當水中Mg2+含量低時,以鉻黑T作指示劑測定水中Ca2+.Mg2+總量,終點不明.

題目：

當水中Mg2+含量低時,以鉻黑T作指示劑測定水中Ca2+.Mg2+總量,終點不明.
常在水樣中先加少量MgY2+配合物再用EDTA滴定,終點就敏銳
這樣做對測定結果有無影響?說明其原理

解答：

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.