如果mg2+的浓度小于ca2+浓度的1/20,则需加入5mlmg2+-edta溶液,为什么?

In titration of Ca2+, add MgY2- to enhance the color change at the end point. When MgY2- mixes with Ca2+, following reactions happen:

1. MgY2- + Ca2+ -----> Mg2+ + CaY2- (they are all colorless)
2. Mg2+ + Black T -----> red complex

at the end point, in order to eliminate the red color due to the Mg2+_black T complex, additional EDTA is needed to replace black T and to form MgY2-. The amount of EDTA required for this replacement is exactly the same as the amount of Y4- passed to Ca2+ from the initially added MgY2-. So the overall mole number of EDTA is exactly equal to the mole number of Ca2+, and is not impacted by the amount of MgY2- added.