Calcium ion with EDTA

Figure 1 7-6 EDTA titration curves for 50.0 mL of 0,00500 M Ca-+ (i cav = 1.75 X 10 ) and Mg2+ = 1.72 X 10 ) at pH 10.0, Note that because of the larger formation constant, the reaction of calcium ion with EDTA is more complete, and a larger change occurs in the equivalence-point region. The shaded areas show the transition range for the indicator Eriochrome Black T.

Consequently, during titration of solution containing magnesium and calcium ions with EDTA in presence of Eriochrome Black T the EDTA first reacts with the free calcium ions, then with the free magnesium ions, and finally with the magnesium-indicator complex. Since magnesium-indicator complex is wine red in colour and the free indicator is blue between pH = 7-11, the colour of the solution changes from wine red to blue at the end-point. 

---