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.常在水样中先加少量MgY2+配合物再用EDTA滴定,终点就敏锐这样做对测定结果有无影响?说明其原理

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.