The complexing agent commonly used in coordination titration of 1. is disodium ethylenediamine tetraacetate (na 2 H2 y 2h2o), and the PH value of its aqueous solution is about 4.8. If the pH value is low, NaOH solution should be used to neutralize to PH=5 to avoid precipitation of EDTA after solution preparation. 2. With chrome black T(EBT) as indicator, it is calibrated in NH3 H2O-NH4C1+0 buffer solution (pH = 10), and EDTA standard solution is generally prepared by indirect method. Because EDTA standard solution can form a stable complex of 1: 1 with most metal ions, it can be calibrated with standard substances containing gold chip ions, such as Zn, Cu, Pb, CaCO3, MgSO _ 4·H2O, etc. Under a certain pH value, an appropriate indicator can be selected. In this experiment, zinc is used as reference and chrome black T(EBT) is used as indicator. The calibration was carried out in NH3 H2O-NH4C1+0 buffer solution (pH = 10).