a fresh solid mixture rather than an old stored solution.
In a water-hardness titration buffered to pH 10, a few grains of Eriochrome Black T turn the sample wine-red; as EDTA is added to the end point, the colour sharpens to a clean sky-blue.
Wine-red while metal is bound, sky-blue once it is all complexed by EDTA.
Eriochrome Black T cannot directly indicate pure calcium, because it binds calcium too weakly to give a sharp change; a little magnesium is often added so the dye can signal the joint calcium-plus-magnesium end point.