Measure 100 ml water sample and put it in the conical flask, add 1 ml borax buffer solution, drop 2 to 3 drops of 0.5% acid chrome blue K indicator, then continuously shake the flask, use  0.0025 mol/LEDTA standard solution for titration, until the water turns to violet, record the volume of the consumed EDTA standard solution a,  the calculation is as follows:
                     2a·M
        YD=                   ×106   umol/L
                       V
        where ：
        a  — showing the volume of EDTA standard solution for titration，mL.
        V — showing the volume of water samples，mL.
        M — showing the molar concentration of EDTA standard solution, mol/L。