FAQs
What is Eriochrome Black T used for?
It is the classic metal indicator for EDTA complexometric titration of total water hardness. At about pH 10, it forms a wine-red complex with calcium and magnesium; when EDTA binds the last free metal ions, the solution turns blue, giving a sharp endpoint for hardness determination.
Why is the titration done at pH 10?
Both the indicator's color change and the EDTA-metal reaction depend on pH. An ammonia/ammonium chloride buffer holds the sample near pH 10, where Eriochrome Black T gives its cleanest wine-red-to-blue transition, and the calcium and magnesium complexes with EDTA are well defined.
How much indicator do I add?
Only a small amount is needed per titration, enough to give a clear color without consuming significant titrant. Many labs use a small measured quantity of a ground mixture or a few drops of a prepared solution. Follow your method's specified amount for reproducible endpoints.
Why does Eriochrome Black T need to be kept dry and dark?
The dye degrades on exposure to light and moisture, which can weaken or shift its color response and blur the endpoint. Store it tightly closed, cool, dry, and out of the light to preserve performance. Discard the indicator if it no longer gives a sharp red-to-blue change.
Can it be used for metals other than calcium and magnesium?
Eriochrome Black T responds to several divalent metal ions and is used in some zinc and other complexometric titrations, but its dominant use is calcium-plus-magnesium hardness. For other metals, follow the specific method, including pH and any masking agents.
