Question

write the reaction between edta and magnesium ion in hard water

Answer 1

EDTA is used to detect the amount of Magnesium and Calcium ions that are present in hard water, sea water etc, by complexometric titration reactions.

EDTA stands for Ethylenediamine tetra-acetic acid. It forms complexes with calcium and magnesium ions which is indicated by the blue indicator Eriochrome Black T (ErioT).

Magnesium ions are allowed to react with excess EDTA during the back titration reactions in which MgCl₂ solution is used. The equation is - 

EDTA⁴⁻ + Mg²⁺ → [Mg-EDTA]²⁻

After the indicator is added, the colour of the solution changes from blue to pink indicating the presence of the ErioT-Mg complexes.