While not as frustrating an indicator as murexide, Eriochrome Black T's wine red to blue transformations provide significant challenges. The change from "fully wine red" to "fully blue with no trace of red" takes around 1mL. It goes through first from the wine red to a purple, which then becomes are deeper blue after several more drops (with still very slight traces of purple and hence red). Addition of several more drops thereafter produces a dark blue colour which appears to have no purple trace (there may be a slight amount), and adding 10 or so drops converts the darkish blue to a beautiful, light blue. Where is the endpoint here? I assume it is at the deep blue, although the wine red to purple transformation is very sharp and abrupt.
Another challenge that this indicator provides is the amount used. It appears the more indicator I put, the more a pure blue solution appears to have a purple trace. The endpoints differ with a tenfold difference in Erio T amount up to 1mL. I have tested this theory - with a dilute solution I titrate to the "deep blue endpoint", add more Erio T, and then hints of purple become visible etc.
Does anyone know the true endpoint of Erio T and the optimal amount to add?
Note: This was for a Mn (II) titration (ascorbic acid added). The problem manifests itself in magnesium titration as well, and prevents obtaining endpoints to any accuracy.
Edit: The concentration of titrant and analyte are both ~0.01M. This is quite standard for EDTA titrations.