UPSC Combined Geo-Scientist, Mains · 2020 · Mains Paper I

Q1(f). Magnesium-assisted EDTA endpoint

Question

Small amount of magnesium chloride is added during the estimation of calcium by EDTA and solochrome black indicator. Justify.

Mg(II) forms a suitably stable wine-red indicator complex, giving a sharper red-to-blue endpoint than calcium alone.

Solochrome Black (Eriochrome Black T) forms a weak Ca–indicator complex, so a calcium-only endpoint is poorly defined at about pH 10.
Added Mg(II) binds the indicator. As EDTA consumes the metal ions, the final increment removes Mg from the Mg–indicator complex, releasing blue free indicator.
Calcium–EDTA is more stable than magnesium–EDTA under the usual conditions, allowing the endpoint to be controlled by Mg.

The full model answer

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