Q1(c). Magnesium and the EBT endpoint

Answer in brief

Magnesium gives a more distinct wine-red indicator complex and a sharper red-to-blue endpoint than calcium alone.

Key points

• At pH about 10, the calcium–EBT complex is relatively weak and its endpoint can be poor. Magnesium forms a clearer coloured indicator complex.
• A small balanced amount of Mg–EDTA is included in the titrant. Calcium displaces magnesium from Mg–EDTA because the calcium chelate is more stable.
• Released magnesium binds EBT. Near the endpoint EDTA removes the final indicator-bound magnesium, leaving blue free indicator.
• Match added magnesium with equivalent EDTA or standardise the modified reagent; unaccounted free magnesium would consume titrant and bias the result.