当水中Mg2+含量低时,以铬黑T作指示剂测定水中Ca2+.Mg2+总量,终点不明.

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.