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。

滴定中Ca2 +，添加MgY2加强在终点的颜色变化。当MgY2与钙混合，发生如下反应：

1。 MgY2+钙+-----> Mg2 +的+ CaY2，（它们都是无色）
2。 Mg2 +的+黑色T----->红色络合物

在终点，以消除由于Mg2 +的_black T复合红色，额外的EDTA的需要更换黑色T和形成MgY2-。正是这种替换所需的EDTA量Y4的传递从最初添加钙离子量相同MgY2。所以整体的EDTA的摩尔数是完全平等的Ca2 +的摩尔数，而不是由量MgY2增加的影响。

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.